Phase transformation and decomposition mechanisms of the β0(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 degrees C. However, the beta(0) and omega-related phases are usually induced by Nb-segregation in cast alloys. It is important to reveal the transformation mechanisms of the beta(0) and omega-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 omega and beta(0) phases and the decomposition process of the beta(0) phase in the as-cast Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y (at.%) alloy are studied. The omega(0) particles grow up by richening Nb and rejecting W to the surrounding beta(0) matrix. The beta(0) and omega(0) phase are separated after annealing at 900 degrees C, whereas the omega(0) phase dissolves into the beta(0) matrix after annealing at 950 degrees C. The beta(0) and omega(0) phases undergo beta(0) -> gamma and omega(0) (B8(2) structure) -> D8(8)-omega (D8(8) structure) -> gamma transformation respectively during annealing, and the beta(0) -> gamma transformation can be achieved in several ways. The omega(0) particles with a high Nb content act as pinning points at the beta(0)/gamma boundaries during the beta(0) -> gamma transformation. The corresponding mechanisms of the transformations mentioned above are discussed. (C) 2014 Elsevier B.V. All rights reserved.