Thermochemical Processing of Titanium Alloys - Springer Link

Overview

Thermochemical Processing of Titanium Alloys F.R. Froes, D. Eylon and C. Suryanarayana As the performance requirements of structures and devices increase, new and improved materials and processes are required. One such technique is thermochemical processing, which involves the use ofhydrogen as a temporary alloying element. Thermochemical processing significantly enhances both the fabricability and mechanical behavior of titanium alloys. ADVANCED MATERIALS AND PROCESSES

The recent National Critical Materials Council report on advanced materials I pointed out that the three most important industries in driving technological change, national security considerations, and economic advances into the next century are information/ communication systems (computers), biotechnology and advanced materials and processes. Of these three, advanced materials and processes are the most far-reaching and vital to advances in the other two fields. Advanced materials have enhanced mechanical and physical characteristics compared to traditional materials, such as aluminum and steel, currently manufactured in large-volume assembly line type production operations. These attributes either allow for very significant improvements in product or device performance or, of perhaps even greater importance, allow for new technologies that are unachievable using the conventional materials. 2 An example of the latter is the high specific strength and specific stiffness of the polymeric compos-

ites which make possible the forwardwept wing of the X-29-a concept not possible with conventional metals. Improvements in materials are key to defense/ aerospace system advances in the 21st century. The extreme demands of these systems will necessitate new materials which are stronger, stiffer, "hotter" and lighter than traditional materials of construction. Additionally, the materials will be "tailored" by use of composite concepts to have the properties required for a given application.3-17 Cost can be a major concern, but an integrated design, manufacturing, and use approach can lead to cost-effective application compared to the use of conventional materials. These new materials must be considered as structures rather than in the same way as traditional materials such as metals. In using advanced materials, it is necessary to balance the so-called "unconstrained" characteristics against the "constraining" characteristics. It is necessary to move from the trend band exhibited by the "basic" materials of today to an enhanced trend band of the "tailored" or "engineered" advanced materials of tomorrow by innovative chemistries, processes and microstructures.lO In many cases, such processing can involve large excursions from equilibrium in what has been termed "far from equilibrium" processing (Figure 1).11 One such innovative process is that of thermochemical processing, or use of hydrogen as a temporary alloying element, which leads to enhanced fabricability and improved mechanical properties in titanium alloysJs,19 THERMOCHEMICAL PROCESSING

Stable Equilibrium

Configuration Figure 1. A schematic of "far from equilibrium" processing. As the free energy difference between metastable and equilibrium states (~GE) increases, the opportunity to attain novel microstructures during subsequent relaxation increases. These novel microstructures can exhibit enhanced mechanical and physical properties.'o,,,

26

Thermochemical processing (TCP) is a term which has been coined to describe the use of hydrogen as a temporary alloying element in titanium alloys to enhance both processability (fabricability) and final mechanical properties. ls In this process, hydrogen is added to the titanium alloy by simply holding the material at a relatively high temperature in a hydrogen environment. The presence of the hydrogen then allows the titanium alloy to be processed at lower stresses flower temperatures (because of the increased amount of beta phase) and heat-treated to produce novel micro-

structures (because the hydrogen allows the material to behave as a eutectoid former system) with enhanced mechanical properties after hydrogen removal. The hydrogen is simply removed by a vacuum annealing treatment to levels below which no detrimental effects occur. Although thermochemical processing has only been applied to titanium alloys to date, there does not seem to be any reason why it should not also be applicable to other hydrogen absorbers, such as niobium and zirconium. Thermochemical processing is particularly amenable to near-net shapes, such as powder metallurgy products and castings, since the TCP treatment does not rely on working the material to modify the microstructures. However, the technique can also be equally applied to ingot metallurgy products. The high solubility of hydrogen in a wide range of titanium-based alloys is shown in Figure 2.20 The material for which the chemical activity is lowest will have the highest solubility for hydrogen. Beta titanium alloys have even lower activity values than commercially pure titanium and a consequent higher hydrogen solubility. In contrast, the equiatomic titanium aluminide (TiAl) has a very high activity, meaning that there is little solubility for hydrogen in this ordered compound. 21 Thus, many titanium alloys are amenable to TCP; a particularly attractive use could be in the processing of the ordered hexagonal close-packed alpha-2 class of titanium aluminides. To date, however, the vast majority of the work reported has been in the Ti-6AI-4V alloy, the most commonly used titanium alloy for both aerospace and nonaerospace applications. 700.------------..., - SOO

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CP-Ti

°0~~~2~==:4~--~S~~J8 Hydrogen Concentration (at.%)

Figure 2. The hydrogen activity for selected titanium alloys at 800°C}O

JOM • March 1990

HYDROGEN·ASSISTED PROCESSING

10

Wrought Products

Early studies indicated that the hot workability of titanium alloys could be improved by the addition of hydrogen. 22- 26 These findings were confirmed by Birla and DePierre27 for the Ti-6Al2Sn-4Zr-6Mo alloy, where a 30-35% reduction in forging flow stress at 730°C was reported when 0.4 wt.% (17 at.%) hydrogen was added and retained in the alloy during processing. A comprehensive study of the effects of hydrogen on the forgeability of wrought Ti-6AI-4V was conducted by Kerr et a1. 28 A plot of peak flow stresses as a function of hydrogen content confirmed the previous finding27 that a minimum in flow stress occurred at 0.3-0.4 wt. % hydrogen (Figure 3).28 Kerr suggested that the reduction in flow stress was a result of the increased proportion of the beta phase (stabilized by the hydrogen) up to 0.4 wt. % hydrogen. At higher hydrogen contents, it was proposed that the flow stress increased due to the presence of hydrides in the microstructure. However, the flow stress is reduced both in the sub-eutectoid temperature singlephase alpha region «0.4 wt. %H) and at super-eutectoid temperatures (>800°C) (Figure 4), suggesting that the mechanism may be considerably more complex than that proposed. 600

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Figure 3. (a) Peak forging flow stress as a function of hydrogen content, for a ram speed of 1.27 x 10-3 cm/sec. and (b) Peak forging flow stress as a function of temperature, comparing forging speeds, at 0% hydrogen with 0.4 wt.% hydrogen.28 Data points are the peak stress values from fitted curves of actual test data.

1990 March e JOM

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Figure 4. A pseudo-binary phase diagram for Ti-6AI-4V, modified from Reference 28. The hydride phase is shown as X since the exact composition of this phase has yet to be defined.29 One TCP treatment, (constitutional solution treatment) is superimposed.

At the present time, TCP is not being used commercially to improve the processability of wrought titanium alloys. However, laboratory-scale studies of the application of TCP to processing of the difficult-to-work alloys IMI-829 and Ti3Al (alpha-2) are in progress in the U.S.s.R. 30

acteristics of alloys such as Ti-6AI-4V and Ti-6AI-2Sn-4Zr-2Mo,36-38 in agreement with prior work by Kearns 39 on a Zircalloy-4 with 0.08-0.3 wt. % hydrogen. This behavior was related to the change in ratio of the volume fraction of alpha:beta phases resulting from a reduction in the beta transus temperature. MODIFICATION OF MICROSTRUCTURE AND PROPERTIES

Powder Metallurgy Products

The use of TCP in the consolidation of titanium alloy powders using the vacuum hot pressing (VHP) technique was investigated by Kao et a1. 31 ,32 Two types of powders with quite different morphologies were studied: a spherical rotating electrode process (REP) powder and angular hydride-dehydride (HDH) powder. It was determined that the presence of hydrogen enhanced processability rather than any differences in particle size or shape. Ka032 suggested that the change in volume due to the hydrides resulted in an increased concentration of vacancies and dislocations in the titanium lattice, which effectively enhanced the plastic flow of the particles. Early work by Yolton et a1,33 demonstrated that hydrogenated powder did not lose hydrogen during hot-isostatic pressing (HIP) in an evacuated mild steel can (which prevents the hydrogen loss). A fine equiaxed microstructure was present after subsequent dehydrogenation. However, no optimization of HIP parameters or comparison with nonhydrogenated powder were conducted in this study. Similar results were obtained by Steele and coworkers21,34,35 in an investigation of HIP of the ordered intermetallic alloy Ti3Al + Nb. Superplastic Forming of Sheet Material

Small additions of hydrogen greatly enhanced the superplastic-forming char-

Although a considerable amount of work has been conducted on processing and mechanical behavior associated with TCP, very little has been done to understand the nature of the microstructures associated with this process and how they develop. It is clear that hydrogen stabilizes the beta phase at the expense of the alpha phase. At sufficient levels, the hydrogen can give rise to an eutectoid ~ --? a + hydride reaction. 4o It is also well established thatthe addition of hydrogen to the titanium matrix results in an expansion of the beta body-centered cubic unit cell by almost 5.35%,41 and that when alpha-titanium transforms to a titanium hydride, a 17.2% volume expansion occurs42 with an associated high strain level in the matrix. 1,000 800 (if

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600 400 200 o~----~--~~--~--~ 4 5 6

10 10 10 Cycles to Failure Figure 5. Comparison of high-cycle fatigue properties of wrought annealed Ti-6AI-4V with castTi-6AI-4V as annealed and after processing via the CST TCP treatment. 52

27

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Figure 6. A comparison of (a) tensile and (b) fatigue data from TCP and conventional material. The letter designations refer to various types of TCP treatments.'8 HVC refers to a hydrogenation. beta solution treatment with water quenching. aging to form hydrides, and, finally, dehydrogenation. BQ-HDH refers to a beta solution treatment with water quenching and hydrogenation and dehydrogenation below the eutectoid temperature. HTH refers to hydrogenation in the beta region, cooling to room temperature and dehydrogenation below the normal eutectoid temperature.

One of the few contributions to understanding of the microstructure development during TCP is the work of Mahajan et al. 41 They demonstrated that orthorhombic martensite transforms to alpha phase and titanium hydrides,28 with a high dislocation density in the alpha phase. 41 When this material is dehydrogenated, a fine alpha phase forms by the decomposition of the hydride to a mixture of alpha and spheroidized beta phase.43,44 The high dislocation density can lead, as expected,45 to relaxation of the alpha phase and an equiaxed morphology. This fine equiaxed alpha is favored by low transformation temperatures and low dehydrogenation temperatures.46 One TCP treatment, constitutional solution treatment (CST) is shown superimposed on the pseudo-binary phase diagram in Figure 4 as an example of how the time-hydrogen content excursion relates to position on the diagram. The microstructural changes described above will take place to varying degrees in each of the TCP treatments. Wrought Product

Work by Kerr et al. 28 determined the tensile properties ofTi-6AI-4V, using 1.0 wt. % hydrogen. As the transformation temperature and dehydrogenation temperature were decreased, strength increased, a result of the finer microstructure produced. An extensive study of the effect of hydrogenation and dehydrogenation temperature in Ti-6AI-4V43 demonstrated that a dehydrogenation temperature below the eutectoid temperature (800°C) was necessary to produce a fine dispersion of spheroidized beta phase and fine alpha regions irrespective of the product type used. The wrought

28

ingot material had a yield strength of985 MPa and an ultimate tensile strength of 1,050 MPa. At the same time, the fatigue ratio (afl a UTS ) was 0.68, a somewhat higher than normal value.47 Application to Net Shapes

Thermochemical processing can be used to control the microstructure in powder material either by adding the hydrogen to the powder prior to compaction or after compaction, in both cases followed by dehydrogenation in the compacted condition. 18 The former method results in a fine equiaxed alpha structure in prealloyed Ti-6AI-4V powder, which has been attributed to formation of "new" grains between former hydride needles upon dehydrogenation. 33 However, no mechanical property data have yet been developed for this material. Conventional blended elemental (BE) product is less than 100% dense and exhibits low fatigue performance, a result of the remnant salt and associated porosity.18 Refinement of the microstructure of this material by TCP leads to little improvement in either tensile or fatigue behavior. 43,47 However, an extra-low chloride (ELCL) BE titanium powder has recently become available, produced by an ingot metallurgy approach; however, it has a higher inherent cost.48-SO Using the TCP method this product results in tensile and fatigue performance superior to that of conventional wrought materiaI43,47-50 providing a moderately priced, high-integrity product which could see use in demanding fatigue-critical applications. Thermochemical processing of HIP' ed Ti-6AI-4V powder produced by the plasma rotating electrode process (PREP) resulted in a refined microstructure and

a substantial enhancement of fatigue behavior of up to 40% over non-TCP' ed PREP powder. 33 Examples of the tensile and fatigue behavior of material given the TCP treatment are shown in Figures 5--6. 51 ,52 A study of the ordered intermetallic Ti3Al + Nb was conducted by hydrogenating PREP Eowder, HIP and dehydrogenation.2 ,34,35 Dehydrogenation at low temperatures resulted in a fine microstructure similar to that observed in Ti-6AI-4V. Mechanical property data is not yet available. Thermochemical processing has also been used to control the microstructure of dispersion-strengthened rapidly solidified elevated-temperature titanium alloys.18,53 This family of alloys conventionally exhibits a dispersion of secondphase particles, an advantage for elevated temperature performance. However, at the same time, they contain a small grain size with an equiaxed morphology, both features which are undesirable for maximizing high-temperature behavior. A beta anneal will both grow the grain size and, subsequently, produce an elongated "transformed beta" alpha morphology. However, this treatment also dramatically coarsens the dispersoid size. Addition of hydrogen lowers the beta transus temperature,54 but even above this reduced temperature, excessive dispersoid coarsening ocTable I. Ratio of Fatigue Strength to Tensile Strength for Ti-6AI-4V Investment Castings5•

Treatment Cast + HIP TCP-I TCP-II It

(JUTS

(MPa) 958 1,055 1,100

cr* f (MPa)

(cr/ a 0JrS)

530 770 745

0.55 0.73 0.68

S

At 5 x 10 6 cycles.

JOM • March 1990

Editions de Physique, 1988). p. 1009. 35. F.H. Froes. D. Eylon. RG. Rowe and CF. Yolton, Bull. Mater. Sci. (June 1989), p . I. 36. R.J. Lederich, s.M. L. Sastry, J.E. O'Neal and W.R. Kerr, Advallced Processing Methods for Titanium, ed. D.F. Hasson and CH. Hamilton (Warrendale. PA: TMs, 1982), p. lIS. 37. RJ. Lederich, s.M.L. Sastry and J.E. O'Neal. Titanium . Science alld Techllology. ed . G. Lutjering, U. Zwicker and W. Bunk (Oberursel, West Germany: DGM, 1985). p. 695. 38. RJ. Lederich et aI., U.s. patent no. 4,415,375 (November 1983). 39. J.J. Keams. J.E. McCauley and F.A Nichols, ]. Nud . Mater. , 61 (1976), p. 169. 40. M. Hansen, Constitution of Binary Alloys, 2nd ed. (New York: McGraw-Hili, 1958), p. 800. 41. Y. Mahajan, S. Nadiv and W.R Kerr, Scripta Met., 13 (1979), p. 695. 42. T.R Gibbs and H.W. Krushwitz,]. Amer. Chern. Soc., 72 (19SO), p . 5365. 43. CF. Yolton, D. Eylon and F.H. Froes, Sixth World Conference on Titanium , ed. P. Lacombe, R. Tricot and G. Berange r (Les Ulis Cedex: Les Editions de Physique, 1988), p. 1641. 44. P. Feng et aI., Proceedings of the First Illternational Confer-

ence ou the Metallurgy and Materials Science o/Tungsten, TUa-

a

30llm

b

Figure 7. The refinement in microstructure of cast Ti-6AI-4V given a TCP treatment. (a) As-cast and (b) after TCP.43.47 Note the elimination of thick continuous grain boundary alpha and the large alpha plate colonies in (b). curred,53 presumably because of the greatly enhanced diffusion rate in the bcc structure compared to the lower temperature hcp structure. At the present time, the product form for which the TCP technique has the most immediate potential for commercialization is castings. 9,18,55 In Ti-6AI-4V, TCP can greatly refine the microstructure of cast product (Figure 7)43,44 with a substantial enhancement in fatigue behavior (Figures

5

and 6b) and higher

strength levels than as-cast material 56 (Table

0.

The improvement in the (J/ (JUTS ratio in Table I suggests that the enhancement in fatigue behavior is not only a consequence of the higher strength levels but also the refinement in microstructure per se.

ACKNOWLEDGEMENTS The au thors would like to acknowledge the assistance of Ms. Theresa Dahmen and Mrs. Susan Coetz in formulating and typing the original manuscript. In addition, helpful discussions with w.R. Kerr, L. Levin, Y.R. Mahajan , P.H. Shingu, L.S. Steele, J. Stinson, R.C. Vogt, I. Weiss and C.F. Yoltonare gratefully recognized. References 1. Executive Office of the President, National Critical Materials Council, "Advanced Materials Program Plan ... the Continu ation of a Presidential Commitment" (1989). 2. ARC Westwood, Met. Trans. B, 19B (April 1988), p . 155. 3. Congress of the U.s., Office 01 Technology Assessment. "Advanced Materials by Design" (June 1988). 4. M.F. Ashby, Phil. Trans. Roy. Soc. London, A322 (1987), p. 393. 5. Scientific American, 255 (4) (Oct. 1986). 6. F.H. Froes, "Aerospace Materials for the Twenty-First Century." Fourth Israel Materials Coni., Beer-Sheba. Israel (Dec. 1988) and to be published in the proceedings. 7. F.H. Froes. Material and Design. X (3) (May/June 1989), p. lIO. S. F.H. Froes, "Aerospace Materials for the Twenty-First Century." Swiss Materials, to be published. 9. F.H. Froes, Materials Edge. no. 5 (May 1988). p . 19. 10. F. H. Froes, "Emerging Technological Developments in Metals, Alloys, and Metal Matrix Composites:' US BOM

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