Motion under stress of a screw superdislocation in the L1₂ structure

Abstract

The transformation of a dissociated screw superdislocation under an external stress is analysed. Analytical expressions of the forces exerted on to superpartials are derived which enable one to check the role of every parameter directly. A graphical method to predict the evolution of a given configuration is presented that can be used to simulate the transformation paths under stress.

The evolution of a twofold (or incomplete Kear-Wilsdorf (KW)) configuration is controlled by a stress-dependent parameter ζ, just as in the unstressed case presented by Saada and Veyssière in 1992, and three distinct domains can be defined: ζ, < α (where α is an elastic constant), the incomplete KW configuration tends towards the KW lock; ζ > 3^1/2, evolution is towards the planar configuration in the octahedral plane; α < ζ < 3^½, the situation is bistable and further evolution towards a planar configuration located either in the octahedral plane or in the cube plane, is controlled by the antiphase-boundary (APB) extent in the cube or in the octahedral plane relative to stress-dependent critical values.

The implications of this analytical study are studied in the case of several processes known to occur during deformation in L1₂ alloys. The contributions of the amplitude and of the orientation of the external load are examined. In particular the conditions for single and repeated APB jumps are discussed and it is shown that repeated APB jumps should occur upon modest external stresses. The stress to destroy a KW lock, the microscopic saturation stress, is calculated and it is shown that, although the magnitude is correct, this stress does not correspond satisfactorily to the flow stress peak.