Microstructural instability in a crept fully lamellar TiAl alloy

Abstract

A fully-lamellar TiAl alloy creep deformed at 760°C with a stress of 138 MPa and a strain of ∼1.5% was employed in the examination of microstructural instability. Fragmentation and spheroidization of α 2 lamellae and formation of recrystallized γ grains ( γ G ) inside the lamellar colony were observed. It was proposed that the fragmentation and spheroidization of α 2 lamellae during creep could be caused by either a subgrain boundary groove mechanism or an intersection of deformation twins with α 2 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 < 1 ̄ 10> γ G // < 1 ̄ 10 γ>// [11 2 ̄ 0] 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.