MOTION UNDER STRESS OF A SCREW SUPERDISLOCATION IN THE L1(2) STRUCTURE

The transformation of a dissociated screw superdislocation under an ex ternal stress is analysed. Analytical expressions of the forces exerte d on to superpartials are derived which enable one to check the role o f every parameter directly. A graphical method to predict the evolutio n of a given configuration is presented that can be used to simulate t he transformation paths under stress. The evolution of a twofold (or i ncomplete Kear-Wilsdorf (KW)) configuration is controlled by a stress- dependent parameter zeta, just as in the unstressed case presented by Saada and Veyssiere in 1992, and three distinct domains can be defined : zeta < alpha (where alpha is an elastic constant), the incomplete KW configuration tends towards the KW lock; zeta > 3 1/2, evolution is t owards the planar configuration in the octahedral plane; alpha < zeta < 3 1/2, the situation is bistable and further evolution towards a pla nar configuration located either in the octahedral plane or in the cub e plane, is controlled by the antiphase-boundary (APB) extent in the c ube or in the octahedral plane relative to stress-dependent critical v alues. The implications of this analytical study are studied in the ca se of several processes known to occur during deformation in L1(2) all oys. The contributions of the amplitude and of the orientation of the external load are examined. In particular the conditions for single an d repeated APB jumps are discussed and it is shown that repeated APB j umps 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 c orrespond satisfactorily to the flow stress peak.