Materials Transactions, Vol. 46, No. 12 (2005) pp. 3001 to 3007 #2005 The Japan Institute of Metals

Elastic Deformation Behavior of Multi-Functional Ti–Nb–Ta–Zr–O Alloys

Tadahiko Furuta, Shigeru Kuramoto, Junghwan Hwang, Kazuaki Nishino and Takashi Saito

Materials Department, Toyota Central Research & Development Laboratories Inc., Nagakute, Aichi 480-1192, Japan

We investigated the effect of cold working on the elastic properties of a newly developed multi-functional titanium alloy, GUM METAL, using in-situ XRD and EBSP analysis. Mechanical and physical properties are changed dramatically by cold working. The alloy has a low elastic modulus (40 GPa), high strength (more than 1100 MPa), high elastic deformability (2.5%) and super-plastic like deformability at room temperature without work hardening. The elastic behavior of the cold worked specimen shows non-linearity, with the gradient of the stress– strain curve in the elastic region continuously decreasing with a stress increase. In-situ XRD measurements during tensile loading show that all peaks shift monotonically to higher 2 angles with increasing tensile strain up to 2.7%. This result suggests that the elastic behavior in the alloy is not accompanied by phase transformations, such as stress-induced 00. Additionally, EBSP analysis reveals that the deformation mode in the alloy does not relate to f112gh111i or f332gh113i twinning. The microstructure of the alloy during deformation is characterized by localized distorted regions ranging in size from several tens of micrometers to submicrometers, with elastic strain located hierarchically in the alloy. It is likely that this microstructure is attributable to its elastic anomaly, which arises at this specific alloy composition of the multifunctional alloy. The above elastic anomaly in the alloy seems to contribute to the development of the unique microstructure during plastic deformation, as well as to its macroscopic elastic behavior.

(Received August 9, 2005; Accepted October 17, 2005; Published December 15, 2005) Keywords: beta-titanium, beta phase stability, titanium–niobium–tantalum–zirconium–oxygen, low elastic modulus, high strength, non-linear elasticity, elastic strain energy

1. Introduction combination of the bond order (Bo value) and the ‘‘d’’ electron-orbital energy level (Md value) based on the DV-X It is well known that the elastic modulus of titanium is method,8) while maintaining an e=a value of 4.24. We reduced by the addition of Va group elements such as determined the optimum combination of the three electronic niobium, tantalum and vanadium.1) Due to these experimen- numbers to be: an e=a of around 4.24; a Bo of around 2.87; tal results, much effort has been made to develop new and an Md of around 2.45. The unique properties such as low titanium alloys having a low elastic modulus with high elastic modulus, high strength and superior cold-workability strength, especially for use in artificial bone or implants.2–4) only appear when all three of these magic numbers are However, none of these alloys were completely satisfactory. satisfied simultaneously, and each alloy system requires Moreover, there have been few attempts to theoretically significant cold working and the presence of at least 0.7% compute and predict elastic moduli of titanium-based alloys. oxygen.6) The composition of the developed alloy is We have recently developed a new titanium alloy, GUM fundamentally expressed as Ti–24 at% (Ta þ Nb þ V)– METAL,5,6) that exhibits a low elastic modulus with high (Zr,Hf)–O. Various alloy compositions are available, such strength, by using a new alloy design method that uses as Ti–23Nb–0.7Ta–2Zr–O alloy and Ti–12Ta–9Nb–3V– theoretical calculation.7) This new theoretical calculation, 6Zr–O alloy (in at%), where each alloy has a simple body- which is based on the ultra-soft pseudo-potential method centered-cubic (bcc) crystal structure. within a generalized gradient approximation to the density In the present paper, we first evaluated quantitatively the function theory, can accurately predict the elastic modulus by effect of cold working on the elastic properties of the Ti– calculating elastic constants ðc11; c12; c44Þ of Ti–X binary 23Nb–0.7Ta–2Zr–1.2O alloy. We also carried out experi- alloys (X ¼ V, Nb, Ta, Mo and W). From the calculation mental verification of the bcc phase stability during elastic results, we found that (c11–c12) is correlated with the deformation and a detailed observation of the microstructure averaged valence electron number (e=a). In Ti–25 at%Nb, after plastic deformation. Finally, we discuss possible whose e=a is 4.25, the elastic anisotropy is significant and the relationships between the characteristic microstructure of elastic modulus in h001i and the shear moduli along h011i on the alloy and its elastic properties in which strong anisotropy {011} and along h111i on {011}, {112} or {123} show a very can be expected to occur. small value.5) This coincides with the result that the value (c11–c12) approaches zero when e=a is close to 4.24. The 2. Experimental results also indicated that the polycrystalline elastic modulus of Ti–X binary alloys, which is calculated from the elastic We used mainly Ti–23Nb–0.7Ta–2Zr–1.2O alloy to constants of the single crystal by the Voigt–Reuss–Hill examine the detailed elastic behavior of the multifunctional method, reaches a minimum value at an e=a of around 4.24.7) alloy. We also used Ti–21Nb–0.7Ta–2Zr–1.2O alloy and Ti– As a result, we believe that an e=a of around 4.24 is one of the 25Nb–0.7Ta–2Zr–1.2O alloy to study the bcc stability. Pure most important requirements for designing a low elastic titanium powder and other powders for alloying, such as modulus titanium alloy. Next, we experimentally investigat- titanium, niobium, tantalum and zirconium, were blended at ed the group IVa elements, which improve the strength of the predetermined compositions in a rotation mill for 7.2 ks. The alloy without increasing the elastic modulus, by changing the mixed powder was compacted into a cylindrical shape by 3002 T. Furuta, S. Kuramoto, J. Hwang, K. Nishino and T. Saito cold isostatic pressing (CIP’ing) at a pressure of 392 MPa, 1600 sintered at 1573 K for 14.4 ks in a vacuum of 103 Pa, and cooled in the furnace to room temperature. The sintered billet 1400 was hot forged at 1423 K and subsequently formed into a round bar. The surface oxidized layer was peeled off the bar, 1200 Elastic Limit 90% Cold Worked / MPa and the bar was then solution treated at 1273 K for 3.6 ks in an B argon atmosphere, quenched in water, and then cold worked σ 1000 As Annealed in a rotary swaging machine. The oxygen content of the material was controlled by mixing in a titanium powder with 800 a high oxygen content (3.0 at% O). Tensile tests were carried out on smooth cylindrical 600 lasticity> specimens 2 mm in diameter with a 10 mm gauge length at a

Tensile Stress, 400 strain rate of 5 104 s1 at room temperature. Tensile strain was measured by the strain gauge method to enable an 200

step of 1.5 mm. XRD was carried out using an X-ray Elongation, (%) diffractometer with Cu-K radiation at a scanning speed of 200 5 Elastic Limit Strength, 0.04 /s. A thin film for TEM was cut into a small disk Ti-23Nb-0.7Ta-2.0Zr-1.2O 0.5 mm thick, mechanically polished to 0.15 mm thick and 0 0 electrolytically polished by the twin-jet method at 10 V. 0 20 40 60 80 100 Observation was made by TEM with an acceleration voltage Cold Working Ratio (%) of 200 keV. Fig. 2 Changes in elastic limit strength and elongation with cold working ratio. 3. Results

Figure 1 shows the change in the tensile stress–strain curve to about 10 after 20% cold working, but then remains at room temperature with cold working for the Ti–23Nb– approximately constant until 90% cold working. 0.7Ta–2Zr–1.2O alloy. The elastic modulus is dramatically Figure 3 shows changes in elastic properties, such as the dropped by the cold working from 70 to 55 GPa at near zero initial and average elastic moduli and the attainable elastic stress and the yield stress increases after cold working. We strain, with cold working. Here, the initial elastic modulus is can confirm non-linearity in elasticity for the cold worked the gradient of the stress–strain curve at near zero stress and specimen, with the gradient of the stress–strain curve in the the average one is the average value of the gradients during elastic region continuously decreasing with the stress elastic deformation with the non-linearity. The attainable increase. As a result of the decrease in the modulus and its elastic strain of the solution treatment material is about 1% non-linearity, the elastic deformability after cold working and increases to 2.5% after 90% cold working. Both elastic reached 2.5%, which is at least double that before cold moduli decrease with increasing cold working ratio, partic- working. Figure 2 shows changes in tensile properties with ularly, the downward trend in average elastic modulus is cold working ratio. The elastic limit strength of the solution preeminent and decreases to below 40 GPa at 90% cold treatment material is about 750 MPa. After 20% cold working. Figure 4 shows effects of cold working on hardness working, the strength reaches 900 MPa. Subsequently, it and reduction in area. The Vickers hardness and the reduction gradually increases with increasing cold working, attaining in area of the alloy both remain approximately constant 1050 MPa after 90% cold working. The elongation decreases regardless of the cold working ratio. On the other hand, the Elastic Deformation Behavior of Multi-Functional Ti–Nb–Ta–Zr–O Alloys 3003

140 400 Ti-23Nb-0.7Ta-2.0Zr-1.2O Conventional β-Titanium Alloy 120 3.0 (Ti-15V-3Al-3Cr-3Sn wt%) E ε / GPa

A 300

E 100 2.5 Hardness , i Elastic Strain E 80 2.0 200 Reduction in Area 80 60 Initial Elastic Modulus 1.5 60

40 1.0 Hardness, Hv 100 40 Elastic Modulus,

Average Elastic Modulus Attainable Elastic Strain, (%) 20 0.5 20 Ti-23Nb-0.7Ta-2.0Zr-1.2O Reduction in area (%) 0 0 0 0 02040 60 80 100 0 20 40 60 80 100 Cold Working Ratio (%) Cold Working Ratio (%) Fig. 4 Effects of cold working on hardness and reduction in area. Fig. 3 Changes in attainable elastic strain and initial and average elastic moduli with cold working ratio.

(a) (b)

30µm 30µm

(c) (d)

30µm 30µm

Fig. 5 Microstructures of Ti–23Nb–0.7Ta–2Zr–1.2O after solution treatment at 1173 K (a) and after subsequent cold working by 40% (b), 80% (c) and 90% (d). hardness of the conventional titanium alloy gradually Optical micrographs of Ti–23Nb–0.7Ta–2Zr–1.2O alloy increases with an increasing cold working ratio. This (Fig. 5) show that the microstructure changes dramatically suggests that the multifunctional alloy has super-plastic like during cold working. The microstructure in Fig. 5(a) (before deformability at room temperature without work harden- cold working) is composed of equiaxed grains 50 to 100 mm ing.5,6) Since the alloy does not show work hardening after in size. Lenticular deformation bands are observed to appear cold working, continuous deformation without annealing is in the coarse equiaxed grain after 40% cold working possible to more than 99.9% under any kind of cold working, [Fig. 5(b)]. Subsequently, as the cold working ratio increas- such as formation of a round bar, wire or thin sheet. es, the deformation bands are gradually distorted as seen in 3004 T. Furuta, S. Kuramoto, J. Hwang, K. Nishino and T. Saito

16000 1600 {011} β Ti-23Nb-0.7Ta-2.0Zr-1.2O {002} β 12000 1400 {112} β 1200 X=23Nb / MPa 8000 B σ 1000 X=25Nb {002} β 4000

X-ray Intensity (cps) (b) 800 (a) X=21Nb 0 600 20° 30° 40° 50° 60° 70° 80° 90° Diffraction Angle, 2θ

Tensile Stress, 400

Fig. 6 X-ray diffraction profiles after solution treatment at 1173 K (a) and 200 after subsequent cold working by 90% (b). Ti-X Nb-0.7Ta-2.0Zr-1.2O 0 01 23 45 Fig. 5(c), and after 90% cold working, the microstructure Tensile Strain (%) finally changes into a characteristic ‘‘marble-like’’ structure [Fig. 5(d)]. The marble-like microstructure is composed of Fig. 7 Change in tensile stress–strain curve at room temperature with Nb assemblies of fine filamentary structure. Although the micro- content in Ti–XNb–0.7Ta–2Zr–1.2O. structure changes dramatically as seen in Fig. 5, the phase configuration doesn’t change at all, even after heavy cold working. Namely, the XRD profile after cold working the change in elastic deformation behavior effected by Nb (Fig. 6) does not show the presence of any peaks other than contents in Ti–Nb–Ta–Zr–O alloys. The stress–strain curve the (bcc) phase. It is also inferred from the XRD profile of the lower Nb containing alloy, Ti–21Nb–0.7Ta–2Zr–1.2O after cold working that the preferred orientation of (011) is alloy, indicates pseudo-elastic deformation. On the other formed by crystallographic rotation as the plastic working hand, the higher Nb containing alloy, Ti–25Nb–0.7Ta–2Zr– proceeds. 1.2O alloy, has a linear stress–strain relationship in the elastic range as is often seen in conventional titanium alloys. The 4. Discussion multifunctional alloy, Ti–23Nb–0.7Ta–2Zr–1.2O alloy, has unique elastic behavior with non-linearity in the elastic 4.1 Elastic deformation behavior and bcc phase stability range. In order to characterize crystallographic information It is well known that the large elastic deformation obtained during the elastic deformation of the alloy, in-situ XRD in ‘‘super-elastic alloys’’ originates from reversible marten- measurements during tensile loading were conducted using sitic transformation, dubbed ‘‘pseudo-elastic deforma- sheet specimens made from the 90% cold swaged material. tion’’,9,10) in which substantial hysteresis can be seen in the Figure 8 shows in-situ measurement results representing stress–strain curve. Conversely, the multifunctional alloy typical reflection peaks, around {011} and {002} , during shows a quite unique elastic behavior with no hysteresis in the tensile loading for the cold worked alloys. A reflection the stress–strain relation.5,6) Therefore, it is likely that the peak of stress-induced 00 martensite is observed in the lower elastic behavior of the alloy is completely different from that Nb containing alloy before tensile loading, as seen in of the conventional ones. Fig. 8(a). Along with increasing strain, the (020) 00 Hence, let us consider the relationship between elastic reflection becomes weaker while the {011} reflection deformation behavior and bcc phase stability. Figure 7 shows becomes stronger. This phenomenon can be interpreted by

5000 0.0 10000 10000 (a) (b) 0.0 (c) 0.0 0.4 β β 0.4 0.4 {011} 0.8 8000 {002} 4000 0.8 8000 0.8 1.2 β 1.7 1.2 {002} 1.2 3000 2.2 6000 1.7 6000 1.7 2.2 2.7 2.2 (020) α" 2.7 2000 4000 4000 ε% ε% ε%

1000 2000 2000 X-ray Intensity (cps) 0 0 0 36° 38° 40° 42° 54° 56° 58° 60° 54° 56° 58° 60° Diffraction Angle, 2θ Diffraction Angle, 2θ Diffraction Angle, 2θ

Fig. 8 Changes in the profiles for {011} reflection of Ti–21Nb–0.7Ta–2Zr–1.2O (a), {002} reflection of Ti–23Nb–0.7Ta–2Zr–1.2O (b) and Ti–25Nb–0.7Ta–2Zr–1.2O (c) during tensile loading. Elastic Deformation Behavior of Multi-Functional Ti–Nb–Ta–Zr–O Alloys 3005

1.5 (a) 1.5 (b) (002) β

1.0 1.0

(112) β (002) β

0.5 0.5 (112) β d Shrinkage Ratio (%)

0 0 02130213 Tensile Strain (%) Tensile Strain (%)

Fig. 9 Changes in the lattice plane spacing estimated from the peak shift values of (002) and (112) as a function of the tensile strain of Ti–23Nb–0.7Ta–2Zr–1.2O (a) and of Ti–25Nb–0.7Ta–2Zr–1.2O (b). considering the orientation relationship between and 00. to that of deformation twins. f112gh111i twinning often Namely, the orientation relationship is known as ð110Þ == occurs in bcc metals and alloys. In addition to this, ð020Þ 00 and ð001Þ ==ð002Þ 00 for transformation from f332gh113i twinning, advocated by Cocker,11) has been to 00 which yields 2.4% expansion in the h110i direction reported to take place in metastable titanium alloys at room and 2.4% compression in the h001i direction, respectively. temperature, by Hanada et al.12,13) Crystallographic misor- Tensile loading generates compressive stress in sheet thick- ientation between matrix and twin in f112gh111i or ness. Hence, (020) 00 on the rolling plane is compressed and f332gh113i twinning is about 70.3 or 50.5 degrees, respec- retransforms to (110) with increasing tensile strain. In the tively, around the h110i axis. First, we carried out a detailed lower Nb containing alloy, obvious rearrangement of variants observation of the microstructural changes with cold work- of 00 occurs, and the stress–strain curve can be explained by ing, using EBSP to investigate the internal constitution of the reversible martensitic transformation, dubbed ‘‘pseudo-elas- unique ‘‘marble like’’ microstructure. tic deformation’’. Figure 8(b) shows the result for the Figure 10 shows an example of an inverse pole figure map multifunctional alloy, in which each peak shifts monotoni- obtained by EBSP for the specimen shown in Fig. 5(b); the cally to a higher 2 angle with increasing tensile strain up to swaging direction is normal to the measured plane. The 2.7%. This result suggests that no phase transformation change in color in each grain corresponds to that in crystal occurs during tensile loading. In the higher Nb containing orientation. We can see that most of the grains are divided alloy, the {011} peak shifts monotonically with increasing into sections by boundaries and/or have a continuous tensile strain up to 1.2%, and then the peak shift stops, as seen orientation change without clear boundaries. Figure 11 in Fig. 8(c). Figure 9 shows the change in the lattice plane shows point-to-point misorientation across the orientation spacing estimated from the peak shift values of (011) and boundaries in the interior of the grains and orientation gaps (112) as a function of the tensile strain in Ti–23Nb–0.7Ta– between the grain-rotation axis and the h110i axis. The 2Zr–1.2O alloy and Ti–25Nb–0.7Ta–2Zr–1.2O alloy. The crystallographic misorientations are less than 30 degrees and lattice plane spacing of the multifunctional alloy, Ti–23Nb– the gap between the axes is in the range between 10 and 30 0.7Ta–2Zr–1.2O alloy, continuously changes as the tensile degrees. Therefore, it seems reasonable to at least conclude strain increases up to 2.7%. This means that the elastic that the boundaries shown in Fig. 5(b) are not twin deformation continues to a tensile strain of as much as 2.7%, boundaries. We also performed a similar analysis on a as shown in Fig. 1. In the case of the higher Nb containing specimen that was cold swaged by 85%. Figure 12 shows an alloy, Ti–25Nb–0.7Ta–2Zr–1.2O alloy, the change in the example of the crystallographic misorientations without any lattice plane spacing is saturated at 1.2% tensile strain, which boundaries in the grain. The point-to-point misorientation is suggests that plastic deformation occurs at more than 1.2% less than 10 degrees. This result also proves clearly that a tensile strain. As observed above, there is no doubt about the deformation mode in the alloy does not relate to twinning. fact that no transformation such as stress-induced 00 or Additionally, the important point in this result is a continuous rearrangement of variants of 00 occurs in Ti–23Nb–0.7Ta– grain rotation of more than 50 degrees across a distance of 2Zr–1.2O alloy. about 45 mm. It is quite likely that a local area of distortion like this has a considerable amount of residual stress. Since 4.2 Microstructure development during plastic defor- the alloy has a very low elastic modulus, quite a lot of mation localized elastic strain would be accumulated in the alloy The unique properties of the multifunctional alloy are after plastic deformation. achieved only after cold working,5,6) which implies that the It was inferred from the XRD analysis results that the internal constitution of the cold worked alloy plays a crucial elastic deformation mode of the multifunctional alloy is role in the development of those properties. Deformation different from that of stress induced martensitic transforma- bands are observed in Fig. 5, and their appearance is similar tion. EBSP results indicated that the elastic strain was 3006 T. Furuta, S. Kuramoto, J. Hwang, K. Nishino and T. Saito accumulated in the cold worked microstructure. Here we will electron beam was parallel to the longitudinal direction of the discuss the internal constitutions of such a unique micro- cold swaged bar. The microstructure shown in Fig. 13(b) is a structure. Figure 13 shows examples of the microstructure of mixture of grains several tens of micrometers to submicrom- a specimen after 90% cold working. In Figs. 13(b), (c), the eters in size. Figure 13(c) shows a magnified image of the flecked contrast. The elastic field seems to be a fractal-like microstructure, which means that the elastic field induced by cold working is located hierarchically in the alloy from micrometer to nanometer. Namely, the assemblies of fine filamentary structure seen in Fig. 13(a) correspond to a

80° Ti-23Nb-0.7Ta-2.0Zr-1.2O 70° Point-to-point misorientation g ° 60 Orientation gap between grain-rotation axis and <110> axis 50°

40° 30° from <110> axis, 20°

10° Misorientation and orientation gap 0° 13274 5910 68 Place of Measurement

Fig. 11 Point-to-point misorientation across the boundary in the grain and Fig. 10 Inverse pole figure map of Ti–23Nb–0.7Ta–2Zr–1.2O after 40% orientation gap between the grain-rotation axis and h110i axis after 40% cold working. cold working.

60° (a) (b) Point-to-origin g 50°

40°

30°

20° Misorientation , 10° Point-to-point 0° 01020304050 Distance, D / microns

Fig. 12 Misorientation in the grain of Ti–23Nb–0.7Ta–2Zr–1.2O (a) and inverse pole figure map after 85% cold working (b).

(a) (b) (c)

100µm 1µm 10nm

Fig. 13 Elastic strain fields in Ti–23Nb–0.7Ta–2Zr–1.2O induced by cold working. Optical microstructure (a) and transmission electron (TEM) images (b) and (c). Elastic Deformation Behavior of Multi-Functional Ti–Nb–Ta–Zr–O Alloys 3007 gathering of localized elastic strain in Figs. 13(b) and (c), relate to f112gh111i or f332gh113i twinning. which is accumulated discretely and hierarchically in the (3) The microstructure of the alloy during deformation is alloy. characterized by a localized distorted region ranging 7) In the previous paper, we have shown that (c11–c12) takes from several tens of micrometers to submicrometers in a value close to zero around the composition of the size, with the elastic strain being located hierarchically multifunctional alloy, namely at an e=a value of around in the alloy. It is likely that such microstructure is 4.24. When (c11–c12) approaches zero, the elastic modulus in attributable to its elastic anomaly, which arises at this h001i and the shear moduli along h011i on {011} and along specific alloy composition of the multifunctional alloy. h111i on {011}, {112} or {123} exhibit a very small value. This suggests that the ‘‘ideal strength’’14) of the alloy is Acknowledgment extremely small. Dislocation glide occurs along h111i on {011}, {112} or {123} in conventional bcc metal when local The authors would like to thank Dr. Y. Senou, Mr. H. stress reaches the critical value. In the case of the alloy, the Ikehata and Mr. N. Nagasako for useful discussions. applied stress can be relaxed by localized elastic deformation due to its extremely low shear modulus in the specific directions. It seems that such elastic relaxation can be related REFERENCES to plastic deformation without deformation twinning. At a 1) S. G. Fedotov: Titanium Science and Technology, ed. by R. I. Jaffe and certain amount of loading, a shear deformation can proceed H. M. Burte, (Plenum Press, Boston, 1973) pp. 871–881. suddenly along the maximum shear stress plane without any 2) M. Niinomi: Metall. Mater. Trans. A 33A (2002) 477–486. aid of dislocations, when the local stress is nearly equal to the 3) K. Nitta, S. Watanabe, N. Masahashi, H. Hosoda and S. Hanada: ideal stress of the alloy.15) As plastic working proceeds, Structural Biomaterials for 21st Century, ed. by M. Niinomi, T. Okabe, highly distorted and localized elastic strain is accumulated in E. M. Taleff, D. R. Lesuer, H. E. Lippard, (TMS, Warrendal, 2001) pp. 25–34. the fine filamentary structure seen in Fig. 13. The above 4) T. Ahmed, M. Long, J. Silvestri, C. Ruiz and H. J. Rack: Titanium ’95 elastic anomaly in the multifunctional alloy seems to Science and Technology, ed. by P. A. Blankinsop, W. J. Evans, F. H. contribute to the development of its unique microstructure Flower, (The Material Society, London, 1996) pp. 1760–1767. during plastic deformation, as well as to its macroscopic 5) T. Saito et al.: Science 300 (2003) 464–467. elastic behavior. 6) T. Furuta, K. Nishino, J. H. Hwang, A. Yamada, K. Ito, S. Osawa, S. Kuramoto, N. Suzuki, R. Chen and T. Saito: Ti-2003 Science and Technology, ed. by G. Lu¨tjering and J. Albrecht, (WILEY-VCH, 5. Summary Weinheim, 2004) pp. 1519–1526. 7) H. Ikehata, N. Nagasako, T. Furuta, A. Fukumoto, K. Miwa and We investigated the effect of cold working on the unique T. Saito: Phys. Rev. B 70 (2004) 174113-1–174113-8. elastic properties of Ti–23Nb–0.7Ta–2Zr–1.2O alloy and 8) M. Morinaga, N. Yukawa, T. Maya, K. Sone and H. Adachi: Sixth World Conference on Titanium 1988, ed. by P. Lacombe, R. Tricot and discussed the accumulation of elastic strain energy during G. Be´ranger, (Les Editions de Phisique, Les Ulis Cedex, 1989) plastic deformation in the alloy. pp. 1601–1606. (1) The elastic modulus of the developed alloy is decreased 9) C. Baker: Metal. Sci. J. 5 (1971) 92–100. dramatically by cold working. After 90% cold working, 10) T. W. Duerig, J. Alberecht, J. Richter and P. Fischer: Acta Metall. 30 the alloy has a low elastic modulus (40 GPa), high (1982) 2161–2172. 11) A. G. Crocker: Acta. Metall. 10 (1962) 113–122. strength (more than 1100 MPa), and high elastic 12) S. Hanada, M. Ozeki and O. Izumi: Metall. Trans. A 16A (1985) 789– deformability (2.5%). The alloy also has super-plastic 795. like deformability at room temperature without work 13) S. Hanada and O. Izumi: Metall. Trans. A 17A (1986) 1409–1420. hardening. 14) C. R. Krenn, D. Roundry J. W. Morris, Jr. and M. L. Cohen: Mater. Sci. (2) In situ XRD revealed that the elastic behavior in the Eng. A 319–321 (2001) 111–114. 15) S. Kuramoto, T. Furuta, J. H. Hwang, Y. Seno, T. Nonaka, H. Ikehata, developed alloy is not accompanied by phase trans- N. Nagasako, K. Nishino, T. Saito, C. Iwamoto, Y. Ikuhara and 00 formations, such as stress-induced . EBSP analysis T. Sakuma: Ti-2003 Science and Technology, ed. by G Lu¨tjering and revealed that the deformation mode in the alloy does not J. Albrecht, (WILEY-VCH, Weinheim, 2004) pp. 1527–1534.