Effects of Heat Treatment on Mechanical Properties and Corrosion Fatigue Strength in Physiological Saline Solution of New Titanium Alloys for Medical Implants

Abstract

Ti-15mass%Zr-4%Nb-4%Ta-0.2%Pd and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd alloys not containing V and At for surgical implantation were cast by plasma electron beam melting. The effect of heat treatment on room temperature strength and corrosion fatigue strength were investigated by aging treatment after solution treatment. The alloys were forged in their α and α + β regions, and then solution-treated at 953∼1173 K for 3.6 ks. 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∼20 vol% of primary α phase. After solution treatment under the optimized conditions, aging treatment was carried out at 623∼873 K for 600 s ∼ 108 ks. 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 these solution treated and aged alloys were about 30% higher than the annealed alloys. It is revealed by transmission electron microscopic observation that, fine α phase precipitates in α' martensitic matrix are responsible for the age hardening. The corrosion fatigue test in a physiological saline solution was carried out under the condition of a tension to tension mode with a sine wave at a stress ratio of 0.1, at a frequency of 10 Hz in Eagle's MEM+fetal bovine serum solution using an environment cell with 90%N 2 +5% CO 2 +5%O 2 gas bubbling at 310 K. The fatigue strength at 10 7 cycles in solution-treated and aged Ti-15%Zr-4%Nb-4%Ta-0.2%Pd and Ti-15%Sn-4%Nb-2% Ta-0.2%Pd alloys were about 650 MPa and 800 MPa respectively, which were about 20∼30% higher than those of the annealed alloys.