Effect of heat treatment on microstructure and mechanical properties of new titanium alloys for surgical implantation

Abstract

Ti-15 mass%Zr-4%Nb-4%Ta-0.2%Pd and Ti-15%Sn-4%Nb-2%Ta-0.2% Pd alloys not containing harmful V and Al for surgical implantation in response to recent concerns about the long term safety of Ti-6%Al-4% V ELI (Extra low interstitial, ASTM specified for surgical implantation) were cast by plasma electron beam melting. The effect of heat treating on room temperature strength was investigated by aging treatment after solution treatment. The alloys were forged in their α and α+β regions, and then solution-treated at 953 to approximately 1173 K for 3.6 ks. Vickers hardness in the range below β transition temperature increases with increasing solution treating temperature and change of Vickers hardness in the range over its temperature is small. Optimized solution treatment conditions are 1038 K-3.6 ks for Ti-Zr alloy and 1098 K-3.6 ks for Ti-Sn alloy containing 15 to approximately 20 vol% primary α phase. After solution treatment under the optimized condition aging treatment was done at 623 to approximately 873 K for 600 s to approximately 36 ks. Vickers hardness in aging below 723 K increases with increasing aging temperature and aging time. Optimized aging conditions for Ti-Zr and Ti-Sn alloys were at 673 K for 36 ks and at 723 K for 36 ks, respectively. The room temperature strengths of those alloys are superior to those of Ti-6% Al-4% V ELI for medical implants. It is revealed by transmission electron macroscopic observation that fine α phase precipitates in the α′ martensitic matrix are responsible for the age hardening.