Fatigue crack growth behavior of a nanocrystalline low Young's modulus β-type Ti–Nb alloy

Abstract

Low Young's modulus β-type titanium alloys, such as the Ti–45Nb alloy, with elastic properties close to that of human bone containing non-toxic elements are very desirable for load bearing implants. Their relatively low hardness and strength can be significantly increased by severe plastic deformation (SPD) methods, for example high pressure torsion (HPT). To gain a better understanding of the fatigue characteristics, the fatigue crack growth (FCG) behavior of Ti–45Nb was investigated in various microstructural states (as-received, HPT-deformed, HPT + cold-rolled, HPT + heat-treated). Compact tension (CT) specimens with different crack orientations were tested at different load ratios under tension-tension loading. The microstructure of the HPT and HPT + cold-rolled material shows highly elongated grains parallel to the principal deformation direction with grain sizes in the nanocrystalline regime. Compared to the majority of studies on FCG of SPD-processed materials this titanium alloy exhibits a remarkably weak anisotropy in the FCG-behavior in all material states but especially in the HPT and the cold-rolled state. This is mainly attributed to the transcrystalline fracture mode. The threshold of fatigue crack growth was found to decrease with increasing load ratio. This is related to a lower contribution of plasticity-induced crack closure at higher load ratios.