Core structure and motion of 〈110〉 screw dislocations in L1₂, alloys with unstable antiphase boundaries on {111} planes

Abstract

The cores of the individual superpartials which comprise a [10l] screw superdislocation dissociated on the (111) and (010) planes in L1₂ ordered materials have been investigated using computer simulation techniques for the case where antiphase boundaries (APBs) on (111) planes are unstable. The superdislocation dissociates in this case on the (010) plane into two 1/2[101] superpartials separated by an APB, and on the (111) plane into a 1/3[112] and a 1/3[211] superpartial separated by a superlattice intrinsic stacking fault (SISF). In both cases the corn of the superpartials are non-planar, and therefore sessile. The cores of the 1/2[71bar;01] superpartials. separated by an APB on (010) planes, are spread into the (111) and the (l11) planes. The 1/3〈112〉 superpartials separated by an SISF on the (111) plane divide into edge and screw components, one above the other: the former component is spread into the (111) plane, but the latter is again spread into both the (111) and (111) planes.

The motion of these dislocations was investigated by applying a resolved shear stress in the [10l] direction along various planes. For APB splitting on (010) planes, it was found that when the maximum resolved shear stress plane deviates by no more than 30° from the (010) plane, the superpartial moves on average on the (010) plane by alternate displacements on the (111) and (111) planes. This occurs at a critical resolved shear stress (c.I.s.s.) of about 0·05G, where G is the shear modulus. When the deviation is greater, the superpartial moves on the (111) plane at a c.r.s.s. of about 0·05G, and an APB is created. For SISF splitting, dislocations always move on the (111) plane. At a c.r.s.s. of about 0·05G the screw component of the 1/3〈112〉 superpartials moves away from the edge component on the (111) plane, leaving a stress-stabilized APB in its wake. However, the c.c.s.s. was found to depend also on the shear stress, which controls the spacing of the edge components of the superpartials on (111) planes, a larger spacing leading to a lower c.r.s.s.; this implies that the c.r.s.s. should depend on the Sense of the applied stress.