Computer simulation of the motion of screw dislocations in Ni 3 Al

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Abstract
The motion of screw superdislocations in Ll2 alloys of the Ni3Al type has been simulated, on the assumptions that the screws glide easily on {111} planes, but that motion on {010} planes occurs in thermally activated steps of b/2, where b is the Burgers vector of the superpartials. The applied stress, crystal orientation, and geometrical constraints imposed by the lattice are found to have important effects on the motion of the dislocations. Changes in dislocation configuration during motion are predicted which would give rise to changes in projected image widths similar to those observed by Molénat and Caillard in in situ deformation transmission electron microscopy experiments at room temperature. For certain crystal orientations cross-slip from one octahedral plane to another can occur at relatively low stresses via intermediate configurations in which the antiphase domain boundary (APB) between the superpartials lies partly on {111} and the {010} cross-slip plane. Below a certain critical stress the final configuration is a Kear-Wilsdorf (or similar) lock. Above that stress (for certain crystal parameters) the screws move in jumps equal to the APB ribbon widths on {111} (‘jerky flow’), and at an even higher stress the screws assume a glissile configuration on {111}.