Design And Study On Mechanical Properties Of Ti-Nb-Zr Alloys With Super-elasticity

This paper aimed at designing and studying on super-elastic Ti alloys composed of non-toxic elements. A Series of Ti-Nb-Zr(-Sn) alloys were designed by d-electron alloy theory. Microstructural changes and deformation switches were detected and analyzed by X-ray diffraction (XRD), transmission electron microscope (TEM) and Optical microscope (OM). Super-elasticity was evaluated by U bending test and tensile loading-unloading test.The addition of Sn element can improve the strength of solutionized alloys, and rarely decrease the plasticity. Sn shows higher capability to stabilizeβphase than Nb and Zr. Although designed alloys locate in metastableβphase area in Bo-Md diagram, alloys with 2 at% Sn addition consist of stableβphase owning to the reducing effect of Sn on Ms temperature. Stress-induced martensitic transformation (SIMT) only occurs in Ti-24at%Nb-2at%Zr and Ti-24at%Nb-4at%Zr alloys during loading tests. U bending test is used to evaluated the super-elasticity of alloys after annealing at 573 K ~ 1073 K. The results show that a small quantity ofωphase forming during annealing at 573 K improves strength significantly, enhances super-elasticity, and reduces plasticity sharply.αphase forming during 823 K shows less dispersion strengthening effect thanωphase. It supplies little contribution to improve super-elasticity.Ti-richωphase forms during aging at 573K after solution treatment. It causes an increase ofβstabilizing elements in matrix. It also supplies a mechanical resistance to shear ofβphase. Thus, the formation and growth restrain {332} <113> twinning and SIMT. The deformation mechanism is considered to initially be coexisting of {332} <113> twinning, SIMT and plastic slip, then be SIMT and plastic slip, and finally be only slip.After aging, precipitation of Ti-richωphase reduces Ms temperature and echances strength of alloys. Good super-elasticity can be obtained due to the appropriate cooperation of Ms temperature and strength of alloys. When the cumulative applied strain more than 6%, the alloys subjected to solution treatment and short time aging show declining recovery strain due to the accumulation of plastic deformation and residual inducedα′′phase. After certain time aging, adequate amount ofωphase can restrain the plastic deformations ofβphase andα′′phase, and avoid residualα′′phase. Thus, the alloys own stable super-elasticity. Excessiveωphase restricts SIMT, and then the super-elasticity will decrease. Agingωphase can improve strength obviously without large plastic loss. After aging at 573 K for 7.2 ks, Ti-24at%Nb-2at%Zr alloy exhibits a Young's modulus of 64.4 GPa, a elongation of 14%, and stable super-elastic recoverable strain of 4.3%. After aging at 573 K for 7.2 ks, Ti-24at%Nb-4at%Zr alloys perform Young's modulus of 61.5 GPa, elongations above 15%, and superior super-elasticity.Ti-24at%Nb-2at%Zr alloys subjected to solution treatment and aging are measured by electrochemical test such as electrochemical impedance spectroscopy (EIS) and polarization test. The results suggests that agingωphase do not causes decrease of corrosion resistance. This alloy exhibits better corrosion resistance than other reportedβtype titanium alloys.Ti-24at%Nb-2at%Zr alloy, with desired super-elasticity, superior mechanical properties and corrosion resistance, is potential to use as biomedical materials in the futhure.