Microstructural evolution and embrittlement of a β-solidifying γ-TiAl alloy during exposure at 700 °C

The microstructural evolution and the room-temperature tensile properties of fully lamellar Ti-43.5Al–4Nb–1Mo-0.5B alloy exposed at 700 °C for 500 h in air were investigated. In bulk microstructure, the precipitation and growth of ellipsoidal ω0 particles were found in equiaxed β0 grains, and the parallel decomposition of coarse α2 lamellae into fine α2 + γ lamellar packets was observed. The α2 lamellae near the surface decomposed first into fine α2 + γ lamellar packets and then blocky γ grains after the full consumption of the α2 phase. A mixed oxide scale with obvious stratification mainly composed of TiO2 and Al2O3 formed on the surface of the specimens. The outward diffusion of Ti atoms on the specimen surface led to the formation of an Al-rich and Ti-lean blocky γ zone between the matrix and the oxide scale. The degradation of room-temperature mechanical properties of the alloy after exposure was primarily caused by the evolution of subsurface microstructure, and the evolution of bulk microstructure had little effect on the mechanical properties at this stage. Under axial stress, microcracks nucleated from the tip of a V-shaped “notch microstructure” at the interface between the blocky γ zone and the matrix, causing premature failure. Removal of the surface layer of the thermally exposed alloy led to room temperature mechanical properties essentially similar to those before thermal exposure.