Turning machining induced microstructural stability of a high Nb-containing TiAl alloy during high temperature exposure

Turning machining induced microstructural instability was investigated in a fully lamellar Ti–45Al–8.5Nb–(W, B, Y) alloy during high temperature exposure. After turning machining followed by thermal exposure at 900 or 1000 °C for 100, 300 and 500 h, a depth-dependent gradient microstructure with random orientations was produced in the region close to the machining surface. Two typical layers, a fine-grained (FG) layer with equiaxed grains and a coarse-grained (CG) layer with elongated grains, are formed in this region in transversal direction. The thickness of the two layers is up to 120 μm after thermal exposure at 1000 °C for 500 h, which is less than the depth of the hardened region (200 μm) after turning machining. Most of the new grains in FG and CG layers are constituted of γ single phase, while short α₂ segments and few B2 particles are precipitated at the γ/γ interface or inside the γ grains. Recrystallization and phase boundary bulging are found to be the major mechanisms responsible for lamellar degradation in FG layer and CG layer, respectively. The residual deformation energy stored is considered to be the main driving force of this process.