Change in fatigue strength of biomedical β-type titanium alloy through heat treatment processes

Relationships between fatigue properties and microstructures of hot-forged Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) in under aged, peak aged, and over aged conditions at aging temperatures of 673K and 723K are investigated. The changes in the fatigue properties of TNTZ subjected to thermomechanical treatments, which includes an aging treatment at 673K or 723K for 259.2ks after a severe deformation process by cold rolling, are also investigated. At an aging temperature of 673K, ω phase precipitates at the early stage of aging, but α phase precipitates at relatively longer aging time. The precipitation site of α phase changes from intra-grain to grain boundary at around peak aging time when TNTZ is aged at 673K and 723K. The elastic modulus of TNTZ increases simply with increasing aging time at both 673K and 723K The fatigue strength of TNTZ increases considerably when α phase precipitates compared with when ω+α phases co-exist. The fatigue strength of TNTZ decreases slightly due to the coarsening of α phase precipitated in β grain and its grain boundary. TNTZ aged at 723K for 259.2ks after cold rolling exhibits the highest fatigue strength in both the low- and high-cycle fatigue life regions. Furthermore, the fatigue limitof about 770MPa (fatigue raito : 0.71) is nearly equal to that of hot-forged Ti-6Al-4V ELI alloy subjected to aging after solution treatment with equiaxed α structure.