Age-hardening behavior, microstructural evolution and grain growth kinetics of isothermal ω phase of Ti–Nb–Ta–Zr–Fe alloy for biomedical applications

Age-hardening behavior, microstructural evolution and the grain growth kinetics of isothermal ω during aging treatments of Ti–25Nb–10Ta–1Zr–0.2Fe alloy were investigated. The results showed that in addition to martensite α″, a small amount of α and athermal ω was observed in the β matrix after solution treatment. The decomposition of martensite α″ and the transformation from athermal ω (ωath) to isothermal ω (ωiso) occurred at the early stage of aging. ωiso firstly competed to grow with α phase, and then dissolved and transformed into α phase. The growth and dissolution of ωiso was accelerated with increasing aging temperature. Finally, the α + β stable microstructure was obtained after aging for 280, 200, 24 and 2 h at 623, 673, 743 and 773 K, respectively. The alloy showed stronger age-hardening response at intermediate temperatures of 673 and 743 K, while exhibited weaker age-hardening response at lower temperature of 623 K and higher temperature of 773 K. The uniform distribution of dense ωiso and α precipitates in the β matrix with moderate size resulted in the peak micro-hardness values. The grain growth of ωiso obeys an asymptotic law, and the grain-growth exponent, n, was computed to be in the range of 0.23–0.26 at temperatures in the range of 623–743 K. The activation energy for ωiso grain growth, Qg was calculated to be 119.7 kJ/mol.