Mechanical properties of Ti-Nb biomedical shape memory alloys containing Ge or Ga

Mechanical properties of Ti-24 mol% Nb-based shape memory alloys (SMA) containing 3 mol% Ga or Ge were characterized in this paper as a part of our systematic work for the development of β-Ti based biomedical shape memory alloys. The alloys, called TiNbGa and TiNbGe, were produced by severe cold-rolling followed by a solution treatment at 1273 K for 1.8 ks. The apparent phase was β (bcc) at RT in both the alloys. It was revealed by X-ray diffraction pole figure analysis that a {112}β〈110〉β recrystallization texture was well developed in TiNbGa. However, a {001}β〈110〉β deformation texture still remained in TiNbGe even after the solution treatment. Martensite transformation temperatures were significantly lowered by the addition of Ge, compared to Ga and Al additions. TEM-EDX observation revealed that (Ti, Nb)5Ge3 particles are formed in TiNbGe regardless of the solution treatment. The (Ti, Nb)5Ge3 particles were judged to be an ineffective strengthener, because significant hardening was not recognized in the flow-stress of TiNbGe. TiNbGa exhibited a large shape recovery of about 2% above RT in the strain-temperature curves during thermal cycles under external stress. The TiNbGe alloy exhibited superelasticity of 3.5% at RT.