Age-hardening behaviour of two metastable beta-titanium alloys

Two novel β-titanium alloys, of compositions Ti–10V–2Ni and Ti–10V–6Cu (wt.%) and with distinct potential for athermal ω-phase formation, have been selected and the sensitivity of the age-hardening responses to the cooling rate post solution treatment investigated. Upon rapid water quenching, the solution-treated Ti–10V–2Ni alloy contained a significantly higher volume fraction of ω-phase than the Ti–10V–6Cu alloy. It was observed that the Ti–10V–6Cu alloy is remarkably insensitive to cooling rates in the range of water quenching to air cooling, in terms of both microstructural evolution and age-hardening behaviour, when aged isothermally at 350 °C and 500 °C. The microstructure is thermally stable and the hardness remains approximately constant for extended aging (48 h) at both temperatures, implying that it may have potential as a deep-hardenable alloy. In comparison, the aging response of the Ti–10V–2Ni alloy was marginally influenced by cooling rate when aged at 500 °C, the effect being ascribed to autocatalytic formation of α-phase in the course of slow air cooling. The results are discussed with respect to the β-phase stability and the time-temperature-transformation (TTT) characteristics of metastable beta-titanium alloys.