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 omega during aging treatments of Ti-25Nb-10Ta-1Zr-0.2Fe alloy were investigated. The results showed that in addition to martensite alpha, a small amount of alpha and athermal omega was observed in the beta matrix after solution treatment. The decomposition of martensite alpha and the transformation from athermal omega (omega(ath)) to isothermal omega (omega(iso)) occurred at the early stage of aging. omega(iso) firstly competed to grow with alpha phase, and then dissolved and transformed into a phase. The growth and dissolution of omega(iso), was accelerated with increasing aging temperature. Finally, the alpha + beta 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 743K, while exhibited weaker age-hardening response at lower temperature of 623 K and higher temperature of 773 K. The uniform distribution of dense omega(iso) and alpha precipitates in the beta matrix with moderate size resulted in the peak micro-hardness values. The grain growth of omega(iso) 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 623-743K. The activation energy for omega(iso) grain growth, Q(g) was calculated to be 119.7 kJ/mol. (C) 2011 Elsevier B.V. All rights reserved.