Weak-beam observation of a dissociation transition in TiAl

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Abstract
TiAl is a L10 ordered alloy which exhibits a flow-stress anomaly peaking at approximately 600°C. In order to correlate the increase in flow stress with changes in deformation microstructure, the nature and dissociation of superdislocations are compared after deformation at 20 and 600°C. It is observed that superdislocations split into a triplet involving an antiphase boundary (APB) and a superlattice intrinsic stacking fault (SISF). In the screw orientation this configuration undergoes a glissile–sessile transition as the temperature of the deformation test is raised, since the APB plane changes from the octahedral plane of the SISF after deformation at room temperature to a cubic plane at 600°C. The abundant evidence for cross-slip onto octahedral planes at room temperature only supports the idea that the high-temperature configuration of the screw superdislocations controls the flow-stress peak in TiAl. The reported change in dissociation configuration is the equivalent, for the L10 structure, of the Kear–Wilsdorf locking mechanism currently thought to explain the flow-stress anomaly of some L12 alloys.