Heterogeneous grain refinement of biomedical Ti–29Nb–13Ta–4.6Zr alloy through high-pressure torsion

The biomedical ββ-type titanium alloy Ti–29Nb–13Ta–4.6Zr (TNTZ) exhibits non-toxicity and a low Young’s modulus that is similar to that of bone. This alloy has a low Young’s modulus because it contains a metastable ββ phase. Strengthening due to grain refinement tends to provide high mechanical strength, while keeping the Young’s modulus low, because it maintains the original ββ phase. In this case, severe plastic deformation, such as High-Pressure Torsion (HPT), is a potential treatment for obtaining these properties simultaneously. Thus, in this study, the effect of HPT on the microstructure and hardness of TNTZ was systematically investigated. On the cross sections of TNTZ subjected to HPT, heterogeneous microstructures, consisting of a matrix and an unetched band not corroded by an etching solution, were observed. Both the matrix and the unetched band were comprised of a single ββ phase with submicron-sized grains, but their grain geometries differed: equiaxed grains and elongated grains are observed in the matrix and the unetched band, respectively. The hardness distribution in the cross section of TNTZ subjected to HPT is also heterogeneous; the hardness is higher in the matrix than in the unetched band, when the number of HPT rotations is small.