Abstract
This study proposed a new alpha-extrusion & annealing route to rapidly break down the coarse as-cast lamellar microstructure of a Nb-rich β-solidifying γ-TiAl alloy. It was found that one-step alpha-extrusion could induce widespread decomposition of lamellar structures via a precipitation reaction L(α/γ) → γ + β internally and along primary lamellar-colony boundaries, leading to formation of a streamlined microstructure consisting of alternately-arranged elongated lamellar structures and bands of βo + γ grains. Large-scale dynamic recrystallization of γ grains was motivated during alpha-extrusion and resulted in bands of fine γ-grains between elongated lamellar structures. A post-extrusion annealing at temperature far below Tα thoroughly broke down the residual lamellar structures and gave rise to a higher content of globularized γ and βo grains. Finally, the resultant fine-grained and high-angle misoriented α2 + γ + βo triphase microstructure rendered an excellent superplastic deformability at 950 °C and especially a highly-promising one-step hot-die forming ability. The related microstructural evolution, phase transformation and recrystallization behaviors during one-step alpha-extrusion & annealing were investigated systematically.
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