Abstract: The β solidified γ-TiAl alloy holds important application value in the aerospace industry, while its complex phase compositions and geometric structures pose challenges to its microstructure control during the thermal-mechanical process. The microstructure evolution of Ti-43Al-4Nb-1Mo-0.2B alloy at 1200 °C/0.01 s^-1 was investigated to clarify the coupling role of dynamic recrystallization (DRX) and phase transformation. The results revealed that the rate of DRX in α₂+γ lamellar colonies was comparatively slower than that in β_o+γ mixed structure, instead being accompanied by intense lamellar kinking and rotation. The initiation and development rates of DRX in α₂, β_o, and γ phases decreased sequentially. The asynchronous DRX of the various geometric structures and phase compositions resulted in the uneven deformed microstructure, and the dynamic softening induced by lamellar kinking and rotation was replaced by strengthened DRX as strain increased. Additionally, the blocky α₂ phase and the terminals of α₂ lamellae were the preferential DRX sites owing to the abundant activated slip systems. The α₂→β_o transformation within lamellar colonies facilitated DRX and fragment of α₂ lamellae, while the α₂→γ transformation promoted the decomposition of α₂ lamellae and DRX of γ lamellae. Moreover, the varied β_o+γ mixed structures underwent complicated evolution: (1) The γ→β_o transformation occurred at boundaries of lamellar colonies, followed by simultaneous DRX of γ lamellar terminals and neighboring β_o phase; (2) DRX occurred earlier within the band-like β_o phase, with the delayed DRX in enclosed γ phase; (3) DRX within the β_o synapses and neighboring γ phase was accelerated owing to generation of elastic stress field; (4) Dispersed β_o particles triggered particle stimulated nucleation (PSN) of γ phase. Eventually, atomic diffusion along crystal defects in β_o and γ phases caused fracture of band-like β_o phase and formation of massive β_o particles, impeding grain boundary migration and hindering DRXed grain growth of γ phase.

Key words: TiAl alloy, Hot deformation, Dynamic recrystallization, Phase transformation