The microstructure evolution and phase transformation behavior of a β-solidifying γ-TiAl alloy during creep

The microstructural evolution and creep behavior of the Ti-43.5Al–4Nb–1Mo-0.1B alloy have been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The excellent creep property was obtained with a fully lamellar (FL) microstructure containing the least grain boundary β_o phase (GB-β_o). TEM results revealed that after creep testing the α₂ →β_o phase transformation was observed in the FL microstructure. The formation β_o phase is associated with the accumulation of Mo element, which is confirmed by the energy-dispersive X-ray spectroscopy (EDS). Moreover, the formation of β_o precipitation in α₂ lamellae effectively decreased the generation of dislocations in (α₂/γ) lamellae, thereby improving the creep resistance. For the near gamma (NG) microstructure of the as-forged sample, a large number of dislocations and dislocation tangles were observed in the globular γ phase (γ-glob), which are considered to be the dominant creep mechanism. Moreover, the ellipsoidal ω_o phase was observed in the GB-β_o phase, accompanying with dislocations and sub-boundaries formation. In sum, the excellent creep property of the β-solidifying γ-TiAl alloy is attributed to the fine FL structure with a small amount of GB-β_o phase and the formation of β_o precipitation in (α₂/γ) lamellae.