Phase transformation and age hardening behavior of new Ti–9.2Mo–2Fe alloy

The effect of aging treatment on the microstructures and hardening behavior of a metastable beta Ti–9.2Mo–2Fe alloy, which is new designed on the basis of Bo–Md molecular orbital method (Bo, the bond order; Md, the metal d-orbital energy level), has been investigated using hardness measurement, X-ray diffraction, optical microscopy and TEM characterization. The results reveal that, the athermal ω phase with the size of 2–5 nm is found in Ti–9.2Mo–2Fe alloy when solution-treated at 860 °C followed by water quench. The isothermal ω phase is also found when Ti–9.2Mo–2Fe alloy aged at temperatures from 300 °C to 500 °C, although the ω phase has a limited time of stability at 400 °C and 500 °C. The athermal ω phase does not lead to attractive hardening due to its too small size and volume fraction, so the hardness of the solution-treated alloy is about 370 Hv. However, the isothermal ω phase with the size of 5–10 nm shows a moderate hardening (470 Hv), while the ω phase with the size of 50–100 nm contributes the best hardening (∼500 Hv). As well, the hardening due to isothermal ω phase precipitation hardly exhibits over-aging response as long as ω phase exists.