Microstructural instability in surface layer of a high Nb-TiAl alloy processed by shot peening following high temperature exposure

• A depth-dependent gradient microstructure was produced near the surface.
• CG layer is composed of incompletely recrystallized γ grains.
• α2 lamellae can undergo dissolution, precipitation and growth.
• Precipitated α2 particles have the same orientation with primary α2 lamellae.

Microstructural instability induced by shot peening was investigated in a Ti-45Al-8.5Nb-(W, B, Y) alloy following high temperature exposure. After shot peening and thermal exposure at 1000 °C for 300 h, fine grains are formed in the outermost (FG layer). Underneath, coarse grains (CG layer) are formed. The FG layer is composed of completely recrystallized γ grains. The CG layer is composed of incompletely recrystallized γ grains, where the critical strain for recrystallization is not reached. During long term thermal exposure, α2 lamellae can undergo dissolution, precipitation and growth. After thermal exposure for 300 h, large α2 grains precipitated at the γ/γ interface or inside the γ grains in both FG and CG layers. The precipitated α2 particles almost have the same orientation with primary α2 lamellae, indicating that nearly no recrystallization phenomena occur for α2 phase. So the γ lamella is easier to recrystallize than that of α2 lamella at the same temperature and residual strain.