Thermal stability and mechanical properties of nanostructured Ti–24Nb–4Zr–7.9Sn alloy

Thermal stability of the nanostructured grains of cold-rolled Ti–24Nb–4Zr–7.9Sn alloy and corresponding variations in mechanical properties were investigated. The activation energy for grain growth was found distinct below and above the (α + β)/β transus of ∼950 K, with values of ∼47 and ∼206 kJ/mol, respectively. Due to the pinning effect of the α precipitates at β grain boundaries, grains sizes can be maintained at less than 100 nm during prolonged annealing at temperatures up to 773 K, and are less than 1 μm for annealing temperature up to 923 K and time up to 2 h. Annealing above the β transus resulted in coarse grains with sizes of tens of micrometers in less than 2 h. Tensile and hardness tests showed rapid strengthening with the increase of annealing time below 773 K, which was attributed to both the rapid formation of nano-sized α precipitates and the slow growth rate of β grains. By adjusting the grain size of the cold-rolled material the high strength/low Young's modulus match desirable for implant applications can be improved over the hot-rolled bars with coarse grains.