Mechanisms and effect of microstructure on creep of TiAl-based alloys

Transmission Election microscopy studies on crept samples of an equiaxed Ti-48Al alloy deformed to strains near the minimum strain rate show a microstructure dominated by unit 1/2[110] type dislocations, These dislocations are pinned by superjogs. The jogged-screw model is adopted, where the rate controlling step is assumed to be the non-conservative dragging of a periodic array of jogs along the screw dislocations. The presence of tall jogs, and the existence of a stress-dependent upper bound on the jog height, have been incorporated into the model. These modifications lead to excellent agreement with experimental data for near-gamma Ti-48Al. In contrast, lamellar structures show a highly inhomogeneous deformation behavior with dislocation activity in lamellae above a critical thickness, and negligible activity below this thickness. This limit is related to the minimum stress required to cause channeling of dislocations. The observation of jogged segments in the thicker lamellae suggests that a modification of the jogged-screw velocity law could be used by incorporating an effective stress approach. These modifications predict lower creep rates and higher stress exponents for lamellar structures, as observed experimentally.