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

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.