Exploration of hypereutectoid compositions for achieving and enhancing superelasticity in biomedical Ti–Au–Mo alloys

Abstract

β (bcc) Ti–Au based biomedical alloys are attractive due to their excellent biocompatibility, X-ray radiographic imaging properties, and shape memory effects. However, increasing the Au content beyond the eutectoid composition (4.2 mol% Au, β-Ti = α-Ti (hcp) + Ti₃Au, in the binary system) presents a challenge because the intermetallic compound Ti₃Au readily forms at hypereutectoid compositions, leading to increased brittleness in the alloys. Thus, the feasibility of hypereutectoid compositions was explored in this work using the Ti–Au–Mo ternary system. The addition of Mo was expected to significantly suppress the eutectoid reaction by enhancing the stability of the β phase. In solution-treated alloys, it was found that Ti–Au–5Mo alloys with up to 6 mol% Au (approximately 20 mass% Au, 5 karat) exhibited good cold workability, achieving a 95 % reduction in thickness. Furthermore, in the Ti–6Au–5Mo alloy, a β single phase was successfully quenched without the formation of Ti₃Au. The stability of the β phase increased with higher Au content, with the reduction rate in the martensitic transformation temperature of −85 K/mol% Au. Additionally, the Ti–6Au–5Mo alloy exhibited clear superelasticity at room temperature, with a 4.8 % shape recovery strain and a 2.7 % superelastic strain. Besides, the effect of Ti₃Au precipitation through heat treatment on mechanical properties was also evaluated. These findings provide a solid foundation for the development of functional low-karat Ti–Au alloys for advanced medical devices.