Microstructural instability in a crept fully lamellar TiAl alloy

A fully-lamellar TiAl alloy creep deformed at 760 °C with a stress of 138 MPa and a strain of approximately 1.5% was employed in the examination of microstructural instability. Fragmentation and spheroidization of α₂ lamellae and formation of recrystallized γ grains (γ_G) inside the lamellar colony were observed. It was proposed that the fragmentation and spheroidization of α₂ lamellae during creep could be caused by either a subgrain boundary groove mechanism or an intersection of deformation twins with α₂ phase lamellae. Possible mechanisms were proposed. The formation of coarsened γ_G grains was frequently observed. The γ_G grains had crystallographic relationships with the lamellar structure as 〈1̄10〉_γ(G)// 〈1̄10γ〉//[112̄0]_α(2) and {111}γ_G⊥{111}γ//(0001) _α(2). Simple thermodynamical analysis was done on the formation of γ_G grains. Deformation and dynamically recrystallized structures in the γ_G grains were also observed.