Rapid decomposition of lamellar microstructure and enhanced hot workability of an as-cast triphase Ti–45Al–6Nb–1Mo alloy via one-step alpha-extrusion & annealing

Highlights: • Alpha-extrusion & annealing creates a fine-grained α₂ + γ + β_o triphase structure. • Alpha-extrusion gives alternately-distributed lamellar structures and β_o + γ grains. • L(α/γ).→ γ + β decomposition dominates the whole microstructural evolution process • α₂ + γ + β_o microstructure by alpha-extrusion & annealing has superplastic ability. • Alpha-extrusion & annealing renders a highly-desirable die-forging formability. 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 β_o + 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 T_α thoroughly broke down the residual lamellar structures and gave rise to a higher content of globularized γ and β_o grains. Finally, the resultant fine-grained and high-angle misoriented α₂ + γ + β_o 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.