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 βophase(GB-βo).TEM results revealed that after creep testing the α2→βophase transformation was observed in the FL microstructure. The formation βophase is associated with the accumulation of Mo element, which is confirmed by the energy-dispersive X-ray spectroscopy(EDS). Moreover, the formation of βoprecipitation in α2lamellae 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 ωophase was observed in the GB-βophase, accompanying with dislocations and sub-boundaries formation. In sum, the excellent creep property of the β-solidifying γ-Ti Al alloy is attributed to the fine FL structure with a small amount of GB-βophase and the formation of βoprecipitation in(α2/γ) lamellae.