Influence of oxygen and plastic deformation on the microstructure and the hardness of a Ti–Nb–Ta–Zr–O Gum Metal

Abstract

In this paper, the Gum Metal Ti–29Nb–13Ta-4.6Zr-xO with two different oxygen contents (2200 and 4600 ppm) was prepared by high-pressure torsion (HPT) and their microstructural evolution, thermal stability, physical and mechanical properties were investigated for specimens prepared under different processing conditions. An increase in the β-phase stability at the expense of αʺ, as well as in the hardness was observed with the combined effect of oxygen addition and HPT processing. The increase in hardness was attributed to the microstructural refinement, to the creation of a high density of defects, and to the stress-induced ω-phase formation, which were all obtained due to the severe plastic deformation. Besides that, oxygen solid solution hardening provides a great increase in hardness. Vickers microhardness of approximately 400 HV was obtained for samples with higher oxygen content and deformed by HPT. The elastic modulus did not change significantly for the different oxygen levels, with values of 53–66 GPa. The typical Gum Metal properties were attained in the present paper, and they are directly related to its deformation mechanisms such as slip, stress-induced αʺ-martensite as well as reverse martensitic transformation (αʺ → β). The unique Gum Metal properties combined with the obtained increase in hardness make these two alloys very well suited for biomedical applications such as implants.