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

While Ti-Nb binary alloys have been extensively studied over the years, there are discrepancies in the literature relating to the beta-alpha '' martensitic transformation that indicate an incomplete understanding. In particular, there are inconsistencies regarding the phase constitution of as-quenched material and the ability for alpha '' phase to be thermally-induced. To resolve these issues, the beta-alpha '' 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 (greater than or similar to 500 degrees C/s) result in the formation of alpha '' phase, and these materials exhibit a reversible thermally-induced beta-alpha '' transformation at low temperatures. In contrast, slightly slower water quench procedures lead to a beta+omega microstructure, and these samples do not exhibit a thermally-induced beta-alpha '' transformation, even when cooled to 168 degrees C. Despite this, room temperature mechanical testing results indicate that alpha '' 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 alpha '' and athermal omega 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 beta-titanium alloys.