Phase transformation and decomposition mechanisms of the βo(ω) phase in cast high Nb containing TiAl alloy

Abstract

High Nb containing TiAl (Nb–TiAl) alloys are thought to be used at higher temperatures than the conventional TiAl alloys, typically up to 900°C. However, the βo and ω-related phases are usually induced by Nb-segregation in cast alloys. It is important to reveal the transformation mechanisms of the βo and ω-related phases at elevated temperatures for better design of the alloy and optimizing the heat treatment process. In this study, the interconversion mechanism between the ordered ω and βo phases and the decomposition process of the βo phase in the as-cast Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at.%) alloy are studied. The ωo particles grow up by richening Nb and rejecting W to the surrounding βo matrix. The βo and ωo phase are separated after annealing at 900°C, whereas the ωo phase dissolves into the βo matrix after annealing at 950°C. The βo and ωo phases undergo βo→γ and ωo (B82 structure)→D88-ω (D88 structure)→γ transformation respectively during annealing, and the βo→γ transformation can be achieved in several ways. The ωo particles with a high Nb content act as pinning points at the βo/γ boundaries during the βo→γ transformation. The corresponding mechanisms of the transformations mentioned above are discussed.