Dislocations in interfaces in the hcp metals - II. Mechanisms of defect mobility under stress

Atomic scale computer simulation of a model of the h.c.p, metal alpha-titanium is used to investigate the mobility of interfacial defects in response to applied shear stress. Interfacial defects in (10 (1) over bar 2) twins and a 90 degrees incommensurate tilt boundary are investigated. Defects with Burgers vector, b, parallel to their host interface can move conservatively in principle, but were found to be mobile only if their step height, h, is small. In such cases, particularly when the defects exhibit wide cores, the atomic shuffles involved in transferring atoms from sites of one crystal to the other are simple. Conversely, defects which exhibit large h generally have narrow cores and require complex shuffles for motion. Applied stress tends to cause core reconstruction and emission of partial dislocations trailing stacking faults from these defects. Defects with b inclined to the interface can move conservatively in response to applied shear stress through a climb-compensated mechanism in some circumstances. This mechanism can lead to limited mobility of defects in both types of interface studied, and involves the generation of additional glissile interfacial defects due to the stress concentrating effect of the riser of the initial defects. Activation of this mechanism is only feasible when the elementary mechanism of motion involves a small number of atoms shuffling from one crystal to the other. Unlike the case for defects with b parallel to the interface, this number is not simply related to h and can be effectively small even when h is relatively Elsevier Science Ltd. All rights reserved.