Fatigue properties of a metastable β-type titanium alloy with reversible phase transformation

Due to recent concern about allergic and toxic effects of Ni ions released from TiNi alloy into human body, much attention has been focused on the development of new Ni-free, metastable β-type biomedical titanium alloys with a reversible phase transformation between the β phase and the α″ martensite. This study investigates the effect of the stress-induced α″ martensite on the mechanical and fatigue properties of Ti–24Nb–4Zr–7.6Sn (wt.%) alloy. The results show that the as-forged alloy has a low dynamic Young’s modulus of 55 GPa and a recoverable tensile strain of ∼3%. Compared with Ti–6Al–4V ELI, the studied alloy has quite a high low-cycle fatigue strength because of the effective suppression of microplastic deformation by the reversible martensitic transformation. Due to the low critical stress required to induce the martensitic transformation, it has low fatigue endurance comparable to that of Ti–6Al–4V ELI. Cold rolling produces a β + α″ two-phase microstructure that is characterized by regions of nano-size β grains interspersed with coarse grains containing α″ martensite plates. Cold rolling increases fatigue endurance by ∼50% while decreasing the Young’s modulus to 49 GPa along the rolling direction but increasing it to 68 GPa along the transverse direction. Due to the effective suppression of the brittle isothermal ω phase, balanced properties of high strength, low Young’s modulus and good ductility can be achieved through ageing treatment at intermediate temperature.