Effect of thermomechanical treatment on the superelasticity of Ti–7.5Nb–4Mo–2Sn biomedical alloy

Effects of thermomechanical treatment on the microstructure and superelasticity of Ti–7.5Nb–4Mo–2Sn biomedical alloy were investigated by using XRD measurement, optical microscope (OM), transmission electron microscope (TEM) and tensile tests. The titanium alloy samples were prepared by annealing at a temperature in the range of 600 to 1000 °C after severe cold rolling; and the samples that were annealed at 800 °C were further aged at 600 and 700 °C. The volume fraction of α phases decreased while that of ω phases increase with increasing annealing temperature. The α → β transformation temperature of the alloy was determined to be between 700 and 800 °C. The alloy that was annealed at 700 °C exhibited a high level of superelasticity with relatively high first yield stress (σSIM) at room temperature because it contained a fine α phase. A certain amount of ω phases also resulted in an increase in σSIM, leading to an improvement in the superelasticity of the alloys that were annealed at 900 and 1000 °C. Aging treatment led to the precipitations of α and ω phases in the alloy after annealing at 800 °C; and the volume fraction of α phases decreased while that of ω phases increased with increasing aging temperature. Excellent superelasticity with high recovered strain (εrecoverable) and strain recovery rate (η) were obtained in the aged alloy due to the reinforcement of α and ω phases induced by aging treatment. The alloy annealed at 700 °C for 0.5 h exhibited the best superelasticity in all the thermomechanically treated alloys due to the strengthening from the subgrain refining and the precipitating of fine α phases.