EFFECT Of Zr, Sn AND Al ADDITION ON MECHANICAL PROPERTIES OF

EFFECT Of Zr, Sn AND Al ADDITION ON MECHANICAL PROPERTIES OF METASTABLE p TITANIUM ALLOYS S.Isiyama, S.Hanada Nippon S~ainless Steel Co., Ltd. Minatocho, Jyoetsu, Niigata-ken, 942 Japan Institute for Materials Research, Tohoku University Katahira, Aoba-ku, Sendai, Miyagi-ken, 980 Japan Abstract Mechanical properties of metastable 8 titaniumu alloys were investigated using Ti-16V and T1-7Cr base alloys doped with Sn.Zr and Al as ternary or quaternary additions. Zr addition had little influence on the mechanical properties, while Sn addition decreased flow stress and increased elongation markedly. In Al added alloys, rapid work hardening followed by secondary yielding was observed after extremely low yielding. This tendency became clear with increasing Al content, which resulted in remarkable deterioration in elongation. These results were explained by considering the effect of Zr.Sn and Al addition on deformation modes and 8 phase stability. Introduction It has been reported in metastable 8 titanium alloys that different types of deformation modes,slip,(332)twinning and deformation induced martensitic transformation operate depending on alloy compositions. The present authors! showed that the dominant deformation mode of metastable 8 titanium alloys was controlled by 8 phase stability referred to martensitic transformation and w phase transformation which varied with concentration and type of alloying element. On the other hand, on the mechanical properties of metastable 8 titanium alloys, Hanada et a1.2-4 found in mainly binary alloys that (332)twinning which operated predominantly in the alloys with large instability of 8 phase led to low yield strength and large elongation, although crystallographic slip in the alloys with small instability of 8 phase resulted in high yield strength and small elongation. Moreover, Duerig et al.5,6 and Flower7 investigated the mechanical properties of the alloys such as T1-10V-2Fe-3Al in which deformation induced a " martens! tic transformation occurred as dominant deformation mode. They also observed the. tensile properties characterized by the similar large elongation and low yield strength to the alloys deform predominantly by (332)twinning. From these results, it seems apparent that both (332)twinning and deformation induced martensitic transformation in metastable 8 titanium alloys cause lower.yield strength and higher elongation than slip does. However, the difference between the effect of Titanium '92 Science and Technology Edited by F.H. Froes and I. Caplan The Minerals, Metals & Materials Society, 1993 1,947

(332)twinning on the mechanical properties and that of deformation induced martensitic transformation ls left unknown. Further, the effect of ternary or quaternary alloying elements on the mechanical properties of metastable ~ titanium alloys is also unclear. The object of this study is to make clear the correlation of mechanical properties, deformation modes, and ternary or quaternary alloying elements in metastable ~ titanium alloys. In this study, therefore, tensile tests were conducted for a series of Ti-16V-(Zr,Sn,Al) and a series of Ti-7Cr-(Zr,Sn,Al) alloys, and the effect of Zr, Sn and Al on the mechanical properties was discussed in connection with the the deformation modes presented in our previous paperl in detail. V and Cr were selected as representatives of isomorphous and eutectoid ~ stabilizer, respectively. Both 16wt%V and 7wt%Cr correspond to a minimum content preventing martensitic transformation-during quenchingb. Zr, Sn and Al were chose as ternary or quaternary additions, since they are extensively used as important alloying elements in most commercial titanium alloys. Procedure Ti-16V-(Zr,Sn,Al) and Ti-7Cr-(Zr,Sn,Al) alloys containing 0/6wt% Zr, 0/6wt% Sn and 0/3wt% Al were prepared by arc melting in an argon atmosphere. The arc melted buttons were hot rolled and cold rolled to lmm thick sheets. Tensile test specimens with the gage size of lmm x 3mm x 16mm were obtained from the sheets by spark-cutting, and were sealed in a quartz tube under a vacuum. They were solution-treated at 1023-1123K for 1.Sks, quenched into ice water and then descaled by both mechanical and chemical polishing. Tensile test were performed on Instron type testing machine at an initial strain rate of 1.04 x 10-4 s-1. All the alloys had an oxygen content in the range of 0.04 to 0.06wt%. Results Tensile Properties of Zr Added Alloys and Sn-Added Ones The effect of Zr addition on the stress-strain curves of Ti-16V and Ti~7Cr alloy systems is shown in Fig.1 and the effect of Sn addition in Fig.2. Although Ti-16V and Ti-7Cr systems show the similar dependence of the stress-strain curves upon Zr and Sn addition, the effect of Zr addition on tensile properties and that of Sn addition are quite different each other. Namely, Zr addition up to 6wt% has no significant influence on the tensile properties and thus Zr added alloys in Fig.1 all show the similar stress-strain curves characterized by 600/650MPa of yield strength, linear work hardening and 25/30% elongation in engineering strain. In contrast with-zr addition, Sn addition changes tensile properties drastically, as shown in Fig.2. Although Sn added alloys also show the similar linear work hardening to Zr added alloys, increase in Sn content.. up to 4/5wt% decreases yield stress and flow stress and increases elongation markedly. The alloys containing 4/5wt% Sn show the low yield strength of about 250MPa and the excellent elongations above 50% in engineering strain. Further Sn addition, however, tends to increase yield strength and decrease elongation. The effect of Sn addition on tensile properties is more distinct in Ti-16V systems than in Ti-7Cr ones. Tensile Properties of Al Added Alloys 1,948

~ phase could not be retained at room temperature in a series of Ti-16V-Al and a series of Ti-7Cr-Al alloys because of martensitic transformation during quenching. The tensile properties of the alloys doped both with Zr and Al were controlled only by Al content, which may be understood from the results on the effect of Zr addition on the tensile properties as mentioned above. Thus the effect of Al addition was investigated using a series of Ti- 16V-6Zr-Al and a series of Ti-7Cr-4Zr-Al alloys in which ~ phase was retained under solution treated conditions except for Ti-16V- 6Zr-1.5Al. Fig.3 shows the effect of Al addition on the stress-strain curves. The similar dependence of the tensile properties on Al content is also found in Ti-16V. and Ti-7Cr systems. The yield stress is lowered extremely by Al addition as in the case of Sn addition. However, different from Sn addition, rapid work hardening followed by second yielding can be seen after first yielding in stress-strain curves of Al added alloys shown in Fig.3. This tendency becomes clear with increasing Al content, which results in remarkable deterioration in elongation. Tensile Properties of The Alloys Doped Both with Sn and Al Under a small amount of Sn content, tensile properties of the alloys containing both Sn and Al were analogous to those of the alloys doped both with Zr and'al stated earlier. However, increase in Al content increased yield strength and decreased elongation in the alloys with a large amount of Sn content. This situation is indicated in the stress-strain curves of a series of Ti-16V- 6Sn-Al and a series of Ti-7Cr-6Sn-Al alloys in Fig.4. In these alloys, rapid work hardening after first yielding and second yielding observed in Al added alloys in Fig.3 disappear. 0 400 200 Ti-16V-Zr 200 Ti-7Cr-Zr o~~~~~~~~~o.2.l.4 0 0.1 O.l O.l 0.4 Fig. 1 Stress-strain curves of the alloys doped with Zr. 1000 OSn 200 SSn 200 Ti-7Cr~Sn 0~~~~~~~~~- 0.2.l.4 0 0.1 0.2 O.J 0.4 e ~ Fig.2 Stress-strain curves of the alloys doped with Sn. 1,949

IOOO QAI 15Al It is pertinent to summarize the results on the effect of Zr, Sn and Al addition on the deformation mode and ~phase stability determined by X-ray diffraction and selected area electron diffraction described in our previous paperl before discussing the present results on the mechanical properties. Zr addition has little influence on deformation modes and ~phase stability and thus the dominant deformation mode in Ti-16V-Zr and Ti-7Cr-Zr systems is {332)twinning in which ~phase is transformed to w phase, as in the case of many binary alloy systems2,9,10. On the other hand, Sn and Al addition change dominant deformation mode from (332)twinning to deformation induced a " or a ' martensitic transformation depending on alloy systems, which is ascribed to the suppression of athermal w phase transformation by these elements addition. Sn addition above 4/5wt% and Al addition above 1.5wt%, however, have a tendency to restrain the deformation induced martensitic transformation. This is considered to be attributed to increase in shear stress for martensitic transformation resulting from solution hardening with higher Sn and Al content. As a result, addition of a large amount of Sn and Al changes dominant deformation mode from deformation induced martensitic transformation to slip. Correlation of Alloying Elements, Deformation Modes and Mechanical Properties As shown in Fig 1, Zr addition has no significant influence on tensile properties of metastable ~titanium alloys. This may be 1,950

understood by the fact that Zr addition does not change B phase stability and thus deformation mode. On the other hand, since Sn addition varies dominant deformation mode from (332)twinning to deformation induced martensitic transformation, tensile properties change drastically with Sn content. It is reported5-7 that yield stress decreases remarkably when deformation occurs by deformation induced martensitic transformation, which is good agreement with the present results on the tensile properties oi Sn added alloys. Both (332)twinning and deformation induced martensi tic transformation have higher work hardenability than slip2-7. Therefore, these deformation modes may increase elongation by means of inhibiting early necking. In this study, while high work hardening rate is also found in both Zr added alloys deforming predominantly by (332)twinning and Sn added alloys deforming preferentially by deformation induced a " martensitic transformation, the elongations of the former are obviously smaller than those of the latter. The lower ductility of the former may be attributed to higher sensitivity to cracking resulting from a large amount of w phase within B phase induced both by quenching and deformation. The decrease in yield stress and flow stress with Sn addition in Ti-7Cr alloy systems is smaller than in Ti-16V ones. This is probably due to the smaller amount of athermal w phase in binary Ti-7Cr alloy than in binary Ti-16V onel, which may result in smaller decrease in yield strength and flow stress with depression of athermal w phase by Sn addition. While both Sn added alloys and Al added alloys deform by deformation induced martensitic transformation, the tensile'properties are quite different each other. Al added alloys show the "double yielding" and have smaller elongation than Sn added alloys, which becomes clear with increase in Al content. A similar phenomenon was observed in a Ti-10V-2Fe-3Al alloy by Duerig et al.5,6 who explained the stress-strain curves as follows: first yielding is due to occurrence of a " transformation, flat portion after first yielding is ascribed to a " transformation continuing under fairly constant stress, rapid work hardening is nearly elastic deformation because of difficulty in producing additional a " phase and second yielding is. caused by operation of slip resulting from increase in applied stress. The difference of the effect of Al addition on tensile properties from that of Sn addition is apparently shown in Fig.5 and Fig.6. The stress-strain curve of Ti-16V-6Zr-3Al alloy obtained by alternating loading and unloading and optical microstructures under unloading after applying a given value of strain are shown in Fig.5. It should be noted that pseudoelasticity is apparently observed in Fig.5. Namely, when the applied stress is below the second yielding, the strain under loading is restored by unloading, which results in no permanent strain in the stress-strain curve and no deformation structure in optical micrographs(a,b). Permanent strain and deformation structure probably involving slip line in addition to a " plates (micrograph:c,d) appear after loading exceeding the second yielding. The stress-strain curve during loading in Fig.5, therefore, can be interpreted according to the. explanations presented by Duerig et al. mentioned above, although they do not describe any pseudoelastic behavior in Ti-10V-2Fe-3Al. The pseudoelasticity in Fig.5 is considered to occur in a similar manner to transformation pseudoelasticity observed in Cu-Al-Ni alloyll. Namely, in Ti-16V-6Zr-3Al alloy, the deformation induced a " phase is stable only under loading unless slip is introduced, and thus the strain under loading below second yielding is restored by unloading according to the reversion of a " phase to B phase. In any event, it is presumed that the "double yielding" in Al added al- 1,951

loys occurs because of the difference in shear stress for martensi tic transformation and for slip and becomes clear due to increase in shear stress for slip with increasing Al content. Fig.6 shows the stress-strain curve and optical microstructures of Ti- 16V-4Sn alloy obtained in the same manner as Ti-16V-6Zr-3Al alloy in Fig.5. Ti-16V-4Sn alloy shows conventionally shaped stressstrain curve without the pseudoelastic behavior found in Ti-16V- 6Zr-3Al alloy, and thus deformation structures in response to applied strain values are observed in micrographs. This is considered to mean that slip as well as a " martensitic transformation also operates simultaneously after yielding. Unfortunately, obvious evidence of slip deformation can not be obtained in optical observation because of difficulty in distinguishing slip lines from a lot of deformation induced a " plates. However, since Sn added alloys have extremely large elongations which can not be explained only by deformation induced martensitic transformation7, it is likely that slip occurs with the martensitic transformation in these alloys. If a combination of slip and a" martensitic transformation operates in Sn added alloys, 700 600 ~ " 500 Ti-16V-3Al-6Zr b400 JOO 200 3 4 5 6 7 8 9 10 11 12 13 1~ E ("/.) Fig. 5 Stress-strain curve of Ti-16Y-6Zr-3Al allor obtained br alternating loading and unloading and icrostructures under unloading. 1,952

high work hardening rate attributed to work hardening of martensi te itself and martensite boundaries acting as barriers to dislocations movement may continue until failure to result in large elongation. Relatively large elongations observed in 1.5wt% Al added alloys in Fig.3 are considered to be obtained in a manner similar to Sn added alloys. However, since higher Al addition restrains deformation induced martensitic transformation, the alloys containing 3wt% Al deform only by slip with poor work hardenabili ty after second yielding, and thus the elongations of these alloys decrease markedly as shown in Fig.3. Since both Sn and Al addition above a given value suppress deformation induced martensitic transformation and change dominant deformation mode to slip, the alloys doped both with Sn and Al tend to increase yield strength and decrease elongation with increasing Sn and Al content, as shown in Fig.4. Therefore,Ti-16V- 6Sn-3Al and Ti-7Cr-6Sn-3Al alloy containing the largest amount of Sn and Al in present work deform almost only by slip, and the stress-strain curves of these alloys are in close agreement with 700 600 c ~ 500 Ti-16V-4Sn 0400 JOO 200 10 11 12 13 14 Pig.& Stress-strain curve of Ti-16Y-4Sn alloy obtained by alternating loading and unloading and icrostructures under unloading. 1,953

those of ~ titanium alloys deform only by slip reported by Hanada et al.4. Summary The effect of Zr, Sn and Al addition as ternary or quaternary alloying element on the mechanical properties of Ti-16V and Ti-7Cr metastable ~ titanium alloy systems was investigated. Zr addition up to 6wt% did not change significantly the tensile properties. Yield strength and flow stress were decreased and elongation was increased markedly with increasing Sn addition up to 4/5wt%, but further addition showed opposite tendency. Al addition up to 1.5wt% indicated the similar effect to Sn addition up to 4/5wt% on tensile properties. However, further Al addition brought about "double yielding", pseudoelasticity, and remarkable deterioration of elongation. A large amount of both Sn and Al addition caused increase in yield strength and decrease in elongation. The similar dependence of tensile properties on Zr, Sn and Al addition was observed in Ti-16V alloy systems and Ti-7Cr ones. These results were explained by considering the effect of Zr, Sn and Al addition on deformation modes and ~ phase stability. Reference I. S. lsiyama, S. Hanada and 0. Izumi. "Effect of Zr. Sn and Al Addi lion on Deformation Mode and Beta Phase Stability of Metastable Beta Ti Alloys. JSIJ Int.. 31(1991). 807-813 2. S. Hanada and 0. Izumi. "Deformation Behaviour of Retained f3 Phase in f3 - eutectoid Ti-Cr Alloys, J.Mater.Sci.. 21(1986), 4131-4139 3. S. Hanada. Y. Yoshio and 0. Izumi. "Effect of Deformation Modes on Tensile Properties of Beta Titanium Alloys, "Trans. Jpn. Inst. Met.. 27(1986), 496-503 4. S. Hanada and 0. Izumi. "Correlation of Tensile Properties, Deformation Modes, and Phase Stability in Commercial f3 -Phase Titanium Alloys, Metal!. Trans., 18A(1987). 265-271 5. T.W.Duerig.G.T.Terlinde and J.C.Williams."Phase Transformations and Tensile Properties of Ti-10V-2Fe-3Al. Metal I. Trans., 11A(1980), 1987-1998 6. T. Ill. Duerig et al.. "Stress Assisted Transformation in Ti-10V-2Fe-3Al. Proc. 4th Inter. Conf. on Titanium. (1980). 1503-1512 7. H.M.Flower, "The Plastic Deformation of Metastable f3 Titanium Alloys, Proc. 5th Inter. Conf.on Titanium. (1984).1651-1658 8. S.Ankem and S.R.Seagle:Beta Titanium Alloys in the 1980's(ed. by R.R.Boyer and H. Ill. Rosenberg, AIME. New York, 1984), 107-126 9. S. Hanada and 0. Izumi. "Transmission Electron Microscopic Observation of Mechanical Twinning in Metastable Beta Titanium Alloys, Metal!. Trans., 17A(1986), 1409-1420 10. M.Hida et al.. "Stress Induced Products and Ductility due to Lattice Instability of f3 Phase Single Crystal of Ti-Mo Alloys," Acta Metal I.. 30(1982).1471-1479 II. K. Otsuka, H. Sakamoto and K. Shimizu, "Successive Stress-Induced Martensi tic Transformations and Associated Transformation Pseudoelasticity in Cu-Al-Ni Alloys, Acta Metal!.. 27(1979), 585-601 1,954