Modelling the flow stress anomaly in γ-TiAl I. Experimental observations of dislocation mechanisms

Abstract

Mechanical tests on a polycrystalline γ Ti₄₇A1₅₁,Mn₂ alloy between ‐ 150 and 1000°C and subsequent microstructure analysis clearly distinguish between three temperature domains which correspond to different deformation mechanisms. At low temperatures, the motion of superdislocations dragging faulted dipoles is rate controlling. At intermediate temperatures where a stress anomaly is observed, screw simple dislocations are observed with cusps, the density of which increases with increasing temperature. A description of the simple dislocation motion based on these observations is developed. The glide of a screw simple dislocation in two planes through a Peierls mechanism is believed to be the intrinsic source for the formation of pinning points. It is proposed that these pinning points can be erased by lateral motion of superkinks: an unzipping process. A model for this pinning-unzipping mechanism will be fully developed in part 11. At high temperatures, the climb of simple dislocations appears to control the deformation.