Effect of Fe and Zr additions on ω phase formation in β-type Ti–Mo alloys

The effect of 1% Fe and/or 5% Zr (mass%) additions on ω phase formation was investigated for the Ti–15Mo alloy by means of X-ray diffraction analysis and hardness testing. Upon water quenching following solution treatment in the β phase region, the athermal ω phase formation could not be observed in all the alloys, regardless of Fe and Zr additions. The lattice parameter of the β phase decreases with Fe addition, while it increases with Zr addition. Solid solution strengthening by Fe and Zr is not recognized for the β phase. The isothermal ω phase formed after aging at 723 K and 773 K for 3.6 ks, which results in a decrease in the lattice parameter of the β phase and an increase in the hardness. The isothermal ω phase formation is suppressed with Fe and/or Zr additions. This is interpreted as the consequence of the increase in the average value of the bond order (Bo) for the Ti–15Mo–5Zr and Ti–15Mo–5Zr–1Fe alloys, and of the decrease in the average value of the metal d-orbital energy level (Md) for the Ti–15Mo–1Fe alloy. In addition, the degree of the suppression of isothermal ω phase can be predicted by the average values of Bo and Md.