Dec 16, 2017 - day and is being applied in biotechnology, information and ...... [58]- Qiang Chen, Dayu Shu, Chuankai Hu, Zude Zhao, Baoguo Yuan, âGrain.
KINGDOM OF SAUDI ARABIA Ministry of Higher Education Qassim University College of Engineering Mechanical Department
Mechanical Properties of Nanostructured Nickel Alloy Processed Using Multiaxial Forging A Thesis Submitted in Partial Fulfilment of the Requirements for the Master Degree in Mechanical Engineering. By Zahid Hussain 361118085
Supervisor Dr. Fahad A.Almufadi Associate Professor of Mechanical Engineering Department, Engineering College, Qassim University, Saudi Arabia Academic Year 2017/2018
KINGDOM OF SAUDI ARABIA Ministry of Higher Education
Qassim University College of Engineering Mechanical Department
Mechanical Properties of Nanostructured Nickel Alloy Processed Using Multiaxial Forging By Zahid Hussain The Committee has approved this dissertation as a partial completion of the requirement for the master Degree in Mechanical Engineering. Examination and Decision Making Committee Committee Members Advisor Internal Examiner
Name
Academic Degree
Specialization
Dr. Fahad
Associate
Materials
A.Almufadi
Professor
Manufacturing
Associate
Materials
Professor
Manufacture
Dr. Osma M.Irfan
and and
Internal
Dr. Hany Ali
Examiner
Sherif
External
Dr. Waleed
Assistant
Materials
Examiner
Hassan
Professor
Manufacturing
Professor
Date: 24/12/2017
Materials Engineering and
Signature
ACKNOWLEDGEMENT I express my deep gratitude to Almighty Allah for giving me strength and the composure to complete my coursework and this research work successfully. I would like to extend my most gratitude to my supervisor Dr. Fahad A. Al-Mufadi (Dean College of Engineering) whose guidance and advice enabled me to take this endeavor. Whenever I needed his support, he was happy to provide all the facilities. Despite being busy, he always welcomed me and did his best in resolving the issues like material procurement, manufacturing of die etc. Also, I am thankful to my co-advisor Dr. Osama M. Irfan for his guidance and patience over the past year to carry out this research. I am indebted to Dr. Abdulaziz Alaboodi Chairman of Mechanical Engineering Department for his continuous support and encouragement. I would like to pay thanks to Prof. Mohammad A. Irfan for his love, affection and support throughout my graduate studies. He always encouraged me to work hard and to hope for the best. The acknowledgement would be incomplete if I don’t mention here Dr. S. Sivasankaran for his sincere help in conducting experiments, discussing results and drawing conclusions. My special gratitude goes to him. My special thanks also go to Prof. Farmarzjavan Roodi for providing assistance to numerical modelling of MAF. Last but not least, special mention to all support, love and care offered by my family. To my mother whose supplications are always with me. And my sister, who always been there for me when I needed her. And particularly noteworthy “thank you” to my wife and daughters for their patience and sacrifice during my studies. In fact, they have contributed a lot in sparing me to pursue my research goals. iv
TABLE OF CONTENTS ACKNOWLEDGEMENT………………………………………………………...iv TABLE OF CONTENTS…………………………………………………………..v LIST OF FIGURES……………………………………………………………...viii LIST OF TABLES………………………………………………………………...xi LIST OF ABBREVIATIONS…………………………………………………….xii LIST OF SYMBOLS…………………………………………………………….xiii ABSTRACT……………………………………………………………………..xiv CHAPTER I………………………………………………………………………1 1. INTRODUCTION………………………………………………………………1 1.1 Background ................................................................................................ …1 1.2 Nano-structured Materials (NS) .................................................................. …1 1.2.1 Classification of NS materials....................................... ………………3 1.2.2 Mechanical Properties of NS materials ................................................. 4 1.3 Synthesis of Nanomaterials ............................................................................. 9 1.3.1 Equal Channel Angular Pressing (ECAP) ........................................... 10 1.3.2 High Pressure Torsion (HPT) .............................................................. 10 1.3.3 Twist Extrusion (TE) ........................................................................... 11 1.3.4 Multiaxial Forging (MAF) ................................................................... 12 1.3.5 Accumulative Roll Bonding (ARB)..................................................... 12 1.3.6 Straightening and Repetitive Corrugation (SRC) ................................ 13 v
1.4 Forging Process ............................................................................................. 13 1.4.1 Material Behavior ................................................................................ 14 1.4.2 Forging Load Calculation .................................................................... 18 CHAPTER II…………………………………………………………………....22 2. LITERATURE REVIEW……………………………………………………...22 2.1 Processing of MAF........................................................................................ 22 2.1 Nickel Processing Using SPD; A Survey ...................................................... 29 CHAPTER III………………………………………………………………….. 33 3. EXPERIMENTAL METHODOLOGY……………………………………….33 3.1 Flow chart ................................................................................................... 33 3.2 Material procurement and sample preparation .............................................. 34 3.3 MAF Die Design ........................................................................................... 34 3.4 MAF Execution of Nickel ............................................................................ 36 3.5 Finite Element Modeling of MAF ................................................................. 39 3.6 Validation of FEA Model .............................................................................. 43 3.7 Characterization Techniques ......................................................................... 45 3.7.1 Mechanical Properties Evaluation....................................................... 45 3.7.2 Microstructural Examination .............................................................. 47 CHAPTER IV…………………………………………………………………...50 4. RESULTS AND DISCUSSIONS……………………………………………...50 4.1 Stress Distribution of MAF Die Using ABAQUS ........................................ 50 vi
4.2 Strain Distribution in SPD Processed Nickel ............................................... 52 4.2.1 Effect of Strain Rate on SPD Processed Nickel……………………...52 4.2.2 Effect of Temperature on SPD Processed Nickel ............................... 53 4.2.3 Effect of Friction on SPD processed Nickel ........................................ 54 4.3 Investigation of MAF on Nickel ................................................................... 56 4.3.1 X-ray diffraction analysis .................................................................... 56 4.3.2 Microstructural examination using optical microscope (OM) ............ 59 4.3.3 Microstructural Examination Using Transmission Electron Microscope .......................................................................................................... 61 4.3.4 Compression Stress-strain Analysis .................................................... 65 4.3.5 Hardness Examination ........................................................................ 67 CHAPTER V ……………………………………………………………………69 5. CONCLUSION………………………………………………………………...69 CHAPTER VI…………………………………………………………………...70 6. FUTURE SCOPE………………………………………………………………70 CHAPTER VII…………………………………………………………………..71 7. PUBLICATIONS……………………………………………………………...71 REFERENCES…………………………………………………………………...72 APPENDIX ‘A’…………………………………………………………………..85 APPENDIX ‘B’…………………………………………………………………..86
vii
LIST OF FIGURES Figure 1.1. (a) Zero-dimensional Clay and Cluster; (b) One-dimensional Nanofibers, wires and rods; (c)Two-dimensional plate and networks; (d) Three-dimensional nanomaterials...3 Figure 1.2. Illustration of the effeext of grain size on yield strength of the material……..6 Figure 1.3. Shear bands in UFG nickel produced by SPD method………………..….…..8 Figure 1.4. Deformation and fracture of UFG material: (a) Plastic flow localization; (b) nanocrack nucleation; (c) final failure………………………………………………….….8 Figure1.5.Cross section of ECAP die with s pecimen and plunger....…………….……..10 Figure 1.6. High pressure torsion-schematic………………...……………….………….11 Figure 1.7. Schematic illustration of twist extrusion…………………………………….11 Figure 1.8. Schematic illustration of MAF process showing how the sizes change during process so that overall volume is constant…………………………………………..…....12 Figure 1.9. Straightening and Repetitive Corrugation…………………………...…...…..13 Figure 1.10. Plane strain upsetting of rectangular billet..…………………………..……19 Figure 1.11. Stresses on small elemnt of width dx due to die pressure…….……………20 Figure 3.1. Flow Chart iluustrating the overall volume of the work…………………….33 Figure 3.2. MAF Die (a) MAF-Die Assembly (b) Illustration of MAF execution (c) MAFed Samples before and after processing………….………………………………….………35 Figure 3.3. The hydraulic press used in present study.……………………..……………37 Figure 3.4. (a) Flow stress vs flow strain (b) Finite Element Mesh System of Nickel sample.................................................................................................................................40 Figure 3.5. Comparison between kinetic energy with total energy of the system…….…44 Figure 3.6. FEM validation graph for MAF Simulation………...…………………..……45 viii
Figure 3.7. Universal hardness measuring device (Zwick/ZHU 250) used in present study……………………………………………………………………..………...…......46 Figure 3.8. MTS universal testing machine used in present study………………..……....47 Figure 3.9. Optical Microscope used for microstructural examination.…………….…….48 Figure 3.10. Mounting press for sample preparation for polishing (b) Polishing and lappingmachine(c)Samples for microstructural examination……………….……………49 Figure4.1. Stress distribution in MAF die during MAF operation………………………..51 Figure 4.2. Figure 4.2. Inhomogeneous factor at (a) 25 °C; (b) 250 °C; (c)500 °C….…....53 Figure 4.3. Strain distribution contour plot (a) at 25 °C; (b) at 250 °C; (c) 500 °C…...…..55 Figure 4.4. Effective strain at (a) 25 °C; (b) 250 °C; (c)500 °C …….……………….........56 Figure 4.5. X-ray diffraction patterns of pure nickel with different number of cycles of MAF………………………………………………………………………...……………58 Figure 4.6. Optical microstructures of pure nickel during MAF: (a) zero cycle (b) one cycle (c) two cycles (d) three cycles (e) four cycles……………………………………...........60 Figure 4.7. TEM bright field images of Ni 200 alloy after MAF of: (a) One cycle; (b) two cycles; (c) three cycles and (d) SAD pattern of (c); (d) four cycles ….……………………62 Figure 4.8. Schematic of microstructure of Ni (a) 0 cycle of MAF; (b) 3 cycle of MAF; (c) 4 cycle of MAF …………………………………..………………………………….......63 Figure 4.9. Compressive engineering stress and strain plots for pure nickel after MAF with different cycles of forging………..………………………..……………………..………66
ix
Figure 4.10. Vicker’s hardness and inhomogeneity factor of pure nickel after MAF with different number of cycles……………………………………………………..…………68
x
LIST OF TABLES Table 1.1. Dependent and Independent variables for the forging process…....................14 Table 1.2. Advantages and disadvantages of forging process………….……………..…16 Table 2.1. SPD of nickel and its alloy: a survey………………………………………….28 Table 3.1. Mechanical Properties of H-13 tool steel………………………….….……….35 Table 3.2. Input parameters for design of MAF die………………………………………36 Table 3.3. Johnson-Cook parameters for nickel………………………………………….39 Table 3.4. Mechanical and thermal properties of nickel…………………….…...............40 Table 3.5. Input and output parameters during compression……………………………..43 Table 4.1. Factor of safety of MAF under different processing conditions………………51 Table 4.2. Structural analysis of Ni during MAF at different cycles ………...………….59 Table 4.3. Mechanical properties of pure nickel during MAF with different cycles…….67 Table 7.1. Research publications details…………………………….……………………71
xi
LIST OF ABBREVIATIONS ARB
Accumulative Roll Bonding
ES
Effective Strain
ECAP
Equal Channel Angular Pressing
EBSD
Electron Back Scatter Diffraction
FEA
Finite Element Analysis
FEM
Finite Element Modelling
FOS
Factor of Safety
HAGB
High Angle Grain Boundaries
HPT
High Pressure Torsion
IF
Inhomogeneous Factor
MAF
Multiaxial Forging
NS
Nanostructured Materials
OM
Optical Microscope
SAD
Selective Area Diffraction
SPD
Severe Plastic Deformation
TE
Twist Extrusion
TEM
Transmission Electron Microscope
UFG
Ultrafine Grained Material
XRD
X-Ray Diffraction
xii
LIST OF SYMBOLS σ
Engineering Stress
ε
Engineering Strain
Yf
Flow Strength
p
Die Pressure
𝜀̇
Strain Rate
v
Velocity of the ram
h
Height of Processed sample
T
Temperature
n
Strain Hardening exponent
m
Thermal softening exponent
μ
Coefficient of friction
γ
Shear strain
ρ
Dislocation Density
H
Hardness
θ
Bragg’s Angle
λ
X-Ray Wavelength
t
Crystallite size
b
Burger’s Vector
xiii
ABSTRACT Nanostructured (NS) materials possess grain size in the range of 10 to 100 nm. These materials are increasingly becoming the center of interest of material scientists because of their novel properties and applications. Fabrication of ultrafine grained (UFG) or NS materials is possible either by severe plastic deformation (SPD) method or by consolidation process using mechanical alloying/mechanical milling. In this work, pure nickel was processed by MAF in a confined channel die. Mechanical properties (compressive yield strength and hardness) and resultant microstructures using x-ray diffraction, optical microscope and transmission scanning electron microscope were investigated and compared with original material. In order to achieve the objectives, various steps were taken. The steps included for the accomplishment of task were design of confined channel die, selection of sizes of the specimen to able to measure all mechanical properties according to ASTM standards, execution of MAF process and then measuring and comparing the properties before and after MAF process. MAF die was designed by analyzing the stresses in it through finite element analysis software for optimization, better design and execution. The observed results were explained that the grain size of the nickel was decreased significantly due to grain refinement. The rate of shifting of the microstructure to ultrafine and nano regimes was variable as it was remarkable after the first cycle of MAF and reduced significantly in subsequent cycles. The average grain size after the third cycle was 220± 8 nm. The results of uniaxial compression and hardness tests were indicated the considerable increase in strength, elastic modulus and hardness when compared to unforged samples. These results also confirmed the deployment of this technique to manufacture NS xiv
for ‘not easily deformed’ materials at room temperature. The distribution of properties along the MAFed samples as the function of temperature, strain rate and friction has also been investigated by simulating the MAF process in ABAQUS.
xv
CHAPTER I 1. INTRODUCTION 1.1 Background UFG have received a special attention due to their potential applications in modern industries [1]. With the refined structure, materials exhibit superior mechanical, electrical, magnetic and chemical properties. A variety of techniques are being used to fabricate such type of materials [1]. Among these, the SPD is a well-recognized technique that has the capacity to minimize the defects associated with other methods while producing UFG / NS [2]. In this method, the materials (metals and alloys) are subjected to very large strain such that there is no change in sample dimensions. Due to dislocation of large grain boundaries, the outcome is a relatively refined structure. SPD in fact is a large family and many methods belong to this widened technique. Each method has its own pros and cons. Therefore, at the end of this chapter, we presented some basic concepts about forging in depth for studying the multiaxial forging (MAF) process. This chapter has been divided into three sections. The first part introduces nanomaterials, properties and applications. The second part is devoted to understanding the SPD methods in current practice. Last part focuses on the forging theory.
1.2
Nano-structured Materials (NS) Nanotechnology can be defined as development of devices, structures and systems
through controlling the properties-responses and functionality under 100 nm range [3-4]. Nanotechnology is an emerging technology that has potential to create new products and devices and bring innovation to already established devices. It has become a topic of the 1
day and is being applied in biotechnology, information and communication technology, medicine industry, cosmetics, food industry, smart materials, electronics and precision mechanism of high degree like achievement of a precise target for a cruise missile [5]. Nanotechnology has the capacity to adequately address the serious energy, biomedical and environmental issues. Not only affecting the engineering industry, this discipline is also a very handful tool to solve humanity problems like diseases, weather climate and energy efficiency [5]. “Nanotechnology, in addition to advancing the competitiveness of the industry, is also responsible for creating innovative products that are certainly bringing positive changes in the society. New avenues of research are being foreseen because due to the emergence of a dawn of nanotechnology and thus leading to new and occasionally surprising applications. The performance of newly developed materials and surfaces are excellent. Fatal diseases such as brain tumor and Alzheimer are being treated by applying nanotechnology [5]. Nanomaterials such as nanoparticles, carbon nanotubes quantum wires have been developed and are available for commercial applications [4]. Commercial products cover a wide range of materials including metal, ceramics, composites, cosmetics textiles, varnishes and paints [4]. However, there persists a need to develop new methodologies and to increase the sphere of knowledge and information about the potential consequences of their use with the perspective of environment, safety and health related topics. New developments also need to be judged on the ethical and moral grounds [5]. The conventional material has minimum grain size of an order of 10 μm followed by the microstructural material with range of 10 μm to 1 μm. UFG materials and NS materials lie within the range of 100 nm to1 μm. 2
1.2.1 Classification of NS Materials Based on dimensions of sizes of the orders of nanometer, nanomaterials can be classified as zero-dimensional (Figure 1.1(a)), one-dimensional (Figure l.1 (b)), twodimensional (Figure 1.1(c)) or three dimensional (Figure 1.1 (d)). Atomic clusters, nanoparticles, filaments and similar spatially confined molecular systems are defined as zero modulation dimensionality (0D or more correctly quasi-zero dimensional) and possess an aspect ratio from one to infinity. Atomic clusters are spherical and composed of several thousand atoms [6]. If nanomaterials are nanoscale in one dimensions like surface films, they can be regarded as one dimensional. Fibers are the example of two-dimensional nano materials. Nano particles have three sizes in the range of nanometer and hence are called three-dimensional nanomaterials. Nanotubes, dendrimers, quantum dots and fullerenes are some common types of nanomaterials [6]. Each type has been displayed in Figure 1.1. Nanomaterials exhibit different physical, chemical, electrical, optical and magnetic properties and have applications in the field of nano technology.
Figure 1.1. (a) Zero-dimensional clay and cluster; (b) One-dimensional nanofibers, wires and rods; (c) Two-dimensional plate and networks; (d) Three-dimensional nanomaterials [6]
3
1.2.2 Mechanical Properties of NS Materials At nano-scale, materials can exhibit different properties from coarse grained materials. What makes nanomaterials different is the size of feature comparable to the physical phenomenon. For instance, the growth and propagation of crack with tip radius in the range of 1-100 nm are likely to be different in nanomaterials from bulk materials [7]. Thus it can be said that mechanical response of the materials is different in two cases. Similarly biological, chemical, magnetic etc. characteristics also become different at this level. For instance, proteins of the size of 1-1000 nm and cell wall of thickness of 100 nm would interact with nanomaterials quite differently as they are supposed to do with materials [7]. The advantage of nanomaterials is that, unlike coarse grained materials, their properties can be tailored in a predictable manner by controlling the dimensions at the nanoscale. In order to be close to our topic, it was aimed to present how nanostructure does affect the mechanical properties. Grain size is a basic microstructural parameter that controls the mechanical properties and responses of the materials. There persists an ever-increasing demand for devolving high strength materials in many applications, so the grain size control is a very useful technique to tailor mechanical properties of the materials according to the specific application. Famous Hall-Petch relation (equation 1.1) indicates that the yield strength of material varies as the function of grain size. Higher is the strength, if smaller is the grain size as shown in the following equation [8]: 1
𝜎 = 𝜎 ° + 𝑘 𝑑 –2
(1.1)
4
Where d is the average grain size; σo and k are constants that depend upon nature of the material. Various theories have been presented to explain the mechanisms of this phenomenon: accumulation of dislocations against the grain boundaries (GBs), dislocations are formed by GBs and presence of necessary dislocations in the zones of GBs to account for the deformation compatibility of polycrystalline metals etc. [9]. It is to be noted that average grain size, grain size distribution and grain boundary structure would influence the mechanical properties materials. There has been observed a significant increase in yield strength and hardness, decreased elongation, improved wear resistance and promising performance under high strain rate applications [7]. The properties which are influenced by nanostructure and its structural applications are given below: (a) Hardness and Strength Hardness and strength of the materials are dependent on grain size. The fine grained materials with high angle grain boundaries ((HAGB) provide resistance to dislocation motion because of necessity in the change of direction of motion in the vicinity of grain boundaries. Pile-up of dislocations is considered as the basic cause of enhanced plastic flow resistance. When microstructure is refined further, the process further breaks down and stress vs grain size relationship significantly deviates from that seen at higher grain sizes as illustrated in Figure 1.2. Normally, yield strength is at its peak when the grain size is 10 nm or so [10]. Further refinement causes to weaken the material and hence called inverse Hall-Patch relationship. The reason behind this unusual phenomenon is not clear yet. However, mechanical properties of metals such as copper, nickel, palladium etc. below 100 nm have been investigated through uniaxial tensile/compression and nanoindentation, and 5
enhanced yield strength has been observed [10]. The hardness of the nanomaterials has also been observed to increase as compared with their coarse grained counterparts. Among many mechanical properties of nanomaterials, high hardness of nanocomposites is one of the most fascinating. Hardness can be increased by the introduction of nitrides, borides and carbides through the use of chemical and physical deposition techniques. Among many excellent mechanical properties of nanomaterials, high hardness of nanocomposites systems is one of the most intriguing. Superhard materials can be developed through the introduction of nano particles like nitride, borides and carbides by modern techniques such as chemical and physical deposition methods. The hardness of the consequent binary system considerably increases as compared with that given by the rule of mixtures in bulk. Nanocrystalline MnNa– Si3N4 (M=Ti, W, V,…) nano-composites with the optimum content of amorphous Si3N4 matrix close to the percolation threshold, for instance, has been found about 50 GPa whereas maximum hardness of individual nitride is 21 GPa only [10].
Figure 1.2. Illustration of the effect of grain size on yield strength [10]
6
A relatively high force would be required to displace the dislocations to cut through the crystallite of nitrides. Ductile amorphous Si3N4 is supposed to act as the barrier for the propagation of possible crack as the result of the application of high stresses thereby minimizing the chances of fracture. These superhard nanocomposites can be applied in hard protective films. Super hardness can also be achieved through pure nanoparticles. For example, bulk silicon possesses the hardness of 15 GPa, however, when its size is reduced to 20-50 nm, its hardness increases to four times [10]. (b) Ductility Nanomaterials have been observed to display low tensile ductility which limits their practical use for the applications where there is a requirement of high ductility. Conventional microcrystalline materials show higher tensile elongation at fracture relative to the nanomaterials [10]. There are three factors responsible for limiting the ductility: structural defects such as cracks or porosity; nucleation of cracks; plastic tensile instability [10]. Defects obviously decrease a fracture strength as they are vulnerable sites for crack initiation and propagation and thus causing a greater chance for brittle failure. During deformation, the dislocation storage and annihilation at grain boundaries are two key competing mechanisms influencing the level of the flow stress. Observation of dimple rupture on fracture surfaces of FCC metals can be attributed to highly localized deformation that results in decreased ductility. Formation of the shear bands on deformed specimens indicates localized deformation as shown in Figure 1.3. Plastic flow localization in materials in the absence of strengthening leads to the macroscopic necking (Figure 1.4.), followed by the stress concentration in the neck region. Nano-scale voids in the structure have been observed to influence the mode of failures. For instance, nickel films made by 7
magnetron sputtered method having nano-voids between the grains fail in a brittle manner whereas film made by laser deposition method has no voids, continuously get thin with the propagation of crack(ductile failure) [10].
Figure 1.3. Shear bands in UFG nickel produced by SPD method
Figure 1.4. Deformation and fracture of UFG material: (a) Plastic flow localization; (b) Nanocrack nucleation; (c) final failure [10]
8
1.3
Synthesis of Nanomaterials There are various methods of synthesis of nanomaterials and these can broadly be
classified as using “Top-Down Approach” and “Bottom-Up Approach”. The top-down approach uses the breakdown of a larger structure into smaller one whereas in bottom-up approach, small particles are joined in a controlled way to create the bulk materials. The SPD is, in fact, the top-down approach. Since we are interested in SPD, it would be useful here to discuss the SPD in details. Sub-micrometers sized grains are formed in initially coarse-grained (CG) material by application of SPD. Consequently, an enhanced mechanical performance (strength, tribological etc.) was observed. It is, however, thought that SPD results in the formation of shear bands which causes subdivision of grains due to deformation process [11-13]. It is well established that enormous deformation of metals results in a distinctive structure of dislocations and extremely fine grains. The important parameters in defining a submicron grain structure are an average spacing of HAGB and proportion of HAGB area [13]. SPD causes major changes of the material’s structure which are reflected in enhanced mechanical and physical properties of metals such as hardness and yield stress. However, the disadvantage of the material deformed by SPD is limited ductility [14]. It is worth mentioned here that most researches showed an increase in ductility and toughness as well as enhancement of the physical properties of other materials. The fine-grained structure of materials can be attained by SPD results in superplastic behavior at lower temperatures [15]. Materials can be severely deformed by employing many techniques each having set of advantages and disadvantages. Different SPD methods which are being used for research purposes are given below. 9
1.3.1 Equal Channel Angular Pressing (ECAP) A variety of metals and alloys have been successfully processed by this method, schematic of which is shown in Figure 1.5. Here a sample is pressed into the channels of a die. High pressure causes the material to flow within the channel. Die channel angle may vary from 60 degrees to 120 degrees. A substantial grain refinement can be obtained by a single pass, however, it needs four to six passes to make the homogeneous structure. To date, many materials and alloys such as copper, Al-Mg alloys, pure titanium have been processed [16-17] through ECAP.
Figure 1.5.Cross-section of ECAP die with specimen and plunger [17]
1.3.2 High Pressure Torsion (HPT) The grain size of 100 nm or less can be obtained by application of HPT [18]. This technique has been successfully applied to a variety of materials such as metals, alloys, composites and more recently on semiconductors [19]. Figure 1.6 shows the schematic of HPT technique. A circular specimen of is placed between two anvils; one being fixed and the other being rotatable. High pressure is applied on the fixed anvil and the rotatable anvil is rotated. The friction between the sample and the fixed anvil results into large strains. The
10
pressing force (high pressure) prevents the specimen from breaking despite application of severe deformation [19]
Figure 1.6. High pressure torsion-schematic [19]
1.3.3 Twist Extrusion (TE) The principle of twist extrusion is shown in Figure 1.7. A billet is twisted in a die and thus intense severe deformation is obtained [20]. The form and the cross-sectional area remains the same along the extrusion axis. Twist extrusion method can be used to process metallic materials and alloys [20].
Figure 1.7. Schematic illustration of twist extrusion [20]
11
1.3.4 Multiaxial Forging (MAF) Multi axial forging is one of the most effective and easiest ways to impose a severe strain on bulk materials [21-22]. No specific devices are needed to apply MAF as shown in Figure 1.8. The appropriate strain rate and temperature are the key parameters that control this process. The material of suitable size is pressed in one direction in a die that may be opened or closed, removed, rotated and repressed in a second orthogonal direction. This is followed by pressing in the third orthogonal direction so that overall size of the specimen does not alter. Pressing in one direction is technically called ‘Pass’ and pressing in three orthogonal directions is termed as ‘cycle’.
Figure 1.8. Schematic illustration of MAF process showing how the sizes change during the process so that overall volume is constant.
1.3.5 Accumulative Roll Bonding (ARB) Accumulative roll bonding is a deformation method in which two metal sheets of the same thickness are simultaneously passed between two rollers. In ARB the two sheets 12
join together to form one solid body and can be halved once again. The rolling process can be repeated many times. In most cases, the process is repeated up to 10 times [23-24].
1.3.6 Straightening and Repetitive Corrugation (SRC) Schematic of straightening and repetitive corrugation method has been shown in Figure 1.9 where a specimen is alternatively subjected to shear and bending stresses when it is passed between two gears like rollers under constraining pressure [25-26].
Figure 1.9. Straightening and Repetitive Corrugation [25]
1.4
Forging Process The basics of forging process play a vital role in understanding the mechanics of
MAF. Forging is a typical type of forming processes where materials are subjected to compressive stresses in a free or confined die to deform them to desired sizes and shapes. Materials should possess certain characteristics to be successfully forged. It should have an appreciable level of ductility and low yield strength. These properties are affected by temperature, strain rate and friction. During the forming process, there are some variables, called independent variables that can be controlled directly whereas dependent 13
variables are the consequences of independent variables. Table 1.1 enlists the independent and dependent variables. The link between independent and dependent variables is truly the most important area of knowledge for a person in metal forging. Experience, experiment and modeling link between dependent and independent variables. Table 1.1. Dependent and Independent variables for the forging process [27] Type of the material to be processed Initial geometry of the sample Geometry of the die Independent Variables
Lubricating conditions Processing temperature Strain Rate Effective Strain Load required to carry out the process Properties of resultant product
Dependent Variables
Temperature after forging Surface finish and dimensional precision Material flow conditions
1.4.1 Material Behavior . The knowledge of material behavior is essential to understand forging requirements, die design, initial temperature, material size and power requirements etc. The plastic region of stress vs strain curve is important to understand as the material is to be
14
deformed plastically. The material behaviour in this region is expressed by the flow curve defined by following equation [28] 𝜎 = 𝐵𝜀 𝑛
(1.2)
Where B is strength coefficient, n is strain hardening exponent, ε is true strain σ is true stress. (i) Flow Stress When a material is deformed its strength increases due to the strain hardening. This also happens during forging process. The progressively increasing strength with the imposed strain requires higher and higher strength. The value of stress at any time during deformation is called the flow stress. It is denoted by Yf. Flow stress can be deemed as the yield strength that is dependent on total strain and is given by the following equation [28]. 𝑌𝑓 = 𝐵 𝜀 𝑛
(1.3)
The average flow stress (also called the Mean flow stress) to carry out the entire deformation is denoted by𝑌̅𝑓 and is given by the following equation [28]. ) 𝐵(𝜀 𝑌̅𝑓 = 𝑚𝑎𝑥 1+𝑛
𝑛
(1.4)
(ii) Temperature Temperature plays an important role during deformation process because generally, the yield strength drops at elevated temperature and hence relatively lower forces and power would be required to achieve the desired deformation. But keep in mind quality of microstructure may get affected at elevated temperatures. Based on temperature, the
15
forging process can be categorized as cold forming, warm working and hot working process. The definition, advantages and disadvantages are summarized in Table 1.2. (iii) Strain Rate Strain rate refers to the amount of strain imposed on a material per unit time during the forging process. Strain rate is related to the speed of ram of a forging press and is defined as [27]
𝜀̇ =
𝑣
(1.5)
ℎ
Where ε̇ is strain rate, v is the velocity of the ram of the press and h is the instantaneous height of the workpiece being deformed. Table 1.2. Advantages and disadvantages of forging process [27] Category Cold Working
Definition 0.3 Tm
Advantages
Disadvantages
Improved quality of
More forces are
the product High yield and
required Heavy
ultimate strangth of
Equipment’s and
the product
tooling required
More refined
Starting material
structure
should be free
No heating
from dirt
equipment is needed Warm Working
0.3 Tm – 0.5 Tm
Relatively lower forces required More intricate geometry possible
Heating of work piece is required Surface finish is not so good 16
Need for annealing reduced Hot Working
0.5 Tm – 0.75 Tm
Grain size increases
Lower forces and power requirements Shape can be
Lower dimensional accuracy
altered significantly Homogeneity in strength is possible in the product
High total energy required Poorer surface finish Shorter tool life
When a material is subjected to high strains (>103 s-1), the calculation of flow stress is not as simple as given by eq. (1.3). The dynamics effects and influence of temperature must be considered to for plastic flow stress determination. Different constituent models have been proposed for this purpose which are valid for specific material under specific conditions. Banerjee et al [29] proposed that Johnson-Cook (J-C) model was well suited for metals like nickel, aluminum and steel etc. J-C constitutive model is described by following governing equation [29]: 𝜀̇
𝑇−𝑇𝑟
𝑜
𝑚
𝑌𝑓 = (𝐴 + 𝐵𝜀 𝑛 ) [1 + 𝐶 ln (𝜀 ̇ )] [1 − (𝑇
𝑚
) ] −𝑇 𝑟
(1.6)
Where Yf is the flow stress, ε is the strain, 𝜀̇ is the strain rate and T is the initial temperature. The flow of materials not only depend upon the value of imposed strain but also on the strain rate and thermal conditions. A, B, C, n and m are the five parameters defining completely the flow behaviour of material, A is yield strength, B is strain hardening coefficient, C is the strain rate hardening coefficient, n is the strain hardening exponent and m is the thermal softening exponent. Also, 𝜀𝑜̇ is the reference strain rate at 17
which yield strength calculated, Tr is room temperature and Tm is the melting temperature of the material. (iv) Friction Friction in metal forming is an important parameter on which forming process depends. It is caused by the close contact between workpiece and tool die along with high pressure during the forging process. In most metal forming processes, the friction is undesirable because of following reasons: i.
Metal flow is not adequate.
ii.
High power is required.
iii.
Wear and tear of the tool occurs at rapid rate Friction creates a severe problem in hot working processes. For high value of
friction, there is the possibility of occurrence of condition known as sticking which causes two surfaces to stick together instead of sliding. To avoid this condition, lubricants are used between the surfaces that also give a better surface finish.
1.4.2 Forging Load Calculation Consider a rectangular billet of height ho, width “2a” (symmetric along y-axis) and unit depth z-axis. Considering the plane strain conditions, when this billet is subjected to upsetting load, it undergoes strain only in x and y directions because z-direction is restrained. The schematic is shown in Figure 1.10. Let the lateral stress is uniform along the height.
18
Figure 1.10. Plane strain upsetting of rectangular billet Consider a small element strip of width dx with unit depth as shown in Figure 1.11. Force balance gives: (𝜎𝑥 + 𝑑𝜎𝑥 )ℎ + 2𝜏𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 𝑑𝑥 − 𝜎𝑥 ℎ = 0 𝑑𝜎𝑥 ℎ + 2𝜇𝑝𝑑𝑥 = 0
(1.7)
Where σx is stress along x-direction, 𝜏𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 is the force of friction, 𝑝 is the instantaneous die pressure required for the flow of material and μ is the coefficient of friction. Applying Von-Mises criterion for plane strain [28] 𝜎𝑥 + 𝑝 =
2 𝑌 √3 𝑓
(1.8)
.
19
Figure 1.11. Stresses on small element of width dx due to die pressure
Substituting after differentiation in equation (1.7) (𝑑𝑝)ℎ + 2𝜇𝑝𝑑𝑥 𝑑𝑝 2𝜇 = 𝑑𝑥 𝑝 ℎ Upon integration above equation, we get 𝑝 = 𝐶𝑒
−2𝜇𝑥 ℎ
Applying boundary conditions, At 𝑥 = 𝑎; 𝜎𝑥 = 0, So from equation (2), we have, 𝑝 =
2 √3
(𝑌)
20
2 √3
(𝑌𝑓 ) = 𝐶𝑒
𝐶=
2 √3
𝑌𝑓 𝑒
−2𝜇𝑎 ℎ
2𝜇𝑎 ℎ
Therefore,
𝑝=
2 √3
𝑌𝑓 (𝑒
2𝜇(𝑎−𝑥) ℎ
)
(1.9)
The average forging pressure can be calculated by integrating above equation from 0 to a. 1 𝑎 𝑝 = ∫ 𝑝 𝑑𝑥 𝑎 0 Substituting 𝑝 from equation (1.9) in above equation, we get 𝑝=
2 √3
𝑌𝑓 (𝑒
2µ𝑎 ℎ
− 1)
(1.10)
𝑌𝑓 is the average flow stress and can be calculated by using equation (1.4).This relation gives the average value of forging pressure. Average load required to forge the material can be found by: 𝐹 = 4 𝑎2 𝑝
(1.11)
21
CHAPTER II 2. LITERATURE REVIEW 2.1 Processing of MAF It has been reported that NS/UFG materials could be produced by the application of severe plastic deformation. Various SPD methods are available each having a set of advantages and disadvantages. For example, ECAP, HPT and TE have been successfully applied to fabricate UFG materials [30-33]. However, disadvantage associated with these is their limited potential to be applied on large scale. It is to be noted that large functional materials require high strength and hardness. There are some SPD methods that can be employed on large scale but their drawback is that the quality of product is not as good as impurities include in the deformed material. This can be observed in case of Accumulative Roll Bonding. [34-37]. Multi axial forging is also one of the SPD methods through which severe strain can be imposed without compromising the quality of the product [38]. Among the processing techniques discussed above, MAF is the easiest to realize as no specific devices are needed. Furthermore, it can be used on industrial level for mass production because it is capable of processing relatively large materials [39-40]. So far, MAF technology has been limited to produce some UFG materials such as copper, titanium and titanium alloys, steel, aluminum and magnesium [39-46]. These studies mainly focus on the function of MAF process on grain refinement and the subsequent mechanical properties of the materials [47-48]. However, a researcher can easily find a gap about this kind of processing and microstructural evolution is not clear yet for many materials [48].
22
MAF can be executed either in open (free) or closed (confined) die. In open channel die the amount of strain is controlled manually contrary to the closed channel die. Kundu et al. [49] emphasized the use of closed channel die to avoid grinding and extra machining work. Pure titanium was first time processed in confined channel die by Kumar et al [44]. The MAF was carried out at high temperature (500 °C) and grain refinement of the order of 0.3 μm was achieved. Also, it was revealed that inhomogeneity Vis: a common problem in SPD processed materials, was reduced to a great extent by subsequent passes in the cyclic channel die. MAF can be considered as one of the best SPD techniques as it provides an opportunity to optimize the mechanical properties. In general, by application of SPD, strength increases at the expense of ductility. However, one can control parameters like the number of cycles, strain rate and temperature to get optimum results as has been done by Somjeet et al. [45] in case of magnesium alloy Mg–3Al–0.4Mn. When materials are deformed to very large strains, in addition to the reduction in grain size, the emergence of crystallographic texture takes place due to the severe strains imposed on the material. It is well-known fact that many mechanical properties such as rate of strength hardening, formability, anisotropy etc. are largely influenced by the deformation texture [46]. To date, limited published data have been found that investigate the texture evaluation during SPD processes other than ECAP. Gurao et al. [46] studied deformation texture evolution of multiaxially forged interstitial free (IF) steel through visco-plastic simulation. MAF can be potentially used for the weakening of the texture in magnesium alloy [50]. Ting et al. [51] observed the evolution of weak texture in Mg-7Gd5Y-1Nd-0.5Zr after two cycles of MAF. 23
MAF is a very useful technique in thixforming procedure. The thixforming involves the forming of material in semi-solid state to near net shaped components. In this process, material looks like a solid but behaves pseudoplastically and thixotropically [52]. The reason behind this usefulness is the ease with which partial re-melting temperature can be applied to the specimen. This has been done for ZK 51 magnesium alloy by Quan et al. [53]. It has now been established that mechanical properties of severely deformed materials are not only influenced upon the amount of strain imposed and the strain homogeneity factor obtained but also, the in-situ temperature and the applied strain rate have an effect on these. The possible solution might be optimizing SPD process so that less number of cycles (or passes) should refine the microstructure to a larger extent. In this regard, Multi-Axial Incremental Forging and Shearing (MAIFS) has been recently introduced and different materials have been processed through this technique [54]. Montazeri et al. [55] used this technique for aluminum alloy 1100. Another way to achieve high quality product is to combine different types of SPD techniques. This would increase the extent of deformation and enhance the microstructure refinement. Shi et al. [56] used ECAP followed by forging and obtained improved results as compared to the applicability of either process individually in case of cast AZ80 magnesium alloy. Similar work has been carried out by Zaharia et al. [57] for copper. In both cases, they observed the increase in strength compared to the original material with a continued high plasticity. There are many problems associated with SPD when processing material is very hard and of large size. Brittle materials cannot be easily processed. A relatively new technique called Multi Axial Temperature Forging (MAFT) has been employed to handle 24
brittle materials. This technique begins at a higher temperature and to achieve the maximum possible grain refinement, the processing temperature is gradually reduced. Chen et al. [58] studied the effect of MAF on the microstructure of AZ51 magnesium alloy. After each pass temperature was gradually decreased to increase the equivalent strain. At a certain stage, dynamic crystallization was achieved, after which no improvement in the grain size was observed. The grain size was reduced to 8 μm with excellent improvement in mechanical properties. UTS and yield strength were enhanced to 60% and 45% respectively. Similarly, Titanium alloy (Ti-6Al-4V), an important material having the biomedical applications, was processed by MATF process [59]. Despite the potential of MAF to be used on large scale, little work has been done on dynamic precipitation. Such a study was carried out by Ting et al. [60] that investigates the dynamic precipitation behavior of Mge7Gde5Ye1Nde0.5Zr alloy. It was revealed that this behavior is caused by dislocations and hence it can be called as dislocation-assistant process. Mechanical properties and microstructure of SPD processed materials have been investigated as the function of cryorolling [61]. However, the influence of MAF on mechanical properties and microstructure at 77 K temperature has been investigated by few researchers. Maruff et al. [62] revealed the effect of very low temperature in case of Al 6063 alloy. During the application of SPD processes, the strength increases due to the decrease in grain size. This happens after each pass of SPD until saturation stage arrived. Although less common but has been observed in some cases that reverse happens after imparting further deformation. The strength decreases with further reduction in grain size. This is 25
called the softening behaviour of metals. Such behaviour was studied by Kapoor et al [63] for Aluminum deformed by MAF process. This behaviour of softening was attributed to the annihilation of grains and dynamic recovery and explained on the bases of dislocation theories [63]. Finite element modeling is a useful tool to simulate engineering system. It has potential to solve many practical problems that are either costly or impossible to execute otherwise. In case of MAF, the stresses in tools or dies can be numerically computed and material flow (strain distribution) can be seen and hence more homogenous structure is possible by controlling independent variables such as temperature, tool shape, ram speed etc. An idea was proposed by Kwapisz et al. [64] who analyzed the impact of the shape of the die on stress and strain distribution in the alternate extrusion and multiaxial compression processes. This distribution would contribute to assessing the homogeneity in mechanical properties of the product. Bylya et al. [65] presented a constitutive model that relate microstructure and mechanical behavior of the metal. They asserted that this is a complex task due to multi-domain areas vis-à-vis material science (microstructure related), solid mechanics (Metal flow related issues) and engineering (tooling, friction, heat etc.). They took grain size D as an internal variable and simulated MAF process. When any SPD method is applied to the material, with the improvement in grain size, one unwanted phenomenon occurs. The strain imposed by the process is not uniformly distributed along the deformed material which results in non-uniform properties. Various attempts have been made to get uniform properties. For example, Roodi et al. [66] studied the influence of punch and die geometry on strain distribution of the material processed by
26
ECAP. They found the combination of input variables to have the optimum results. However, such type of study can’t be found in the literature for MAF. Up till now, various metals and alloys have been processed by MAF. For example, Xiang et al. [67] multiaxially forged the AZ 81 magnesium alloy. Łyszkowski et al. [68] studied mechanical properties and microstructure of Fe3Al-base intermetallic alloy processed by MAF. Despite a lot of work on exploring MAF on various metals and alloys, a researcher can still find a gap in the systematic study of MAF process itself and also on the product of MAF process. For instance, no simulation or experimental study exists that investigates the effect of the die design, shape and parameters on process execution, resultant micro structure and mechanical properties whereas the same task has been accomplished in case of ECAP [69-70]. Also, there is no clear understanding about the effect of processing temperature on the resultant properties of processing materials. Another aspect that needs researcher’s attention is the effect of strain rate during MAF on mechanical and microstructure properties of the material to be processed. Similarly, the strain rate sensitivity of MAF processed material is another parameter that has not been studied yet. Although an appreciable number of metals and alloys have been processed and their mechanical properties have been evaluated but not for all. For instance, nickel is an important metal possessing applications in electrical and chemical industry. It has not been processed by MAF yet. Therefore, it is intended to focus present work mainly on MAF processed by the nickel and investigating some of afore mentioned aspects related to the MAF process and its product.
27
2.2 Nickle Processing Using SPD; A Survey It would be imperative to have a literature survey on the prospect of SPD done on the nickel. Pure Nickel and low alloying of Nickel based materials can be used in structural applications. Further, the mechanical properties of Nickel based material can be improved by grain refinement process which would further enhance the corrosive resistance. In addition, the properties and characteristics of nickel and nickel based alloy are well suitable to manufacture the micro-electromechanical systems (MEMS) applications. Moreover, it is also used in chemical and electrochemical industry. Some researchers have processed pure nickel and its alloys through SPD techniques other than MAF [71-78]. Table 2.1 describes the processing of nickel through various SPD techniques. Table 2.1. SPD of nickel and its alloys: a survey Material/Alloy Title & Reference
Procedure
Results
Used Local texture-microstructure Pure Nickel
10x10x50
mm Strong correlation
correlation
billet
was was
due
to
found
deformation localization in
processed
ECAP-processed
ECAP at room microstructure
(2016)
nickel.
by between
local
temperature at a and local texture [71]
deformation of 5 developing
after
mm/s using rout plastic BC4.
deformation. `Properties can be tailored according to application
28
Effects of annealing on Polycrystalline microstructure mechanical
and nickel
properties
18x15x100
based billet
of alloy
was
mm NG
Ni-based
cold alloy with average
rolled and 98 % grain size of about
nano-grained Ni-based alloy
thickness
50
produced by severe cold
reduction
was obtained
rolling. (2015)
achieved.
[72]
nm
was via
severe
cold-
(Severe
Cold rolling at room
Rolling)
temperature
Microstructure evolution of INCONEL®
Alloy
a
deformed
by down to 50 nm
processed by severe plastic
application
of was obtained as a
deformation. (2014)
HPT and Multiple result of HPT.
multiphase
super-alloy Alloy 718
[73]
was NS with grain size
Forging (MF) at MF processing at room temperature high temperature and
°
900
C yielded grain size
respectively.
of about 300 nm.
Microstructural analysis of Pure Nickel
Microstructure
Strongly
the and nickel
was examined in microstructure
single crystals subjected to
case of two single (300
severe plastic deformation
crystals of nickel obtained.
by hydro extrusion. (2013)
subject to hydro HE,
[74]
extrusion
refined
nm)
was After
(HE, retained
its
ε=2.4) at room orientation while temperature.
orientation
of
observed
was to
transformed two
be to
other
principal orientations.
29
Microstructure mechanical nickel
and Pure Nickel
properties
processed
2 mm thick strip Microstructure
of
of pure nickel was and
mechanical
by
subjected to total properties
accumulative roll bonding.
strain of 6.4 by investigated
(2013) [75]
Accumulative Roll
were and
compared.
Bonding Comparison
(ARB)
showed
that
microstructure were more refined in case of ARB as compared
to
conventional rolling (CR). The strength of sample
after
8
ARB cycles were 740 MPa Nano crystallization process Super
alloy Nickel based alloy Surface
and mechanism in a nickel C2000®
C2000®
alloy subjected to surface
plastically
severe
deformed on the
plastic
was structure
nanowas
obtained
deformation.(2009) [76]
surface
Microstructural evolution of Fe–32%Ni
14x14x14
Fe–32%Ni
cubic block was was investigated
large
alloy
strain
during
multi-axial
forging. (2006) [77]
mm3 Micro-structure
multi-axially
by
forged at 773 K.
resolution
MAF
was electron
employed
and backscatter
High-
diffraction
30
strain value for (EBSD)
and
each pass was 0.5. transmission electron microscope (TEM). A
mechanism
knows continuous dynamic recrystallization (CDRX)
was
proposed
for
structure refinement on the bases
of
observations
of
HAGB. Tensile
strength
ductility
of
and 98.7 % pure 20 mm diameter Grain size up to
ultra-fine- nickel
rod was processed 300
nm
grained nickel processed by
by ECAP at room obtained.
severe plastic deformation.
temperature
(2005) [78]
was
Higher
yield
strength
and
ductility
were
observed. To best of our knowledge, no published data has been found for application of MAF on Ni and investigation of resultant microstructure evolution and mechanical properties. Therefore, the present research was carried out and the main objective of the present research work is to examine the microstructural evolution and its effect on the mechanical behaviour of nickel. The objectives of present study are:
31
The execution of MAF on nickel and inevestigation of microstructure and mechanical properties as fumction MAF cycles. Numerical modeling of MAF on nickel and investigation of strain disribution and properies inhomogenity as the function of strain rate, initial temperture and friction.
32
CHAPTER III 3. EXPERIMENTAL METHODOLOGY 3.1 Flow Chart In this chapter, a complete description of the research objectives and steps intended to take in pursuit of these objectives are presented. A flow chart shown in Figure 3.1 presents a better understanding of the set goals and the way these are achieved.
Figure 3.1. Flow chart illustrating the overall volume of the work
33
3.2 Material Procurement and Sample Preparation Commercially pure nickel was purchased from Yakun manufacturing industries, China. Its chemical purity was ensured and chemical analysis report is attached as Appendix A. The material procured had dimensions of 16x16x32 mm. The size was reduced to 15x15x30 mm by milling operation and all samples were mechanically polished to suit with the size of the anticipated die and MAF processing. Compression test and hardness test were conducted for original samples. The microstructural analysis was also carried out.
3.3 MAF-Die Design As discussed in the previous chapter, there are two types of dies that can be used for MAF. Closed channel die is often preferred over open channel die because of better microstructure evolution and relatively easier processing. Closed channel die was selected and designed for the present study. The 3-D modeling was made in Solidworks and the forging process was simulated in ABAQUS to investigate the stresses generated in the die during MAF processing. The mechanical properties of H-13 tool steel selected for MAF die are shown in Table 3.1. The behavior of die was also investigated for different strain rates, temperatures and lubrication conditions as flow stress normally depend upon these independent parameters. The coefficient of friction during SPD usually ranges from 0.1 to 0.15 [64, 66]. Also, stresses in the die were calculated under quasi-static and dynamic conditions. 3-D models of the die components are shown in Figure 3.2. After making sure that die will work in actual condition by simulation, 2-D manufacturing drawings were generated (Appendix B) and sent for manufacturing. There are five components in MAF
34
die assembly namely male die, female die, plunger, stud and the base plate. Figure 3.2 (c) shows the macrograph of MAFed samples before and after the MAF processing.
Figure 3.2. MAF Die (a) MAF-Die Assembly; (b) Illustration of MAF execution; (c) MAFed samples before and after processing
Table 3.1. Mechanical Properties of H-13 tool steel [79] Elastic Modulus Yield Strength Ultimate Tensile Strength Poisson’s Ratio (GPa)
(MPa)
(MPa)
215
1400
1590
0.3
Tool die was designed under conditions given in the Table 3.2. Finite element analysis was carried out to calculate the stress in the die in ABAQUS simulation software. The factor of safety was calculated by finding the ratio of yield strength and equivalent von Mises stresses [28].
Factor of Safety =
Permissible Stress Working Stress 35
=
Yield Strength Max. Von mises stress Table 3.2. Input parameters for design of MAF die
Independent Input Parameters Coefficient of friction
Output Result
Strain Rate (s-1) 6666.67
0.1 & 0.15
666.67
Factor of Safety
66.67
3.4 MAF Execution on Nickel Hydraulic press of 150 ton capacity (shown in Figure 3.3) was used to process the pure nickel in an MAF die shown in Figure 3.2. The load required to forge the specimen has been calculated using equation 1.4, 1.10 and 1.11. The whole process was carried out at room temperature. The several samples were prepared using the milling machine and the surface of the samples were mechanically polished to have smooth surface. The prepared samples were annealed in the electric induction furnace under nitrogen atmosphere. During annealing, the samples were heated to 750°C with heating rate of 40°C per hour, maintained at this temperature for 60 minute (soaking time) and then put it for furnace cooling [80].
36
Figure 3.3. The hydraulic press used in the present study To carry out MAF, closed MAF die was designed and fabricated from H13 tool steel material. The samples were multiaxially forged in three orthogonal directions using MAF die which is shown in Figure 3.2. The forging speed was kept 3mm/minutes (Quasistatic conditions). The nano grease was used as lubricant to minimize the friction between the sliding surfaces. Before forging, the sample height was 30mm and it was forged to 15 mm by applying severe mechanical stress on the material as illustrated in Figure 1.8. It is to be noticed here that the width of the sample remains constant and 50% normal strain was imposed on the material during each pass. The effective strain imposed on the material during each cycle of MAF under plane strain condition can be determined using the following equation [27]:
37
True effective strain,
eff
2 2 2 2 ( 2 ) ( 2 3 ) ( 3 1 ) 9 1
12
(3.1)
Where ε1, ε2 and ε3 are normal true strains in the pressing direction (x-axis), the flow direction (y-axis) and the transverse direction (z-axis) respectively. It is to be noticed here that:
h
h
1 ln 1 , 2 ln 0 where h1 = 30 mm; h0 = 15 mm h1 h0 Further, during MAF, ε1 = - ε2, ε3 = 0 (The sample is restrained to flow along third perpendicular direction)
eff
2 ( 1 ( 1 )) 2 ( 1 0) 2 (0 1 ) 2 9
2 (4 12 ) 12 12 9
2 6 12 9
12
1/ 2
1/ 2
1/ 2
12 12 9 2 1 3 2 h ln 1 3 h0
The above relation gives the strain for single pass of MAF cycle and 1 cycle contains three similar passes. Therefore, effective strain for one cycle is given by:
38
2 h ln 1 3 h0
eff N
2 30 3 ln = 2.40 3 15
. During MAF of each pass, the sample has to be rotated by 90° and then put it into the die for its forging. In the present investigation, 0 cycle (annealed), 1 cycle, 2 cycle, 3 cycle and 4 cycle of forging was carried out.
3.5 Finite Element Modeling of MAF MAF process was simulated in ABAQUS 6.14 explicit dynamics. The plunger and stud were considered as rigid. Material of nickel sample was defined according to Table 3.3 and Table 3.4 respectively. Since MAF is symmetric about half plane of die, half symmetric problem was defined in ABAQUS to save computational time. Symmetric boundary conditions were applied on the plane of symmetry. Hard contact was defined between the surfaces to avoid penetration. The coefficient of friction was varied from 0 to 0.1 with increment of 0.05. In this model, plastic deformation behavior has been described by Johnson-Cook constitutive model as given below [29] 𝜀̇ 𝑇 − 𝑇𝑟 𝑚 𝑛) (𝐴 𝑌𝑓 = + 𝐵𝜀 [1 + 𝐶 ln ( )] [1 − ( ) ] 𝜀𝑜̇ 𝑇𝑚 − 𝑇𝑟
(3.2)
All parameters have been defined for nickel elsewhere [87] and has been presented in Table 3.3. Table3.3. Johnson-Cook parameters for nickel [81] A
B
C
n
m
Melting Temperature
163.4 MPa
648.10 MPa
0.006
0.33
1.44
1465 °C
39
This relation can handle the material over a wide range of strain rates, temperature and plastic deformation [81]. Other elastic and thermal properties have been represented in Table 3.4. The flow behaviour of nickel under quasi-static conditions is shown in Figure 3.4. The plunger speed and step time period were varied so as to compress the material to the desired strain level. Optimum mesh size was determined by plotting mesh sensitivity diagram and an optimum number of elements were 27000. Tetrahedral mesh elements were selected. Also automatic remeshing was applied to account for the large deformation using global remeshing having an absolute interface of 0.4 mm. Table 3.4 Mechanical and thermal properties of nickel [82] Elastic Modulus (GPa)
Poisson’s ratio
Density (g/cm3)
Specific heat (J/g-°C)
210
0.3
8.908
50
(a)
(b)
Figure 3.4. (a) Flow stress vs flow strain (b) Finite element mesh system of nickel sample The plunger was given a displacement of 15 mm to press the specimen’s vertical dimension (30 mm) to 15 mm. Equivalent stresses in die and strains specimen were investigated. The purpose of simulation is two-folds.
40
(i)
First objective is to design a die for MAF that can withstand loading under different conditions. For this, MAF process has been simulated and maximum equivalent stresses on the die have been calculated under different parameters such as temperature, strain rate and friction conditions.
(ii)
The SPD processed materials exhibit inhomogenity in properties. The nonhomogeneous distribution of strain along the processed sample results into nonhomogeneous mechanical and other properties which is undesirable. Various attempts have been made to get uniform properties. For example Roodi et al. [66] studied the influence of punch and die geometry on strain distribution of the material processed by ECAP. They found combination of input variables to have optimum results. However, such type of study can’t be found in literature for MAF. Inhomogeneity Factor (IFSD) can be a useful to investigate the strain distribution and is given by following equation [66].
𝑛 1 ∑𝑖=1(𝜀𝑖 −𝜀𝑎𝑣𝑒 )2 𝑛−1
√
𝐼𝐹𝑆𝐷 =
𝜀𝑎𝑣𝑒
(3.3)
Where ɛave is the average strain, εi is the effective strain in each element and n is the number of data. It is well known that lesser value of IF value mean, the material would have more homogeneity consequently it would exhibit improved properties. This factor is calculated and compared for the various MAF modeled types in this research. In case of MAF, the stresses in die parts can be numerically computed and material flow (strain distribution) can be seen and hence more homogenous structure is possible by controlling independent variables such as temperature, tool shape, ram speed etc. An idea was proposed by Kwapisz et al. [64] who analyzed the influence of the shape of die on the stress and 41
strain distribution in the alternate extrusion and multiaxial compression processes. Strain rate is an important controllable variable that can be set to get uniform strain distribution for a specific material. At different strain rates, the response of materials is different. Contrary to quasi-static processes, materials response may differ due to high inertial and thermal forces. Similarly, lubricating conditions may also influence the strain distribution. Further, generally MAF is carried out at high temperatures to process brittle materials and to impose large strains. MAF process has been simulated to get the impact of strain rate, temperature and friction on strain distribution. The effective strain has been calculated by taking average of strain undergone by each element for each simulation. Figure.3.4 shows the meshed sample and flow characteristics of nickel under quasi-static conditions. Effective strain and coefficient of standard deviation or Inhomogeneous factor have been calculated for all combinations of temperature, strain rate and friction provided in Table 3.5. There are three input values for each variable which are enlisted in Table 3.5. Since one cycle of MAF consists of three passes which are identical in terms of process parameters such as strain rate, friction and temperature. Therefore, one-third of MAF process has been simulated through FEM to minimize computational cost. For the strain rate of 66.7 s-1, the velocity of the plunger was kept 1000 mm/s and step time was fixed to 0.015 s. So the plunger would traverse down 15 mm and thus the sample would fill the space in the die such that initial height of 30 mm would reduce to 15 mm and initial width of 15 mm will become 30 mm. All other strain rates were imposed in this manner. While applying the loading conditions, the initial temperature was set to one of the values given in Table 3.5. Friction between the sliding surfaces was defined while setting the contact properties.
42
Table 3.5. Input and output parameters during compression Independent Parameters
Values
Outcomes
66.7 s-1 (Quasi-static) Strain Rate
666.67 s-1 6666.67 s-1 25 °C
Temperature
°
Inhomogeneous Factor
250 C
(IF)
500 °C
Via strain distribution
0.05 Coefficient of friction
0.1 0.15
3.6 Validation of FEA Model It is well known that quasi-static conditions would exist in explicit dynamics analysis if kinetic energy of the system is less than 5% of the total energy. Inertial effects can be ignored and process can be reasonably assumed as quasi-static (very slow dynamic load). Kinetic energy vs total energy plot (shown in Figure 3.5) taken in case of the process when strain rate was 66.7 s-1 indicated that kinetic energy is less than 5 % of total energy of the system for each increment. Therefore, at such strain rate, the process can be reasonably considered as quasi-static. This process was validated by experiment. Nickel material of size 15x15x30 mm was pressed under the same conditions experimentally. As a validation, the required pressing force calculated through FEA can be compared with the experiment. Since during experiment, quasi-static load was applied, so comparison was made for the process where strain rate was 66.7 s-1 (Quasi-static loading). If the pressing 43
force in both cases is same for same plunger/ram position while other processing conditions are similar, it can be assumed that FEM system is correctly modelled. Figure 3.6 shows that the experimental data match well with the simulation data. Also average load was calculated analytically using equations 1.4, 1.10 and 1.11 (µ=0.15, a=7.5 mm, h=15 mm, B=650 MPa). The average load required to press material to desired strain level was 110 kN. FEM also gave the approximately same value for average load (125 kN) for pressing to same strain level. The discrepancy is only 13 % which is acceptable. Only temperature and strain rate were varied and other settings were kept the same for other processes. Johnson-cook constitutive model was used to account for these changes and hence finite element model was validated.-
Figure 3.5. Comparison between kinetic energy and total energy of the system
44
Figure 3.6. FEM validation graph for MAF simulation
3.7 Characterization Techniques 3.7.1 Mechanical Properties Evaluation (i) Hardness Evaluation It is evident that microstructure refinement results into a remarkable enhancement of hardness of a material. Vickers hardness tests were carried out using “universal harness testing” device as shown in Figure 3.7. The hardness test was conducted at ten different places along three orthogonal directions and the average was used for investigation. Before carrying the hardness test, all samples were mechanically ground with abrasive papers of progressing grit sizes to minimize the effects of possible surface cracks.
45
(ii) Compression Test Three samples of sizes of 6 x 6 x 9 mm were prepared so as to keep height to width ration 1:1.5 as per ASTM standard (E9-09) for each condition for the compression test. Three samples were used in each condition during compression test and the average was used for examination.
The compression test was carried out using servo-controlled
universal testing machine supplied by MTS, USA (Model No: 370.25) with a capacity of 250 KN. Figure 3.8 shows the mounting of a sample for compression test by MTS machine. During compression test, force-displacement data of each sample was extracted from the machine for further investigation. The test was conducted before and after the MAF execution to observe the influence of the process on material’s strength.
Nickel Sample
Figure 3.7. Universal hardness measuring device (Zwick/ZHU 250) used in present study
46
Upper Platen
Nickel Sample
Lower Platen
Figure 3.8. MTS universal testing machine used in present study
3.7.2 Microstructural Examination Commercial pure nickel sample of dimensions 5x5x8 mm extracted from large rod through EDM. These samples were mounted in compaction machine and were encapsulated for easy handling during fore coming grinding and polishing, lapping and etching processes. All the samples were ground with emery papers of 600, 1000 and 2000 grit size in the mentioned pattern. Grinding was followed by lapping using alumina and diamond pastes. Mirror polished surface was etched using suitable etchant for the nickel (Marbels’ reagent). The microstructural evolutions were carried using the optical microscope (OMAX A35140U) of Olympus ((shown in Figure 3.9), China and the transmission electron microscope (TEM, JEOL JEM 2100). Before TEM investigations, multiaxially forged samples were cut into small thin pieces and mechanically polished slowly until the thickness reached to around 500 nm as thin foils. Ion milling was done on the thin foils up 47
to around 100 nm and then perforation was made on the thin foils. 10x10x5 mm specimen in each multiaxially forged sample was polished, cleaned using acetone, dried and then it was used for x-ray diffraction test. Qualitative and quantitative investigations of microstructure after MAF were carried out by X-ray diffraction (XRD, Rigaku Corporation, Japan) analysis. Mounting press, polishing machine and prepared samples are shown in Figure 3.10.
Figure 3.9. Optical Microscope used for microstructural examination
48
(a)
(b)
(c)
Figure 3.10. (a) Mounting press for sample preparation for polishing; (b) Polishing and lapping machine; (c) Samples for microstructural examination
49
CHAPTER IV 4. RESULTS AND DISCUSSIONS In this chapter simulation and experimental results have been presented and discussed in details. In the first part, simulation results of the MAF die have been presented under various processing conditions. This is followed by the experimental investigation of the nickel specimen before and after processing of MAF. Microstructural and mechanical characteristics of CP pure nickel and UFG nickel fabricated by first, second, third and fourth cycles have been presented and compared.
4.1 Stress Distribution of MAF Die Using ABAQUS Table 4.1 lists factor of safety of MAF under different processing condition which was prepared to analyze the stresses generated during the process. For each iteration, the results were obtained as shown in Figure 4.1. It can be seen from the Table 4.1 that factor of safety in each case is more than unity, hence the design of the die is safe under prescribed conditions. The influence of friction is also observed. At lower strain rate, the factor of safety is high at low friction value. In general, the more frictional force would cause more stresses in the die resulting in lowering factor of safety. In contrast, safety factor would be higher for the lower value of frictional force inside the die. For instance, safety factor is 3.55 and 3.21 when friction was kept 0.1 and 0.15 respectively. However, the converse trend has been observed at high strain rates where factor of safety is relatively high for high friction values as indicated in the graph. This is probably due the fact that at high strain rates effect of friction becomes insignificant.
50
Table 4.1. Factor of safety of MAF under different processing conditions Max. Strain Rate (s-1)
Friction Equivalent
Factor of Safety
Stresses (MPa) 66.7 (Quasi-static)
0.1
387.9
3.55
66.7 Quasi-static)
0.15
429.1
3.21
666.67
0.1
550.1
2.5
666.67
0.15
447.1
3.08
6666.67
0.1
1050
1.3
6666.67
0.15
1008
1.2
Figure 4.1. Stress distribution in MAF die during MAF operation
51
4.2 Strain Distribution in SPD Processed Nickel The magnitude of effective strain and inhomogeneous factor were investigated under varying conditions of strain rate, temperature and coefficient of friction. Results are given below.
4.2.1 Effect of Strain Rate on SPD Processed Nickel In ABACUS software, the plunger speed was varied to apply different strain rates as explained in section 3.0. It has been observed that as the strain rate increases, the inhomogeneous factor (IF) reduces and thus structure becomes more homogeneous. This is clear from Figure 4.2. At given lubricating and thermal conditions, IF reduces sharply in the start and levels off with the passage of time. This can be demonstrated by looking graph of Figure 4.2(a). At room temperature, for a given friction coefficient value, say 0.05, IF was 62 % for strain rate of 66.67 s-1 (Quasi-static) and it reduces 50 % at 666.67 s-1 under same conditions. The same trend can be seen for other friction and temperature conditions (Figure 4.2 (b) and Figure 4.2 (c)). This behaviour can be verified by looking at contour plots (Figure 4.3) of strain distribution. From left to right, strain rate increases and the distribution of strain within the specimen is more uniform from left to right in Figure 4.3. The effective strain has been observed to increase with the strain rate as indicated in Figure 4.4. For instance, effective strain increases from 83.2 % to 87.2 % when strain rate is increased from 66.67 s-1 to 666.67 s-1 for friction coefficient 0.05 and initial temperature 500 °C as depicted in the graph of Figure 4.4 (c).
52
Figure 4.2. Inhomogeneous factor at (a) 25 °C; (b) 250 °C; (c)500 °C
4.2.2 Effect of Temperature on SPD Processed Nickel As described before, the temperature of the sample was 25 °C, 250 °C and 500 °C. Maximum temperature was below one-fourth of melting temperature which is a favorable condition for MAF. It was observed that for a given coefficient of friction and at a certain strain rate, the structure is more homogenous at high temperature as compared to low temperature. Comparison of graphs of Figure 4.2 reveals the inverse relation between inhomogeneous factor and temperature. For instance, at temperature 500 °C in Figure.4.2 (a) inhomogeneous factor is 32 % when strain rate and friction coefficient were kept 66.7 s-1 and 0.1 respectively. By decreasing temperature to 250 °C, under the same conditions
53
inhomogeneous factor (Figure 4.2(b)) becomes about 65% which is almost double of the previous value. When the temperature is reduced further (25 °C) IF becomes 73% under same conditions as shown in Figure 4.2(a). Contour plot (Figure 4.3) shows similar behaviour. In the set of contours Figure 4.3 (a) the temperature is 25 °C, in contours of Figure 4.3 (b) the initial temperature is 250 °C and it is 500 °C in the contours of Figure 4.3 (c). A larger variation in distribution of strain is obvious at low temperatures. It is to be noted that the effective strain has a direct relation with temperature. The maximum effective strain of 87.2 % at strain rate of 6666.67 s-1 and friction coefficient of 0.05 was observed at temperature 500 °C (Maximum temperature) as shown in Figure 4.4 (c).
4.2.3 Effect of Friction on SPD Processed Nickel Higher the friction, the greater is the inhomogeneous factor (IF). This is clear from the graphs of Figure 4.2. As an example, consider Figure 4.2 (a), for each value of strain rate, IF is smaller when friction is larger. Strain distribution plots (Figure 4.3) also show the same results. In Figure 4.9, friction increases downward as indicated by arrow mark for each set of contours. An increase in variation can be observed as one moves in the downward direction. The opposite is true for effective strain, indicating an inverse relation with the friction coefficient. In Figure.4.2 (a), the effective strain is 79% when the friction coefficient was 0.05 at strain rate and temperature of 66.67 s-1 and 25 °C respectively. It reduced to 77% when the friction coefficient was increased to 0.15 while keeping other parameters constant. A similar trend can be seen in Figures 4.2(b) and 4.2 (c)
54
Figure 4.3. Strain distribution contour plot (a) at 25 °C; (b) at 250 °C; (c) 500 °C
55
Figure 4.4. Effective strain at (a) 25 °C; (b) 250 °C; (c)500 °C
4.3 Investigation of MAF on Nickel 4.3.1 X-ray Diffraction Analysis The X-ray diffraction analysis on as-received and MAFed samples was examined and is shown in Figure 4.5. The observed values of peak intensity and full-width half maximum from XRD results would help to examine the grain refinement mechanism occur in the metals/alloys [83]. The decreasing of peak intensity would indicate more structural defects due to increasing in dislocation density and twins. From Figure 4.5, it was clearly observed that the peak intensity at (1 1 1) plane was started to decrease as the function of number of cycles of MAF. The peak intensity at (1 1 1) plane of as-received nickel sample was 10,000 cps whereas the intensity of one cycle of MAFed sample was 6000 cps which 56
was decreased by 1.7 times when compared to as-received annealed nickel. In addition, the observed peak intensity was around 5000 cps, 4300 cps and 4150 cps for second, third and fourth cycles of MAFed samples respectively. Moreover, the peak broadening and slight shifting of peak towards lower angle were also noticed by increasing the number of cycles of MAF. The right side of Fig. 4 shows the magnified view of most diffraction peak at (1 1 1) plane for zero and three cycles of multiaxial forged samples. The three cycle multiaxially forged sample was exhibited the peak shifting towards lower angle when compared to zero cycle sample. This was attributed to grain refinement occurred in the structure. It can be noted here that the rate of decrease of peak intensity was less after one cycle of MAF. This was expected because of the annihilation of grains in the material occurs when subjected to drastic strains [84]. Further, based on full-width half maximum and diffraction angles, the crystallite size and lattice strain were determined using Williams-Hall method [85] (Table 4.2). Following equation was used for analysis [85].
K 4 sin hkl t
hkl cos hkl
(4.1)
Here K is the shape factor (0.9), is the X-ray wavelength (0.15406 nm), θhkl is the Bragg angle and t is the effective crystallite size normal to the reflecting planes and is the lattice strain. The instrumental broadening corrected line profile breadth, βhkl as a full width at half-maximum (FWHM) was calculated by computer software (XRD-analyzer) based on each reflection of 2θ.
57
In addition, the dislocation density of each sample was calculated as below [86] and the same is reported in Table 4.2.
ρ
16.1 2 b2
(4.2)
where ‘b’ is the burger vector (0.249 for Ni), ‘ε’ is the lattice strain. The structural analysis in terms of grain size, lattice strain and dislocation density of multiaxially forged samples from XRD results are illustrated in Table 4.2.
Figure 4.5. X-ray diffraction patterns of pure nickel with different number of cycles of MAF It was clear that the grain size started to decreases considerably as the function of number of cycles of forging up to three cycles. However, the lattice strain was started to increase with the function of number of cycles of forging. These results were expected to structural modification occurred in the sample resulted by the application of MAF process. It is to be noted here that the slight increase in grain size and lattice strain for four cycle of MAF sample was observed which was attributed to dynamic recrystallization and 58
dislocation annihilation [13]. The same phenomenon was observed in the dislocation density. Table 4.2. Structural analysis of Ni during MAF at different cycles By TEM analysis
By X-ray analysis S.No
Sample condition 1
1 2 2 3 3 4 4 5 5
0 cycle of MAF
Grain size (t), nm
Lattice strain (ε), % --
--
Dislocation density (ρ), m-2 --
Grain size (t), nm 805±22
1 cycle of MAF
452±14
0.220
1.256 x1015
475±15
2 cycle of MAF
305±16
0.260
1.755 x1015
320±13
3 cycle of MAF
212±15
0.310
2.495 x1015
220±16
4 cycle of MAF
255±12
0.290
2.183 x1015
275±11
4.3.2 Microstructural Examination Using Optical Microscope (OM) It is well known that the grain morphology orientation can be examined clearly through an OM. However, the size of grains cannot be determined exactly using OM [87]. To find the grain size of any material exactly, it needs TEM investigation. Therefore, in the present work, the grain morphology reduction was carried out using the optical microscope and to confirm the MAFed samples under nano level, TEM investigation was also carried out and an exact grain size was determined. Figures 4.6 (a)-(e) show the optical microstructure of annealed, one, two, three and four cycles of MAFed samples respectively. The observed grain morphology through OM of Figures 4.6 (a)-(e) was clearly shown that
59
the reduction of grain size occurred drastically with the function of applied cycles of MAF. This would help to enhance the mechanical properties of nickel and nickel alloys.
Figure 4.6. Optical microstructures of pure nickel during MAF: (a) zero cycle; (b) one cycle; (c) two cycles; (d) three cycles; (e) four cycles
60
4.3.3 Microstructural Examination Using Transmission Electron Microscope Figure 4.7 (a)-(f) show the bright field TEM images for zero, one, two, three and four cycles MAFed samples. The average crystallite size was 805±22 nm, 475±15nm, 320±13 nm, 220±16 nm, and 275±11 nm for zero, one, two, three and four cycles of MAFed samples respectively (Table 4.2). The average grain size was calculated based on several bright field images and the average was taken for investigation. In each sample around 400 grains were counted. These results were demonstrated explicitly that the grain size after MAF was in sub Nano level. To confirm further, Selective Area Diffraction (SAD) pattern was taken for the third cycle of MAFed sample which is shown in Figure 4.7 (e). The formation of continuous ring pattern was observed in SAD which indicated that the grain refinement was under Nano level. Further, more dislocations and twins were observed in three cycles of the forged sample (Figure 4.7(d)) when compared to the one cycle of forging (Figure 4.7 (b)). However, there was no dislocation line and or twin observed in zero cycle of MAF sample (Figure 4.7 (a)) due to annealing. In addition, less number of dislocation lines and disappearance of twins were observed in four cycle of MAF sample (Figure 4.7 (f)) when compared to three cycle of MAF sample (Figure 4.12 (d)) due to dynamic recovery and dislocation annihilation [11]. The measured grain size using XRD was in good agreement with TEM results (Table 4.2).
61
Figure 4.7. TEM bright field images of Ni: (a) before MAF; (b) after one cycle of MAF; (c) After two cycle of MAF; (d) after three cycle of MAF; (e) SAD pattern of (d); (f) after four cycle of MAF. For clear understanding, Figures 4.8 (a)-(c) show the schematic diagram representing the arrangement of grains, formation of dislocation lines, formation of twins and disappearing of dislocations and twins. In Figure 4.8, the continuous line indicates the grain boundaries (GBs), dotted line indicates the dislocations and the intersection of the dotted lines indicates the twins. Figure 4.8(a) shows the schematic of the arrangement of Ni grain under zero cycle of forging in which there was no dislocation lines and twins. However, the formation of more dislocation lines and twins after three cycles of forging as schematic is shown in Figure 4.8 (b). Similarly, the disappearing of twins and a slight 62
increase of grain size due to dynamic recrystallization and flow softening as schematic is also shown in Figure 4.8 (c). This was attributed to imposing of more strain and severe shear in the material. Now, it is clear from Figure 4.5, Figure 4.6 and Figure 4.7 that the SPD technique of MAF
Figure 4.8. Schematic of microstructure of Ni (a) 0 cycle of MAF; (b) 3 cycle of MAF; (c) 4 cycle of MAF was influenced more to refine the grain size of pure nickel. It is interesting to note that grain size was severely reduced to 475 nm after first cycle and the rate of refinement for subsequent cycles was significantly reduced. The grain size was reduced to the lowest level of 220 nm after third cycle of MAF. The rate of reduction in the grain size during MAF
63
process follows an admissible trend. This can be explained by the dislocation theory equation known as Orowan equation [88]: γ̇ = ρm bv
(4.3)
Where 𝛾̇ is the rate of imposed shear strain, 𝜌𝑚 is the density of mobile dislocations, 𝑣 is the average glide speed of dislocation and 𝑏 is the magnitude of Berger’s vector. The density of mobile dislocation after a time increment Δt becomes 𝜌𝑚 = 𝜌̇ + 𝑚 Δt where 𝜌̇ + 𝑚 is the mobile dislocation density rate increment. This increase in mobile dislocation is associated with the amount of strain increment. So for a specific displacement d, rates of mobile dislocation density and imposed strain can be related as [88]: Δγ Δt
= 𝜌̇ + 𝑚 𝑥 𝑏𝑥𝑑
(4.4)
Equation (4.4) suggests that rate of the grain size reduction should be constant for a reasonably slow rate of applied shear strain. However, the rate of mobile dislocation density (
∆𝜌𝑚 ∆𝑡
) is reduced by the other dislocation processes such as formation of dislocations
of dipoles, dislocation locks and dislocation annihilation. Roters et al. [89] have developed a relationship between the rate of the reduction of the mobile dislocations and each of these three processes. They proved that rate of dislocation density reduction is proportional to mobile dislocation. As strain hardening results into increase in mobile dislocation density, so the rate of dislocation density reduction also increases proportionally. Therefore, the trend of reduction in the rate of grain refinement is understandable.
64
4.3.4 Compression Stress-strain Analysis The stress-strain curves obtained from compression tests are shown in Figure 4.9 for different cycles of forging at strain rate of 10-2 s-1. The results indicate a significant increase in yield stress for one cycle followed by a gradual increase for subsequent passes. It can be concluded from Figure 4.7 that nanocrystalline nickel (Figure 4.7(a)-(f)) exhibited superior strength. This can be in part due to high resistance to crack progression by severe plastic deformation [90] and low work hardening rate (low value of n in Hoffman’s relation) [91]. Therefore, further deformation requires high applied stress resulting in higher strength. The decreased strain hardening due to SPD is probably due to the dislocation tangling which is responsible for faster dislocation spreading kinetics [95]. Increase in strength results from microstructure refinement as evident from Hall-Patch relation. The increase in strength was high during the first cycle of forging and then relatively small increase in strength was observed for the second and third cycles followed by rather decrease in strength for the fourth cycle. This was attributed to the grain refinement in the structure which was imposed by the process. In general, during SPD of MAF, dynamic recovery and annihilation would dominate after certain number of cycles of forging which decrease the rate of increment in the strength. The strength would reach at maximum level when the amount of dislocations would dominate more in the structure [84]. However, the strength would start to decrease when the dynamic recovery dominates. Therefore, after the four cycles of MAF, an unusual trend has been observed when increase in cumulative strain causes strength to decrease rather to increase. The same trend has also been observed by researcher for other metals during SPD [93-96]. Although this pattern of strength reduction as the result of further processing needs to be further explored, some researchers
65
have attempted to explain this phenomenon. One of the mechanisms found in the literature is micro-cracks formation on the surface [92]. However, with the use of proper lubricant, micro-cracks can be avoided. Another mechanism is “flow softening” [96] during the final stages that cause in the reduction of strength. When cumulative strain further increases, the strain hardening ability tends to restore due to dislocation annihilation and this may result in strength reduction as explained by Kumar [97].
Figure 4.9. Compressive engineering stress and strain plots for pure nickel after MAF with different cycles of forging at strain rate of 10-2 s-1 The elastic modulus was observed to increase significantly after the first cycle of MAF as compared to zero cycle of MAF. It was also noticed that further increase in the number MAF cycles does not alter the value of elastic modulus. These results are quite conformable with the basic theory of material sciences since the elastic modulus intrinsically arises from net energy (resultant of attractive and repulsive energies) for two
66
isolated and adjacent atoms. On an atomic scale, macroscopic elastic strain is manifested as small changes in the interatomic spacing and the stretching of interatomic bonds. As a consequence, the magnitude of the modulus of elasticity is a measure of the resistance to separation of adjacent atoms, that is, the interatomic bonding forces. This modulus is proportional to the slope of the interatomic force–separation curve) at the equilibrium spacing. Under the effect of applied forces, the vacancies between the atoms decrease to the equilibrium of repulsive and attractive atomic forces. After the application of MAF, the equilibrium of atomic forces will not change. In Table 4.3, the effect of different MAF cycles on mechanical properties has been numerically described. Table 4.3. Mechanical properties of pure nickel during MAF with different cycles S.No Sample condition 1 2 3 4 5
0 cycle of MAF (as-received) 1 cycle of MAF 2 cycle of MAF 3 cycle of MAF 4 cycle of MAF
Compressive yield strength (σy) at 0.2% strain, MPa
Vicker’s hardness (HV30)
Inhomogeneity factor (IF), %
345
121.2
--
615 720 805 765
260.6 305.9 330.7 315.2
48.7 35.8 21.7 29.3
4.3.5 Hardness Examination Figure 4.10 shows the results of Vickers hardness tests. A significant increase in the hardness has been observed for first cycle (Cumulative strain = 2.40) and then there is a gradual increase in hardness for the second and third cycles of MAF followed by a slight decrease in fourth cycle. Increase in the hardness is due to the microstructure refinement and the strength boosting as explained earlier. The decrease in hardness at high strains is due to the “flow softening” mechanism as has been reported by Kumar [97]. 67
Figure 4.10. Vicker’s hardness and inhomogeneity factor of pure nickel after MAF with different number of cycles Another parameter called inhomogeneity factor (IF) was calculated that gives an idea about the variation of hardness within the sample. IF is defined as [59] 2
(I. F)𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠 =
√∑i=n(Hi −H) i=1 n−1
H
x 100
(4.3)
Where n is the number of measurements taken along the height of each sample, 𝐻𝑖 is the ith measurement, and 𝐻 is average hardness value. Ten measurements were taken along each orthogonal directions to assess the IF. It is well known that lesser value of IF means, the material would have more homogeneity consequently it would exhibit improved properties. Figure 4.10 shows the results of IF of nickel after MAF. It can be observed that IF tends to decrease as number of cycles of MAF increase. This means that strain homogeneity is improved and attained after MAF process. 68
CHAPTER V 5. CONCLUSIONS MAF is one of the SPD techniques that can produce UFG materials. In this research, the nickel metal was multiaxially forged up to four cycles of MAF. Following conclusions can be drawn. It was concluded by finite element simulation that the IF is low at high strain rate and high temperature. While increase in the friction causes the IF to increase. There was a severe grain size reduction during the first cycle of MAF and then the rate of the grain refinement was reduced. The average grain size after third cycle of forging was 220 nm. A great increase in the compressive yield strength and elastic modulus was observed after MAF processing. An unusual drop in the yield strength was seen after the application of the fourth cycle of MAF. The results of hardness measurement show that there was a drastic increase in hardness after the first cycle of MAF and then the rate of increase was significantly reduced. Also, there was a slight decline in hardness after the fourth cycle of forging. The trend of variations in hardness was correlated with that of variations in microstructure refinement. Inhomogeneity factor calculated on the basis of hardness, was observed to decrease with the cycles of MAF. Rate of reduction in the I.F was decreased with the applied cycles of MAF. A slight rise in the I.F was observed after fourth cycle of MAF.
69
CHAPTER VI 6. FUTURE SCOPE Based on the present research work, the following work as a function of future scope can be carried out to enrich/reveal this field further. Execution of MAF at high temperature on nickel to study the MAF process parametres such as strain rate, friction and the size of starting material. Mechanical properties investigation of nickel at elevated temperatures. Electron Back Scattered Diffraction (EBSD) analysis of MAFed nickel samples. Fracture analysis of MAFed nickel samples and investigation of the effect of MAF on fracture behaviour. Tribological and corrosion behaviour of nickel at room and elevated temperature. Finite element modeling of MAF for different number of cycles and assessment of properties as the function of cummulative strain.
70
CHAPTER VII 7. PUBLICATIONS Table 7.1. Research publication details S. No
Title Microstructure properties
1
Journal name and
mechanical Materials
investigation
on Science
Status and Accepted, In press, and 13 Dec,2017
nanostructured Nickel 200 alloy using Engineering: A multi-axial forging process Investigation on strain behavior and
2
its impact on properties of nickel alloy 200 during multiaxial forging through finite element modeling
Transactions of the Submitted on
Dec
Indian Institute of 16,2017 Metals (TIIM)
71
REFERENCES [1]- Yi Huang
Terence
G.Langdon “Advances in ultrafine-grained materials”
materialstoday ,16( 3), 85-93, 2013 [2]- Kenong Xia, Wei Xu, Xiaolin Wu, Shouqie Goussous, Edward Lui and Yi Sun Wu, “Powder Consolidation by Severe Plastic Deformation and its Applications in Processing Ultrafine and Nanostructured Aluminium and Composites”, Proceedings of the 12th International Conference on Aluminium, 1141-1146, 2010 [3]- K.D. Sattler, Handbook of Nanophysics, Principles and Methods ,CRC, New York, 2010 [4]- B. Bhushan, Handbook of Nanotechnology ,Springer, Berlin, 2004 [5]- F. Simonis S. Schilthuizen, “Nanotechnology Innovation Opportunities For Tomorrow’s Defense” TNO Science & Industry, 2006 [6]- Jitendr N. Tiwari , Rajanish N. Tiwari, Kwang S. Kim, “Zero-dimensional, onedimensional, two-dimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices”, Progress in Materials Science 57 724– 803, 2012 [7]- Rainer Kassing, Plamen Petkov, Wilhelm Kulisch and Cyril Popov, “Functional Properties of Nanostructured Materials”, Part of the Nato Science Series book series (NAII, volume 223),2006 [8]-
Ning Wang, Zhirui Wang, K.T.Aust, U.Erb, “Effect of grain size on mechanical properties of nanocrystalline materials”, 43(2)519-528, 1995
72
[9]- Xue, Q.; Beyerlein, I.J.; Alexander, D.J.; Gray, G.T. “Mechanisms for initial grain refinement in OFHC copper during equal channel angular pressing”. Acta Mater. 55, 655–668, 2007. [10]- K.S. Kumar, H. Van Swygenhoven, S. Suresh “Mechanical behavior of nanocrystalline metals and alloys” Acta Materialia 51,5743–5774, 2003 [11]- R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer, and Y.T. Zhu, “Producing bulk ultrafine-grained materials by severe plastic deformation” JOM 58 (4), 33, 2006 [12]- Mohsin Talib Mohammed and Shahir Hussain, “Mechanical Properties of ECAPBiomedical Titanium Materials: A Review” Int. J. of Innovative Research in Science, Eng.and Tech. 4(5), 2700 – 2704, 2015 [13]- Valiev, R. Z., “Nanostructuring of metals by severe plastic deformation for advanced properties” Nature Materials, 3, 511–516, 2004. [14]- N. Tsuji, R. Ujei, Y. Ito, H. W. Kim, Ultrafine Grained Materials IV., Ed. by Y. T. Zhu, T. Langdon, Y. Horita, M. J. Zehetbauer, S. I. Semiatin, T. C. Lowe, TMS, The Minerals, Metals & Materials Society, 81, 2006 [15]- N. Tsuji, R. Ujei, Y. Ito, H. W. Kim, Ultrafine Grained Materials IV., Ed. by Y. T. Zhu, T. Langdon, Y. Horita, M. J. Zehetbauer, S. I. Semiatin, T. C. Lowe, TMS, The Minerals, Metals & Materials Society, 81, 2006. [16]- Terry C. Lowe, Ruslan Z. Valiev “The use of severe plastic deformation techniques in grain refinement” JOM, 56(10), 64–68, 2004
73
[17]- M.H. Shaeri, M.T. Salehi, S.H. Seyyedein, M.R. Abutalebi, J.K. Park “Effect of ECAP temperature on microstructure and mechanical properties of Al–Zn–Mg–Cu alloy” 57, 250–257, 2014 [18]- R. Z. Valiev, A.V. Korznikov, and R. R. Mulyukov, “Structure and properties of copper processed by HPT at room temperature” Mater. Sci. Eng., A, 186, p.141, 1993. [19]- R.Z. Valiev, R.K. Islamgaliev, and I.V. Alexandrov, “Bulk nanostructured materials from severe plastic deformation”, Prog. Mater. Sci., 45(2), 103-189, 2000 [20]- Yan Beygelzimer Dmitry Orlov, Alexander Korshunov, Sergey Synkov, Victor Varyukhin, Irina Vedernikova, Alexey Reshetov, Alexandr Synkov, Lev Polyakov and Irina Korotchenkov, “Features of Twist Extrusion: Method, Structures & Material Properties” Solid State Phenomena , 114, 69-78, 2006 [21]- Zahid Husaain, Awais Ahmed, Osama M. Irfan, and Fahad Al-Mufadi “Severe Plastic Deformation and its Application on Processing Titanium: A Review”, IJET 9(6), 426-431, 2017 [22]- C. Kobayashi, T. Sakai, A. Belyakov, H. Miura, ““Ultrafine grain development in copper during multidirectional forging at 195 K” Philos. Mag. Lett. 87 751–766, 2007. [23]- P.J. Hurley, F.J. Humphreys, “The application of EBSD to the study of substructural development in a cold rolled single-phase aluminium alloy” Acta Mater. 51: 1087–1102, 2003
74
[24]- N. Tsuji et al., Proceedings Second International Conference on Nanomaterials by Severe Plastic Deformation: Fundamentals-Processing-Applications, ed. M.J. Zehetbauer et al. (Weinheim, Germany: Wiley-VCH, p. 479, 2004 [25]- J. Richert and M. Richert, “A new method for unlimited deformation of metals and alloys Aluminium, 62, p. 604. 1986 [26]- V.M. Segal, “Materials processing by simple shear Mater. Sci. Eng. A, 197, p. 157, 1995 [27]- Mikell P.Groover, “Fundamentals of Modern Manufacturing: Material Processes and Systems,4th edition, John Wiley & Sons, Inc [28]- William D. Callister, “Materials Science and Engineering :An Introduction” 7th edition, John Wiley & Sons, Inc [29]- A.Banerjee S.Dhar S.Acharyya D.Datta N.Nayak “Determination of Johnson cook material and failure model constants and numerical modelling of Charpy impact test of armour steel” Materials Science and Engineering: A ,640, 200-209, 2015 [30]- Y. Iwahashi, Z. Horita, M. Nemoto, and T. G. Langdon, “The Process of Grain Refinement in Equal-Channel Angular Pressing,” Acta Mater. 46, 3317–3331, 1998. [31]- F.J. Humphreys, P.B. Prangnell, J.R. Bowen, A. Gholinia, C. Harris, Phil. Trans. R Soc. London A357, 1663–1681, 1999. [32]- F. Djavanroodi, M. Ebrahimi, “Effect of die parameters and material properties in ECAP with parallel channels” Mater. Sci. Eng. A 527 7593–7599, 2010 [33]- F. Djavanroodi, M. Ebrahimi, B. Rajabifar, S. Akramizadeh, “Fatigue design factors for ECAPed materials” Mater. Sci. Eng. A 528 745–750, 2010
75
[34]- A. Belyakov,W. Gao, H. Miura, T. Sakai, .“Strain-induced grain evolution in polycrystalline copper during warm deformation” Metall. Trans. A 29A, 2957– 2965, 1998 [35]- A. Belyakov, T. Sakai, H. Miura, and K. Tsuzaki, “Grain Refinement in Copper under Large Strain Deformation,” Philos. Mag. A 81, 2629–2643, 2001 [36]- G.I. Rosen, D. Juul Jensen, D.A. Hughes, N. Hansen, “Microstructure and local crystallography of cold rolled aluminium”, Acta Metall. Mater. 43, 2563–2579, 1995 [37]- P.J. Hurley, F.J. Humphreys, “The application of EBSD to the study of substructural development in a cold rolled single-phase aluminium alloy” Acta Mater. 51 1087–1102, 2003 [38]- C. Kobayashi, T. Sakai, A. Belyakov, H. Miura, ““Ultrafine grain development in copper during multidirectional forging at 195 K” Philos. Mag. Lett. 87 751–766, 2007. [39]- O. Sitdikov, T. Sakai, A. Goloborodko, H. Miura, R. Kaibyshev “Effect of Pass Strain on Grain Refinement in 7475 Al Alloy during Hot Multidirectional Forging” Mater. Trans. 45, 2232–2238, 2000 [40]- A. Belyakov, T. Sakai, H. Miura, R. Kaibyshev, K. Tsuzaki, “Continuous recrystallization in austenitic stainless steel after large strain deformation” Acta Mater. 50 1547–1557, 2002. [41]- O. Sitdikov, T. Sakai, A. Goloborodko, H. Miura, R. Kaibyshev, Philos. “Grain refinement in coarse-grained 7475 Al alloy during severe hot forging” Mag. 85, 1159–1175, 2005.
76
[42]- A. Gholina, P.B. Prangnell, M.V. Markushev,“The effect of strain path on the development of deformation structures in severely deformed aluminium alloys processed by ECAE” Acta. Mater. 48, 1115–1130, 2000 [43]- X. Yang, H. Miura, T. Sakai, “Dynamic Evolution of New Grains in Magnesium Alloy AZ31 during Hot Deformation” Mater. Trans. 44, 197–203, 2003. [44]- S.S. Satheesh Kumar, K. Priyasudha, M. Sudhakara Rao, T. Raghu, “Deformation homogeneity, mechanical behaviour and strain hardening characteristics of titanium severe plastically deformed by cyclic channel die compression method” Materials and Design 101, 117–129, 2016 [45]- Somjeet Biswas, Satyam Suwas, “Evolution of sub-micron grain size and weak texture in magnesium alloy Mg–3Al–0.4Mn by a modified multi-axial forging process” Scripta Materialia 66, 89–92, 2012 [46]- N.P. Gurao, P. Kumar, A. Sarkar, H.-G. Brokmeier, and Satyam Suwas, “Simulation of Deformation Texture Evolution During Multi Axial Forging of Interstitial Free Steel”, JMEPEG 22:1004–1009, 2013 [47]- X. Yang, H. Miura, T. Sakai, “Dynamic Evolution of New Grains in Magnesium Alloy AZ31 during Hot Deformation” Mater. Trans. 44, 197–203, 2003. [48]- I. Mazurina, T. Sakai, H. Miura, O. Sitdikov, R. Kaibyshev, “Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP” Mater. Sci. Eng. A 486, 662–671, 2008 [49]- A. Kundu, R. Kapoor, R. Tewari, J.K. Chakravartty, “Severe plastic deformation of copper using multiple compression in a channel die” Scripta Materialia, 58, 235– 238, 2008
77
[50]- Jiawei Yuan, Kui Zhang, Ting Li, Xinggang Li, Yongjun Li, Minglong Ma, Ping Luo, Guangqiu Luo, Yonghui Hao, “Anisotropy of thermal conductivity and mechanical properties in Mg–5Zn–1Mn alloy” Materials & Design, 40, 257-261, 2012 [51]- Ting Li , Kui Zhang, Zhi Wei Du, Jia Wei Yuan, Xing Gang Li, “Production of Fine-Grained and Weak Texture Structure in an Mg-7Gd-5Y-1Nd-0.5Zr Alloy by Multi-Axial Forging”, Applied Mechanics and materials. 633-634, 120-124, 2014 [52]- CHEN Q, ZHAO Z D, ZHAO Z X, HU C K, SHU D Y. “Microstructure development and thixextrusion of magnesium alloy prepared by repetitive upsetting-extrusion”. Journal of Alloys and Compounds, 509: 7303−7315, 2011 [53]- TAO Jian-quan, CHENG Yuan-sheng, HUANG Shao-dong, PENG Fei-fei, YANG Wen-xuan, LU Mei-qi, ZHANG Zhi-ming, JIN Xin, “Microstructural evolution and mechanical properties of ZK60 magnesium alloy prepared by multi-axial forging during partial remelting”, Trans. Nonferrous Met. Soc. China 22 ,428−434, 2012 [54]- M. Montazeri-Pour, M.H. Parsa, A. Khajezade, H. Mirzadeh, “Multi-Axial Incremental Forging and Shearing as a New Severe Plastic Deformation Processing Technique” Adv. Eng. Mater. 7 (18), 1197-1207, 2015 [55]- M. Montazeri-Pour, M.H.Parsa,, H.R.Jafarian, S.Taieban, “Microstructural and mechanical properties of AA1100 aluminum processed by multi-axial incremental forging and shearing” MaterialsScience&Engineering A 639, 705–716, 2015
78
[56]- Shi BQ, Chen RS, Ke W (2012) Effects of forging processing on the texture and tensile properties of ECAEed AZ80 magnesium alloy. Materials Science & Engineering A 546, 323-327, 2012 [57]- Zaharia L, Chelariu R, and Comaneci “Multiple direct extrusion: a new technique in grain refinement. Mater Sci Eng 550:293, 2012 [58]- Qiang Chen, Dayu Shu, Chuankai Hu, Zude Zhao, Baoguo Yuan, “Grain refinement in an as-cast AZ61 magnesium alloy processed by multi-axial forging under the multitemperature processing procedure” Materials Science and Engineering A 541, 98– 104, 2012 [59]- Behzad Fallah GhanbariEmail authorHossein ArabiS. Mehdi AbbasiS. Mohammad Ali Boutorabi, “Manufacturing of nanostructured Ti-6Al-4V alloy via closed-die isothermal multi-axial-temperature forging: microstructure and mechanical properties” Int J Adv Manuf Technol 87, 755–763, 2016 [60]- T. Li, K. Zhang, X.G. Li, Z.W. Du, Y.J. Li, M.L. Ma, G.L. Shi, J. “Dynamic precipitation during multi-axial forging of an Mg-7Gd-5Y-1Nd-0.5Zr alloy” Magnesium Alloys 1 , 47-53, 2013 [61]- Young Bum Lee, Dong Hyuk Shin, Kyung-Tae Park, Won Jong Nam, “Effect of annealing temperature on microstructures and mechanical properties of a 5083 Al alloy deformed at cryogenic temperature”, Scripta materialia,, 51, 355-359, 2004 [62]- Maruff Hussain, P. Nageswara Rao, Dharmendra Singh, R. Jayaganthan, Surendra Singh, “Comparative Study of Microstructure and Mechanical Properties of Al 6063 Alloy Processed by Multi Axial Forging at 77K and Cryorolling”, Procedia Engineering 75, 129 – 133, 2014
79
[63]- R.Kapoor A.Sarkar R.Yogi S.K.Shekhawat
I.Samajdar J.K.Chakravartty,
“Softening of Al during multi-axial forging in a channel die”, Materials Science & Engineering A 560, 404–412, 2013 [64]- Kwapisz, M. Knapiński, M. / Dyja, H. Laber, K., “Analysis of the Effect of the Tool Shape on the Stress and Strain Distribution in the Alternate Extrusion and Multiaxial Compression Process”, Archives of metallurgy and materials volume 56(2), 487-493, 2011 [65]- O I Bylya1, M K Sarangi, N V Ovchinnikova, R A Vasin, E B Yakushina, P L Blackwell, “FEM simulation of microstructure refinement during severe deformation” IOP Conf. Series: Materials Science and Engineering 63(01) ,20-33, 2014 [66]- Mahmood Ebrahimi, Bahram Rajabifar and Faramarz Djavanroodi, “New approaches to optimize strain behavior of Al6082 during equal channel angular pressing”, J Strain Analysis 48(6) 395–404, 2013 [67]- Xiang-sheng XIA, Ming CHEN, Yong-jin LU, “Microstructure and mechanical properties of isothermal multi-axial forging formed AZ61 Mg alloy” Trans. Nonferrous Met. Soc. China, 23 (11), 3186-3192, 2013 [68]- Radosław ŁyszkowskiEmail authorTomasz CzujkoRobert A. Varin, “Multi-axial forging of Fe3Al-base intermetallic alloy and its mechanical properties”, J Mater Sci. 52:2902–2914, 2017 [69]- F Djavanroodi, M Ebrahimi, “Effect of die channel angle, friction and back pressure in the equal channel angular pressing using 3D finite element simulation”, Materials Science and Engineering A 527, 1230–1235, 2010
80
[70]- F Djavanroodi, M Ebrahimi, “Effect of die parameters and material properties in ECAP with parallel channels”, Materials Science and Engineering A 527, 7593– 7599, 2010 [71]- Jörn Leuthold, Gerrit Reglitz, Matthias Wegner, Gerhard Wilde, Sergiy V. Divinski, “Local texture-microstructure correlation due to deformation localization in ECAP-processed nickel”, Materials Science Engineering 669, 196–204, 2016 [72]- Yanle Sun, Songqian Xu, Aidang Shan, “Effects of annealing on microstructure and mechanical properties of nano-grained Ni-based alloy produced by severe cold rolling”, Materials Science & Engineering A 641, 181–188, 2015 [73]- Xavier Sauvage, Shamil Mukhtarov, “Microstructure evolution of a multiphase superalloy processed by severe plastic deformation”, IOP Conf. Series: Materials Science and Engineering 63, 012173, 2014 [74]- J. Zdunek, J. Mizera, K. Wąsik, K.J. Kurzydłowski, “Microstructural analysis of the 〈111〉 and 〈110〉 nickel single crystals subjected to severe plastic deformation by hydroextrusion”, Materials Science & Engineering A 569, 92–95, 2013 [75]- Y.B.Zhang, O.V.Mishin N.Kamikaw, A.Godfrey, W.Liuc, Q.Liu, “Microstructure and mechanical properties of nickel processed by accumulative roll bonding”, Materials: Properties, Materials Science and Engineering A: Structural Microstructures and Processing, 576, 160-166, 2013 [76]- J.C.Villegas L.L.Shaw, “Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation”, Acta Materialia, 57(19), 5782-5795, 2009
81
[77]- Baojun Han, Zhou Xu, “Microstructural evolution of Fe–32%Ni alloy during large strain multi-axial forging”, Materials Science and Engineering: A 447(2), 119-124, 2007 [78]- N.Krasilnikov, W.Lojkowski, Z.Pakiela, R.Valiev“Tensile strength and ductility of ultra-fine-grained nickel processed by severe plastic deformation” Materials Science and Engineering A 397, 330–337, 2005 [79]- Allegheny Ludlum H-13 Tool Steel, UNST20813, DATA SHEET [80]- Online sources[http://www.totalmateria.com/Article32.htm] [81]- A. M. Rajendran, D. G. A Dynamic Failure Model for Ductile Materials, 1991 [82]- Online sources [https://www.azom.com/properties.aspx?ArticleID=2193] [83]- S. Sivasankaran, K. Sivaprasad, R. Narayanasamy , P.V. Satyanarayana “ X-ray peak broadening analysis of AA 6061100 −x−x wt.% Al2O3 nanocomposite prepared by mechanical alloying”. Materials Characterization, 62, 61– 67, 2011 [84]- M. Montazeri-Pour, M.H.Parsa , H.R.Jafarian, S.Taieban, “Microstructural and mechanical properties of AA1100 aluminum processed by multi-axial incremental forging and shearing”, MaterialsScience&EngineeringA639, 705–716, 2015 [85]- Williamson G K and Smallman R E 1955 P. Phys. Soc. Lond. B 68 577-585 [86]- Y Miyajima, T Ueda, H Adachi, T Fujii, S Onaka1 and M Kato, Dislocation Density of FCC Metals Processed by ARB. IOP Conf. Ser.: Mater. Sci. Eng., 63, 012138, 2014 [87]- S. Sivasankaran, K. R. Ramkumar, Abdulaziz S. Alaboodi, “Strengthening Mechanisms on (Cu–10Zn)1002x–x wt% Al2O3 (x 5 0, 3, 6, 9 and 12)
82
Nanocomposites Prepared by Mechanical Alloying and Vacuum Hot Pressing: Influence of Reinforcement Content” Trans Indian Inst Met. 70(3), 797-800, 2017 [88]- A.Ma, F.Roters, D.Raabe. Studying the effect of grain boundaries in dislocation density based crystal-plasticity finite element simulations. International Journal of Solids and Structures, 43, 7287-7303, 2006 [89]- [92]- Roters F, Raabe D, Gottstein G. “Work hardening in heterogeneous alloys— a microstructural approach based on three internal state variables”. Acta Mater 48(17), 4181–4189, 2000. [90]- Meyers MA, Mishra A, Benson DJ, “Mechanical properties of nanocrystalline materials”. Prog Mater Sci 51(4), 427–556, 2006 [91]- Dieter GE, Bacon D, “Mechanical metallurgy”, vol 3. McGraw-Hill, New York, 1986 [92]- Kumar SS, Raghu T “Mechanical behaviour and microstructural evolution of constrained groove pressed nickel sheets”. J Mater Process Technol 213(2), 214– 220, 2013 [93]- Shin DH, Park J-J, Kim Y-S, Park K-T, “Constrained groove pressing and its application to grain refinement of aluminum”. Mater Sci Eng A 328(1), 98–103, 2002 [94]- Krishnaiah A, Chakkingal U, Venugopal P , “Applicability of the groove pressing technique for grain refinement in commercial purity copper”. Mater Sci Eng A 410, 337–340, 2005
83
[95]- Khodabakhshi F, Kazeminezhad M, Kokabi A, Constrained groove pressing of low carbon steel: nano-structure and mechanical properties. Mater Sci Eng A 527(16), 4043–4049, 2010 [96]- R. Kapoor, A.Sarkar, R.Yogi, S.K.Shekhawat, I.Samajdar, J.K.Chakravartty, Softening of Al during multi-axial forging in a channel die, Materials Science & Engineering A 560, 404–412, 2013 [97]- Kumar SS, Raghu T “Tensile behaviour and strain hardening characteristics of constrained groove pressed nickel sheets”. Mater Des 32(8), 4650–4657, 2011
84
APPENDIX ‘A’
85
APPENDIX ‘B’
86
87
88
89
90
