Physical and mechanical properties of stir-casting

DOI 10.1515/secm-2013-0118 Sci Eng Compos Mater 2014; 21(4): 505–515

Aykut Canakci*, Fazli Arslan and Temel Varol

Physical and mechanical properties of stir-casting processed AA2024/B4Cp composites Abstract: In this study, metal matrix composites of an aluminum alloy (AA2024) and B4C particles with volume fractions 3, 5, 7, and 10 vol% and with sizes 29 and 71 μm were produced using stir-casting technique. The effects of B4C particle content and size of boron carbide on the mechanical properties of the composites such as hardness, 0.2% yield strength, tensile strength, and fracture were investigated. Furthermore, the relation between particle content, microstructure, and particle distribution has been investigated. The hardness of the composites increased with increasing particle volume fraction and with decreasing particle size, although the tensile strength of the composites decreased with increasing particle volume fraction and with decreasing particle size. Scanning electron microscopic observations of the microstructures revealed that dispersion of the coarser sizes of B4C particles was more uniform while the finer particles led to agglomeration of the particles and porosity. Keywords: casting; mechanical properties; metal matrix composites (MMCs). *Corresponding author: Aykut Canakci, Engineering Faculty, Department of Metallurgical and Materials Engineering, Karadeniz Technical University, Trabzon, Turkey, e-mail: [email protected] Fazli Arslan: Engineering Faculty, Department of Mechanical Engineering, Avrasya University, Trabzon, Turkey Temel Varol: Engineering Faculty, Department of Metallurgical and Materials Engineering, Karadeniz Technical University, Trabzon, Turkey

1 Introduction Aluminum metal matrix composites (Al MMCs) are being considered as a group of newly advanced materials for their light weight, high strength, high specific modulus, low co-efficient of thermal expansion, and good wear resistance properties. A combination of these properties is not available in a conventional material [1]. The use of Al MMCs has been limited to specific applications such as aerospace and military weapons due to its high processing costs. Recently, Al matrix composites have been used in automobile products such as engine pistons, cylinder liners, brake disc/drums, etc. [2]. Processing techniques

for Al MMCs can be classified as: (1) liquid state processing; (2) semisolid processing; and (3) powder metallurgy [3, 4]. Particulate-reinforced Al composites can be processed more easily by the liquid state, i.e., melt-stirring process. Melt-stir casting is an attractive processing method as it is relatively inexpensive and offers a wide selection of materials and processing conditions. Boron carbide is an attractive reinforcement for aluminum and its alloys. It exhibits many of the mechanical and physical properties required of an effective reinforcement, in particular high stiffness (445 GPa), hardness (3700 Hv) [5], matching and even surpassing those of conventional reinforcements such as Al2O3 and SiC. These factors, combined with a density of 2.520 g/cm3 [5], less than that of solid aluminum’s, i.e., 2.7 g/cm3 [6], indicate that large specific property improvements are possible and specific properties will improve with increasing particle addition. The small density difference between B4C and molten Al means that particle sedimentation rates are low, thereby minimizing settling problems observed during solidification processing. Boron carbide-aluminum composites with low density and high toughness have wide applications, such as in light and hard disc substrates, brakes with high wear resistance, disc drive actuators, and armor plate with high ballistic efficiency [7, 8]. The mechanical behavior of particle-reinforced MMCs has been found to be influenced by the following properties of the matrix alloy: heat treatment [9–13], volume fraction of reinforcement [13–17], size of reinforcement [14, 17, 18], and residual stresses resulting from the differential coefficient of thermal expansion [13, 17, 19–22]. In view of the above points, the main purpose of the present study was to: (a) produce the B4C particle-reinforced MMCs using stir-casting method and (b) investigate the effect of B4C particles content and size on the microstructure and mechanical properties of AA2024/B4Cp composites.

2 Experimental In this study, AA2024 alloy with a theoretical density of 2.78 g/cm3 was used as the matrix material. The B4C

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506 A. Canakci et al.: Stir-casting processed AA2024/B4Cp composites particles with an average particle size of 71 μm which varies between 53 and 90 μm, and 29 μm which varies between 16.5 and 49 μm and a density of 2.52 g/cm3 were used as reinforcements. The B4C particles were supplied by Wacker Ceramics Kempten Gmbh (Kempten, Germany) [23]. The AA2024 alloy was supplied commercially with a chemical composition (in wt%) of 4.850% Cu, 1.310% Mg, 0.667% Mn, 0.254% Fe, 0.110% Si, 0.079% Zn, 0.033% Cr, 0.008% Ti, and balance Al. All samples used in this study were produced using the stir-casting technique and coded as shown in Table 1. Details of the experimental setup and production process can be found in the previous study [24]. Since boron carbide is slowly attacked in hydrofluoric/sulfuric acid (HF/H2SO4) or hydrofluoric/ nitric acid mixtures [23–25], pretreatment of B4C particles was performed using an acid mixture (HF/H2SO4), which contributes to the wetting and dispersion improvements of B4C particles. After the chemical pretreatment holding time of the etching process in HF/H2SO4 mixture for 3 min of coarser B4C particles and 2 min of finer B4C particles, the resulting mixture of acid-B4C particles was diluted with ethanol and particles were ultrasonically cleaned in ethanol. Later, the cleaned particles were air dried for 6 h at room temperature and then heated for 12 h at 150°C. Finally, the heated particles were milled and calcinated for 4 h at 400°C [26]. An unreinforced AA2024 matrix alloy sample was also produced using the same method for comparison. Thus pretreated B4C particles were incorporated and dispersed into molten AA2024 alloy using the vortex method under an argon atmosphere to prevent oxidation and to avoid alteration of the interface properties between the particles and the melt [27]. The alloy was melted in a resistanceheated furnace and heated to 700°C, which was above the liquidus temperature of the Al alloy, and the dross was skimmed from the surface of the melt before stirring, as shown in Figure 1. Temperature control of the melt was done with a precision of ± 1°C. The molten alloy was stirred Table 1 Characteristics of the composites tested. Code C0 C3 C5 C7 C10 F3 F5 F7 F10

Coarse group Fine group

Composition

Al2024 Al2024+3 vol% Al2024+5 vol% Al2024+7 vol% Al2024+10 vol% Al2024+3 vol% Al2024+5 vol% Al2024+7 vol% Al2024+10 vol%

Average size of B4C particles (μm) 71 71 71 71 29 29 29 29

at a fixed speed of 450 rpm at which vortex was formed during the addition of particles using a preheated threebladed impeller made of stainless steel. The impeller was driven by an AC motor with changeable speed. Stirring was continued for 3 min before the addition of pretreated and preheated B4C particles. Particles were added using a vibratory leader at a rate of approximately 5–10 g/min. Argon gas was also blown into the crucible during the operation. After the completion of particle feeding, stirring was continued for a further 5 min at a lower speed of 350 rev/min. The molten mixture was then cast into a water-cooled cylindrical steel mold at 680°C by bottom pouring and cooled to room temperature. The cast ingots had a conical shape with a changing diameter, between 40 and 70 mm and a length of 185 mm. The conical steel mold can be used to help feeding in a cone-shaped ingot and to assist in the removal of ingot from the mold. The B4C particle-reinforced AA2024 aluminum alloy matrix composites containing 3, 5, 7, and 10 vol% B4C with varying sizes (29 and 71 μm) have been successfully produced using pretreated B4C particles and a stir-casting method. The experimental density of the composite sample was measured using the Archimedes method. In this technique, density is determined by measuring the difference between the weight of the samples in air and in distilled water at room temperature. The theoretical density was calculated using the mixture rule according to the volume fraction of the B4C particles. The porosities of the composites were then evaluated based on the difference between the theoretical and the observed density of each sample. Before the machining of all the heat-treatment specimens, the materials in block form were solution heat treatment at 495°C for 4 h and quenched into cold water. After the solution treatment, specimens were machined to standard tensile dimensions: 25 mm gage length with a cross section of W × T = 6 × 5 mm2 (ASTM E8M-91). The solution-treated samples were aged at 177°C for 12 h. The heat-treated samples were evaluated for hardness and tensile properties. A tensile test was carried out on a 3-ton Zwick machine (Zwick GmbH & Co. KG, August-Nagel, Germany) with a cross-head speed of 0.2 mm/s. Two tensile test samples were machined from all the as-billets and tested for 0.2% yield strength, ultimate tensile strength (UTS), and total elongation. After heat treatment, all specimens were subjected to a hardness test to confirm their precipitation hardening behavior and to set up the specimen hardness database. Hardness tests, carried out on polished surfaces of specimens, were conducted using a Brinell hardness testing machine (Bulut Machine, İstanbul, Turkey) with a 125-kg load and 20 s indentation time. For each test condition, at least five tests were performed and their average was used.


A. Canakci et al.: Stir-casting processed AA2024/B4Cp composites 507

Stirrer motor

Reinforcing addition unit

Argon gas entry

Electrical resistance

Molten metal+particles mixture Graphite crucible

Temperature controller Water cooled mold

Figure 1 Schematic diagram of experimental set-up for manufacturing AA2024/B4Cp composites [26].

Specimens for metallographic observation were prepared by grinding through 800 grit papers followed by polishing with 6 and 0.25 μm diamond paste. Some of the samples were etched with Keller’s reagent (1 ml hydrofluoric acid, 1.5 ml hydrochloric acid, 2.5 ml nitric acid, and 95 ml distilled water) prior to microscopic examination [28]. The microstructure of the produced ingots was examined using a Jeol JSM 3600 (JEOL Ltd., Tokyo, Japan) scanning electron microscope (SEM) to determine the distribution of the B4C particles in the matrix and the presence of porosity. For this purpose, the samples were sectioned first and then prepared using the time standard metallographic technique. Fractographic investigations were performed on the fractured tensile specimens using an SEM to provide insight into the deformation and fracture behavior of both the monolithic matrix materials and composites.

3 Results and discussion In the present study, AA2024 alloy MMCs reinforced with varying sizes (29 and 71μm) and volume fractions (3, 5, 7,

and 10 vol%) of B4C particles were successfully produced using a stir-casting method. As a result of various trials, in the production process of investigation, the optimum process parameters were found to be as follows – pouring temperature: 680°C; stirring speed: 450 rev/min; stirring time: 5 min after the completion of particle feeding; and particle addition rate: 5–10 g/min. Appropriate chemical pretreatment of different-sized B4C particles is a new operation for successful fabrication of AA2024/B4Cp composites. The pretreated B4C particles can be easily incorporated and dispersed in AA2024 alloy melt and do not agglomerate before addition to the melt. Etching with an acid mixture, cleaning, drying, milling, and calcination are stages of pretreatment that predominantly affect the incorporation and dispersion of the B4C particles. The acid mixture concentrator must be 50 vol% HF+50 vol% H2SO4 and B4C particles must be exposed to acid mixture for 3 min of coarser B4C particles and 2 min of finer B4C particles. Approximately, the same values of process parameters have been found in our previous studies [24, 26]. The graphs of theoretical and experimental densities and porosities of the composites according to the volume


508 A. Canakci et al.: Stir-casting processed AA2024/B4Cp composites

3

Porosity (%)

fractions of B4C particles are shown in Figures 2 and 3, respectively. Figure 2 shows that the theoretical density decreases linearly as expected from the rule of mixtures. Although a linear decrease was also seen in the experimental densities, the values are lower than those of the theoretical densities. The density measurements also showed that the composites contained some porosity. The porosities of the composites were evaluated from the difference between the expected and the observed density of each sample. The variations of porosity level in these composites are also shown in Figure 3. This figure indicates increasing the volume fraction of B4C particles, especially for finer particle size (29 μm) of composites because of the decrease in the inner-particles spacing. In other words, with increase in the volume fraction of composites during the production stage, it is required that the longer particle addition time is combined with decrease in the particle size. The porosity level increased, as the contact surface area in contact with the air was increased. These results have been observed in previous investigations [29–32]. The Brinell hardness of the unreinforced AA2024 alloy and the composites is shown as a function of particle volume fraction in Figure 4. Data are given for the material before heat treatment (as-casted) as the well as for the T6 condition. The results show that the hardness varies linearly with volume fraction in both the T6 and as-casted conditions. However, the effect is approximately 22% greater for the material in the T6 condition than for the as-casted material for two different particle sizes. It can be seen from Figure 4 that the hardness of the composites increased with increasing particle volume fraction and decreasing particle size. This was attributed to the fact that the fine particles had more surface area in the matrix

2

Coarse group Fine group

1 0

8 4 6 B4C particle content (vol.%)

2

10

Figure 3 Variation of porosity with B4C particle content and size.

[32]. Sevik and Kurnaz assume that this slight increase may be caused by an increased plastic constraint as the particle spacing decreases [33]. As compared to the Al2024 matrix alloy, the hardness of the MMCs was found to be higher, and it increased with the addition of B4C particles [25, 26, 31]. Hamid et al. [34, 35], Hutching [36], and Lloyd [37] explored the significance of hard ceramic particles in increasing the bulk hardness of Al MMCs. Howell and Ball [38] and Vencl et al. [39] suggested the improvement of the hardness of the composites to the increased particle volume fraction. Wu and Li [40] and Deuis et al. [41] attributed this increase in hardness to the decreased particle size and increased specific surface of the reinforcement for a given volume fraction. Kim et al. [42] suggested that the increase in hardness of the composites is due to the increased strain energy at the periphery of particles

120

3 Theoretical density Coarse group Fine group

Density (g/cm3)

Hardness (BSD)

115 110 Coarse group (as-casted) Fine group (as-casted) Coarse group (T6) Fine group (T6)

105 100 95 90

2

0

2


10

Figure 2 Variation of theoretical and experimental density with B4C particle content and size.

85

0

2


10

Figure 4 Hardness values of composites with B4C particle content and size in the T6 and as-casted conditions.



dispersed in the matrix. Deuis et al. [41] concluded that the increase in the hardness of the composites containing hard ceramic particles not only depends on the size of reinforcement but also on the structure of the composite and good interface bonding [41]. The higher value of hardness in the T6 condition of the composites compared to the same composites in as-casted condition explains why the composites of the as-casted condition exhibit a lower hardness than in T6 condition of the composites. Similar results of hardness values have been found in previous studies [43, 44]. Typical SEM micrographs of the unreinforced alloy and the 3, 7, and 10 vol% composites with particle sizes 72 and 29 μm are shown in Figure 5A–H, respectively. The properties of the MMCs depend not only on the matrix, particle and volume fraction, but also on the distribution of reinforcement particles and interface bonding between the particle and matrix [32]. The most important factor in the fabrication of MMCs is the uniform distribution of the reinforcements. The parameters giving such distribution of B4C particles were determined in our previous studies [24, 45], and optimum parameters were used in the present study. In general there appears to be a reasonably uniform distribution of the B4Cp within the matrix. As shown in Figure 5, fairly uniform dispersion of the particles was achieved in the composites reinforced with 71 μm particle size (Figure 5B–D), whereas distribution of 29 μm particle size was not uniform and some of them were agglomerated and clustered (Figure 5E–H). Figure 5F,G presents the SEM micrographs of 7 and 10 vol% B4C particle-reinforced composites with 29 μm particle size in which the particle clustering and agglomeration are clearly shown. The clustering of B4C particles is also observed in comparatively higher volume fractions of 7 and 10 vol% B4C particles with 29 μm particle size. It is due to the increase in particle interaction and settling velocity that result in nonhomogeneous distribution of particles. The incorporation of the reinforcement particles will immediately increase the viscosity of the matrix melt due to the increased solid content [46]. The viscosity reduces as the shear rate increases. At higher shear rates, the clusters of B4C particles are broken, thereby reducing the resistance to flow. Higher viscosity helps to enhance the stability of the slurry by reducing the settling velocity, but also creates resistance to flow in mold channels during casting [47]. As a result, SEM observations of the microstructures revealed that the dispersion of the coarser sizes was more uniform, while the finer particles led to agglomeration and segregation of the particles, and porosity. The reason for the particle segregation is proposed as follows: the Al

dendrites solidify first during solidification of the composite, and the particles are rejected by the solid-liquid interface, and, therefore, are segregated to the inter-dendritic region. This event occurred more easily with finer particles [48]. Some porosity types can also be seen in the micrographs, as shown in Figure 5H. The microstructural analysis also showed that different types of porosities are formed inside the composites: (a) porosities associated with individual particle; (b) porosities related to the B4C particles cluster [49]; and (c) microporosities inside the matrix metal [49]. Investigations using SEM revealed that the dispersion of the particles with coarse sizes was distributed fairly uniformly while the fine particles led to cluster and segregation of the particles and porosity. The main reason for the segregation of the particles is that aluminum alloy dendrites solidify first during solidification of the composite, and the particles are rejected by the solid-liquid interface; therefore, they are segregated to the inter-dendritic region. Similar results have also been reported in previous studies [26, 31, 50]. From the application point of view, the mechanical properties of the composites are of immense importance. Although there is no clear relation between the mechanical properties of the composites, volume fraction, type of reinforcement [51, 52] and surface nature of reinforcements [53], the reduced size of the reinforcement particles [54] is believed to be effective in improving the strength of the composites. While the hardness of the composites increased progressively with volume fraction (Figure 4) the strength did not, at least for the material age hardened to the T6 condition. The mechanical properties of the composites are presented in Figure 6. Adding the reinforcement to the matrix alloy results in decreased yield stress, UTS, elongation of fracture, and increased Young’s modulus. This decrease is due to the formation of B4C clusters in the composites, when the reinforcement concentration exceeds some critical value specified by the reinforcement content and size. This critical concentration of reinforcements is higher for the composites containing 71 μm. However, the value of the critical concentration decreases to 3 vol% B4C for the composites containing particles sized 29 μm. Figure 6 shows the variation of the mechanical properties of the composites with B4C particle content and size at room temperature. It can be seen that the incorporation of B4C into matrix alloy leads to a reduction in the yield strength and the UTS. A comparison of the data for the composites reinforced with 29 and 71μm B4C particles for the concentrations where the reinforcements are not distributed



A

B

E

F

G

C

H D

Figure 5 SEM micrographs of (A) unreinforced alloy, (B) C3 composite, (C) C7 composite, (D) C10 composite, (E) F3 composite, (F) F7 composite, (G) F10 composite, and (H) F7 (PC: porosity associated with the B4C particle cluster; IP: porosity associated with the individual particle).



A

240

Coarse group (as-casted) Fine group (as-casted) Coarse group (T6) Fine group (T6)

5


4

200 Elongation (%)

Yield strength (MPa)

220

C

180 160 140

3

2

120 100

2

340



320 300

1 0

10

4

6

8

10

D

94 92 90 88

280 260 240

86 84 82 80 78 76

220

Corse group (as-casted) Fine group (as-casted) Coarse group (T6) Fine group (T6)

74

200 180 0

2

B4C particle content (vol.%)

Modulus (GPa)

Ultimate tensile strength (MPa)

B

0

72 2


10

70

0

2


10

Figure 6 Mechanical properties of the matrix alloy and composites as a function of the B4C content and size in the as-casted and T6 conditions: (A) yield strength; (B) ultimate tensile strength; (C) elongation; and (D) modulus.

than in unreinforced alloy. The described features and trends are observed for both as-billet (as-casted) and T6 conditions. 105 Halpin-Tsai Mital Hashin-Shtrikman Mixture Rule Coarse group (as-casted) Fine group (as-casted) Coarse group (T6) Fine group (T6)

100 Elastic Modulus (GPa)

sufficiently uniformly (especially for 29 μm B4C particles) leads to the conclusion that both particle sizes provide a reduction in the yield strength, the UTS and the elongation to fracture, while the composites containing both the particle sizes possess higher Young’s modulus. Therefore, the B4C particles bear no load during plastic deformation. The decrease in the yield strength and the UTS of the composites with increasing B4C particle content and size may be attributed to the fracture of B4C during the tensile loading test. Conversely, B4C particles in this study have a sharp corner shape and surface area for finer particles. These shaped particles with a larger surface area behave like a notch effect and decrease the yield strength and UTS. Furthermore, lower yield strength and UTS values were obtained in the composites containing 29 μm particles. This was due to the fact that finer particles could have numerous flaws with respect to coarser ones and are easily crashed under loading. However, similar effects have been reported elsewhere [33, 43]. Ravi Kumar and Owarakadasa [55] observed lower yield and tensile strengths in Al-Zn-Mg composite

95 90 85 80 75 70

0

2


10

Figure 7 Comparison of measured elastic moduli with moduli predicted using upper and lower bound [57], Halpin-Tsai [58], and Mital [60] models.



A

D

B

E

C

F

Figure 8 Representative SEMs showing fracture surface characteristics of different samples: (A) and (B) unreinforced AA2024 alloy, (C) C10 (fractured particle), (D) F10 (fractured particle), (E) C10 (clustered particle region), and (F) F10 (clustered particle region).

The ductility decreased with particle volume fraction in both the as-casted and T6 conditions (Figure 6C). The heat-treated composites had lower ductility than both the as-casted composites and unreinforced matrix

alloy (Figure 6C), which was similar to the observations obtained in previous studies [43, 56]. The results show that the addition of particulate and the T6 heat treatment condition increases the elastic



modulus, thereby the effect becoming greater with increasing volume fraction (Figure 6D). The experimental data is compared with the values predicted by a number of models [57–60] in Figure 7. The predicted values were calculated using the modulus of the matrix alloy and the volume fractions of reinforcement [23, 61]. The results show reasonable agreement with the Halpin-Tsai model [58] at low volume fractions, although they exhibit a decreasing deviation from this model as the volume fraction increases. Metallographic examination revealed that broken particles were present in the composites and that the level of particle fracture increased with volume fraction, as shown in Figure 8. The differences in the fracture behaviors between the AA2024 alloy and the B4C-reinforced composite can be seen from the SEM fractographs in Figure 8A–E. A typical fracture surface of AA2024 alloy is shown in Figure 8A, which is dominated by uniform dimples and indicates an overall ductile fracture mode for the alloy. The fractograph with higher magnification, Figure 8B, shows that some of the dimples are associated with the cracked particles of the α phase. The fracture behavior of the AA2024/B4C composite is, however, quite different. Much fewer dimples were found on the fracture surface of the composite (Figure 8C–E). Extensive matrix cracking and particle cracking were observed on the fracture surface of the C-groups MMC specimens (Figure 8C), which leads to a premature failure at relatively low strains. On loading the composite, stress is transferred to the particles. The damage begins when the applied stress exceeds the critical stress required to initiate failure by interface decohesion or particle fracture. In general, the interface between B4C particles and the AA2024 alloy matrix is strong and, therefore, possibility of the particle fracture is much greater than the interface decohesion [62]. The initiation of this damage influences the failure process further by creating an internal defect which acts as an incipient void and also by the fact that the damaged particles no longer support the load [12]. This situation increases the stress on the undamaged particles which in turn results in acceleration of the overall failure process. The fracture surface of the C-group MMC (Figure 8C) exhibits large number of fractured B4C particles. The finer size of B4C particles (F-groups) in MMC ensures fracture of the particles at an applied stress larger than that required for C-groups (71 μm) B4C particles. Furthermore, the fracture of 19 μm B4C particles (F-groups) is expected to influence the failure process of the composite to a lesser extent than the failure process of the coarser (71 μm) particles. Figure 8D exhibits relatively less-cracked B4C particles on the fracture surface of the F-group as compared to the C-group. For

this B4Cp-reinforced AA2024 aluminum alloy matrix the damage is associated with the reinforcing B4Cp, in the form of: (1) cracking and decohesion at the interfaces; (2) tear ridges; and (3) fine microscopic voids, which have formed around the cracked B4C particles. The fracture initiates by B4Cp cracking coupled with decohesion of the matrix surrounding and between the particles. In the case of the F-group MMC, the damage process involves three phenomena: interfacial failure, damage at clustered regions, and fracture of some of the much coarser B4C particles found in this material. Figure 8E,F shows a region of clustered B4C particles for both C-group and F-group on the fracture surface where damage accumulation ahead of the crack tends to occur more easily. Typical fracture surfaces (Figure 8) consisted of a bimodal distribution of dimples – larger dimples associated with the B4C particulates and smaller dimples associated with the ductile failure of the AA2024 alloy matrix. Similar observations have also been reported previously by other investigators [63–69].

4 Conclusions The following conclusions can be drawn from the present study: 1. AA2024 alloy MMCs reinforced with different size and volume fractions of B4C particles up to 10 vol% have been successfully fabricated by the vortex method. 2. Microstructural examination indicates that the coarser particles (71 μm) were generally homogeneous in the AA2024 alloy matrix while the finer particles (29 μm) led to agglomeration and segregation with porosities. 3. The density of composites is decreased with increasing particle volume fraction and decreasing particle size, although the porosity and hardness of the composites increase with increasing particle content and decreasing particle size. 4. The mechanical testing results showed that the presence of B4C particles leads to significant improvement in macrohardness and elastic modulus, although it reduces the UTS and elongation. 5. It was found that heat-treated AA2024 alloy and AA2024/B4Cp composites showed better strength, hardness, and elastic modulus. 6. The results of fracture surface characterization studies revealed that typical fracture surfaces consisted of a bimodal distribution of dimples – larger dimples associated with the B4C particles and smaller dimples associated with the ductile failure of the AA2024


514 A. Canakci et al.: Stir-casting processed AA2024/B4Cp composites alloy matrix. However, the fracture surfaces exhibit relatively less-cracked B4Cp on the fracture surface of F-group MMCs as compared to C-group MMCs.

Technical University (No: 21.112.003.7). The author would like to thank Wacker Ceramics for their kind support in supplying the B4C particles.

Acknowledgments: The present study was mainly supported by the Scientific Research Projects of Karadeniz

Received May 30, 2013; accepted September 29, 2013; previously published online December 5, 2013

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