Heterogeneous grain refinement of biomedical Ti29Nb13Ta4.6Zr alloy through high-pressure torsion

The biomedical -type titanium alloy Ti29Nb13Ta4.6Zr (TNTZ) exhibits non-toxicity and a low Youngʹs modulus that is similar to that of bone. This alloy has a low Youngʹs modulus because it contains a metastable  phase. Strengthening due to grain refinement tends to provide high mechanical strength, while keeping the Youngʹs modulus low, because it maintains the original  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  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.