Superlattice dislocations in Ti-rich polysynthetically twinned crystals of TiAl

Abstract

The behaviour of superlattice dislocations in polysynthetically twinned (PST) TiAl crystals deformed from room temperature to 850 degrees C is studied by transmission electron microscopy. The dissociation modes undergo significant changes with increasing deformation temperature. At room temperature, the [011] superlattice dislocations dissociate into superpartials including either an antiphase boundary and a superlattice intrinsic stacking fault (SISF), or superlattice extrinsic stacking faults (SESFs), forming faulted dipoles, and are sessile. The 1/2[112] superlattice dislocations dissociate into faulted dipoles and are also sessile. While the dislocation configurations in the crystals deformed at 400 degrees C and 600 degrees C are identical with those in the crystals deformed at room temperature, one important variation is that the density of faulted dipoles decreases with increasing deformation temperature. In-situ heating observations show that self-annihilation of the SESF-bounding hairpin superpartial occurs within a temperature range that compares with that of the brittle-ductile transition of PST TiAl crystals. At 850 degrees C, most [011] superlattice dislocations dissociate to form Kear-Wilsdorf locks. The 1/2[112] superlattice dislocations are edge in character, dissociated into superpartials bordering a SISF and are glissile. The experimental observations suggest that the behaviour of 1/2[112] superlattice dislocations is an important factor controlling the mechanism of brittle-ductile transition of PST TiAl crystals.