Redefining the β-Phase Stability in Ti-Nb-Zr Alloys for Alloy Design and Microstructural Prediction
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β-Phase stability is a key consideration for the design of β-titanium (β-Ti) alloys and subsequent heat treatment and/or thermo-mechanical processing. The concept of the Mo equivalence (Mo-Eq), proposed by Molchanova (Phase diagrams of titanium alloys, 1965), has been commonly used as a general guideline to gauge the stability of a β-Ti alloy. A critical literature review has shown that all four existing Mo-Eq expressions deviate substantially from experimental observations and the well-established d-electron theory in predicting the β-phase stability of Ti-Nb-Zr alloys. The reasons are that existing Mo-Eq expressions either completely neglect or significantly overestimate the β-stabilizing effect of Zr. In this study, a new Mo-Eq expression, i.e., (Mo-Eq)Ti-Nb-Zr = 0.238Nb (wt.%) + 0.11Zr (wt.%) + 0.97, has been defined for Ti-Nb-Zr alloys in order to properly address the β-stabilizing effect of Zr. This new Mo-Eq expression showed proven consistency with both experimental observations and the d-electron theory in predicting the β-phase stability of various Ti-Nb-Zr alloys. With necessary modifications, the approach developed is expected to be also applicable to the assessment of the β-phase stability in other Zr-containing Ti alloys.
This work was supported by the Australian Research Council (ARC) through DP150104719. The authors are grateful to Rodney Boyer for drawing our attention to E. K. Molchanova’s original contribution to the concept of the Mo equivalence (see Ref. 3).
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