The effect of quench rate on the β-α″ martensitic transformation in Ti Nb alloys

Abstract

While Ti-Nb binary alloys have been extensively studied over the years, there are discrepancies in the literature relating to the β-α″ martensitic transformation that indicate an incomplete understanding. In particular, there are inconsistencies regarding the phase constitution of as-quenched material and the ability for α″ phase to be thermally-induced. To resolve these issues, the β-α″ phase transformation is studied in a Ti-24Nb (at.%) alloy subjected to two different water quench conditions. It is demonstrated that only the fastest quench rates (≳500˚C/s) result in the formation of α″ phase, and these materials exhibit a reversible thermally-induced β-α″ transformation at low temperatures. In contrast, slightly slower water quench procedures lead to a β+ω microstructure, and these samples do not exhibit a thermally-induced β-α″ transformation, even when cooled to -168˚C. Despite this, room temperature mechanical testing results indicate that α″ can be induced by stresses as low as 200 MPa. To explain these results, it is proposed that quenching stresses play a role in the competition between ″ and athermal ω phases at rapid quench rates. The present results are then reconciled with the body of literature and a broader understanding of phase stability in metastable β-titanium alloys.