Mobility of dissociated mixed dislocations under an Escaig stress
N Burbery and R Das and G Ferguson, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, 25, 045001 (2017).
In FCC metals, the structure of a mixed dislocation core consists of two Shockley Partials which have different screw and edge character. The interactions between the partial dislocations can influence the stacking fault width (SFW). The SFW can also be manipulated by controlling the non-glide component of the total shear stress within the glide plane, commonly referred to as the 'Escaig stress', or tau(e). Molecular dynamics simulations were used to reproduce the dynamic behaviour of the atomistic core and stacking fault of a moving mixed 30 degrees dislocation in copper and with several magnitudes of te stress. Results showed that the te must be relatively large to cause a significant effect on the SFW and that once the SFW is changed it also has a corresponding effect on the drag coefficient for a dislocation moving at steady-state. The reduction of the SFW, to the extent that the partial dislocations come within close proximity (i.e., partially merge into an imperfect full dislocation), changed the linear curve-fit of the stress- velocity curve and could be associated with a 'quasi-Peierls barrier' effect. The SFW was also shown to change under a pure glide stress without the addition of a te stress when the velocity approached the supersonic limit, and caused an increase of the SFW in one direction and a reduction of the SFW in the other direction. This result demonstrates a unique characteristic of mixed dislocations which cannot be observed in the traditionally representative core structures of pure screw and pure edge dislocations because of their perfect symmetry. Surprisingly, the effects of the glide stress on the SFW were similar in magnitude to the effects of te, and also were found to have a corresponding effect on the drag coefficient. The unique characteristics observed are summarised in terms of a unified equation for the force equilibrium at steady- state, which considers the drag coefficient of the two partials as independent functions of the velocities. This relationship is rationalised in terms of a drag force imbalance between the two partials of a mixed 30 degrees dislocation, caused by a slightly higher drag force in the 'mildly non planar' 0 degrees partial than in the 60 degrees partial.
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