Phase transformation and dynamic recrystallization behavior of a β-solidifying γ-TiAl alloy and its wrought microstructure control

Abstract

This study deals with phase transformation and dynamic recrystallization (DRX) behavior of a β-solidifying alloy Ti–45Al–4Nb–2Mo–B (at.%). Its phase transformation and DRX modes strongly depended on temperature/specific phase field. At low forging temperature within α/α2+γ+B2/β phase field, decomposition of lamellar structures via L(α/γ)→γ+β along with DRX of γ and B2/β grains prevailed around prior α/γ colony boundaries. Conversely, when forging close to α-transus (Tα), closely following quick transition of γ+B2/β→α, DRX was totally localized within α grains. All these processes could be promoted greatly by lowering strain rate. Based on this study, large-scale forging and its microstructural design were realized successfully. Besides anisotropic flow behavior of prior lamellar structures, insufficient decomposition time was also considered as the reason of remnant lamellar structures. Finally, post-forging annealing and an alternative superplastic straining within α2+B2/β+γ phase field were proposed to entirely eliminate the remnant lamellar structures, in order to produce full fine-grained B2/β+γ microstructure.