The behaviour of superlattice dislocations in polysynthetically twinned crystals, which were deformed between room temperature and 850C, was studied by means of transmission electron microscopy. The dissociation modes underwent significant changes with increasing deformation temperature. At room temperature, the <011] superlattice dislocations dissociated into super-partials which included either an antiphase boundary and a superlattice intrinsic stacking fault, or superlattice extrinsic stacking faults which formed faulted dipoles and were sessile. The ½<112] superlattice dislocations dissociated into faulted dipoles and were also sessile. The dislocation configurations in crystals which were deformed at 400 and 600C were identical to those in crystals which were deformed at room temperature. An important variation was that the density of faulted dipoles decreased with increasing deformation temperature. The use of in situ heating observations showed that self-annihilation of the superlattice extrinsic stacking fault bounding a hair-pin super-partial occurred within a temperature range that was similar to that of the brittle-ductile transition of polysynthetically twinned crystals. At 850C, most <011] superlattice dislocations dissociated to form Kear-Wilsdorf locks. The ½<112] superlattice dislocations were edge-like in character, dissociated into super-partials which bordered a self-interstitial stacking fault and were glissile. The experimental observations suggested that the behaviour of ½<112] superlattice dislocations was an important factor which controlled the mechanism of brittle-ductile transitions in polysynthetically twinned crystals.

Superlattice Dislocations in Ti-Rich Polysynthetically Twinned Crystals of TiAl. Y.Liu, D.Lin, Z.Liu, Z.Wang: Philosophical Magazine A, 1999, 79[12], 2965-78