Characteristics Of Cold Deformed And Superelasticity Of TiNbZrTa Beta Titanium Alloy

Titanium alloys are extensively used in medical fields due to their good biocompatibility, low density, high strength and high corrosion resistance to body fluid. However, theβtitanium alloys with lower elastic modulus and non-toxic alloying elements have much more important applications in medical fields. From 1990s, new kinds ofβtitanium in medical fields were studied by many researchers. Recently, Nb,Ta,Zr and Sn show good biocompatibility and low toxicity to human being. The shape memory effects and superelasticity are important to the application of this alloy. Generally, martensite twin, grain orientation, precipitation phase and grain size play an important part on the superelasticity. While, as for these kinds ofβtitanium alloys, the studies of the mechanisms of cold deformation and superelasticity were not so deep. The research of the transformation of martensite and microstructure during cold deformation is very important to learn the superelasticity and shape memory effects of this alloy. TiNbZrTa alloy is studied in the paper. Different kinds of deformation methods are carried out. The mechanism of cold deformation, the transformation of martensite and the mechanical properties are studied. In addition, the textures of cold-deformed and heat-treated specimens are also discussed. The effects of textures to anisotropy of superelasticity are studied. The cold-deformed specimens are treated at different temperature and the influence of precipitation phase on shape memory effects and mechanical properties are also discussed.On the base of the study of the characteristic of cold-deformed TiNbZrTaβtitanium alloy, the rare earth elements are added to this alloy. The rare earth elements added in the alloy can clean the matrix, refine grains and improve the shape memory effects of the alloy. At the same time, the content of oxygen in matrix of titanium alloy can be reduced because of the appearance of rare earth oxide. As the former studies showed, TiB as one of the most important reinforcements in titanium matrix composites can improve the strength of titanium matrix composites strongly. However, few reports focus on the effect of reinforcement on shape memory effects ofβtitanium alloys. In order to receive good strength and shape memory effects, vacuum arc melting method is carried out to synthesize titanium matrix composites with reinforcements of TiB and La2O3. The effect of the content of LaB6 additions on superelasticity and mechanical properties of TiNbZrTaβtitanium alloys is studied in this paper. In situ synthesized titanium matrix composites with reinforcements of TiB and La2O3 are melted by acuum arc melting method through the chemical reaction of Ti and LaB6. The effect of reinforcements on refining grains is studied and the actions of different content of LaB6 on mechanical properties and shape memory effects are also discussed.The main results are showed as follows:Direct rolling and cross rolling at different cold reduction ratio are carried out to investigate the microstructures and the transformation of martensite. The results show, in the processing of direct rolling, with the increase of cold reduction, acicular martensite with straight boundary changes to butterfly-shaped martensite gradually. The microstructure of the martensite is coarse variant and with the increase of cold reduction, crossed martensite tending to rolling direction. The characteristic of the change of martensite during cross rolling is just the same as the characteristic of martensite during direct rolling. However, because of the shear stress of cross rolling, no serious directivity of growth of martensite can be observed. During direct rolling, when the cold reduction ratio is 99%, the transformation amount of martensite is 78.93%. However, during cross rolling, when the cold reduction ratio is 40%, the transformation amount of martensite is 79.63% and with the increase of cold reduction ratio, the amount of martensite keeps the same as the specimen deformed by 40%. The characteristic of shear stress during cross rolling is in favor of the transformation of martensite and reduce the anisotropy of the microstructure. During cold rolling, with the increase of the cold reduction ratio, strain-induced martensite appears, grows up and develops to stable microstructure. The deformation mechanisms of martensite transformation can be summarized as follows: the appearing and growing up of twins of martensite variant, the combination and reorientation of twins and the appearance of new twins variant inside martensite.Theβtexture and strain-inducedα" martensite texture appearring during cold deformation play an important role on elastic modulus and superelasticity. The anisotropy of elastic modulus and superelasticity are coursed by the anisotropy of texture. During direct rolling, with the increase of the cold reduction ratio, preferred {100}β<011>βtextures are obtained. In the specimen at the cold reduction ratio of 90%, obvious strain-induceα" martensite texture exhibits as preferred (200)α"<010>α" texture. The appearance of different kinds of texture along different directions contribute much to the anisotropy of strain transformation during tensile processing.When the cold-rolled specimens are treated at 573K,ωphase appears. Meanwhile,αphase can be observed when the aging temperature is improved from 673K to 873K. The appearance ofωphase at 573K increases the driving force of the transformation of martensite, which increases the superelasticity. Similar textures is obtained in the cold-rolled specimen at cold reduction of 99% and the cold-rolled specimen followed by heat treatment at 873K for 1.2ks. Preferred {100}<011> texture appeares is also observed. When the cold-rolled specimen is treated at 1223K for 1.2ks, recrystallization texture can be observed. The recrystallization texture is {112} <011> texture. During heat treatment, {100} <011> texture changes to {112}<011> recrystallization texture gradually. The same orientation of cold-rolled grains and recrystallized grains is attributed to the mechanism of recrystallization. During recrystallization process, when subgrain boundaries migrate and grow up, the orientation of the nucleation is just the same as the cold-rolled specimen. Plate-shaped deformation twins and strip-shaped stress-inducedα″martensite phase transformation are observed in tensile specimen with the strain of 5.5% followed by heat-treated at 1223K. Thisα″martensite phase occurring during tensile test contributes much to shape memory effects, which changes toβphase after heating.In order to improve the shape memory effects and superelasticity, In situ techniques is carried out to synthesizeTiB and Re2O3 inβtitanium alloy. Ti-35Nb-3Zr-2Ta is considered as the matrix alloy. The mass fraction of LaB6 power is 0.1%,0.2%,0.3%,0.4%, and 0.5%, respectively. The refinements of TiB and La2O3 can be synthesized during chemical reaction. TiB whiskers appearing around grain boundaries strengths the matrix rapidly in 0.1% LaB6-additioned specimen. With the increase of the mass fraction of LaB6, many more TiB whiskers and La2O3 pariticles are investigated. Much more La2O3 pariticles refine grains and the equal grains sizes are around 12um in 0.5% LaB6-additioned specimen. In 0.1% and 0.2% LaB6-additioned specimens, the specimens exhibit better superelastic strain and elastic strain. In 0.5% LaB6-additioned specimen, because of grain refining, the superelastic strain shows the highest value. TiB whiskers increase the critical slip stress and make the transformation fromβphase to martensite come true under lower applied stress. This is very important to the applications of superelastic components for this alloy.The main micromechanisms of the tensile deformation of (TiB+La2O3)/Ti-alloy composites are evaluated during in situ experiments. Screw dislocations are activated along tensile direction at the low strain. After dislocations slipping, { 111 } self-accommodated martensite twins is activated, meanwhile (001) twins can also be observed. With the increase of tensile strain, (001) twins grow up and extend to { 111 } twins. Gradually, the (001) twins have the similar shape as { 111 } twins.