Surface effects on dislocation dissociation in thin foils

Abstract

The equilibrium width of a dislocation dissociated into two partials lying parallel to the surfaces of a thin foil has been computed for free and rigid surface boundary conditions and the case where only the screw components of the partials interact. These conditions approximate the situation in transmission electron microscopy specimens with and without a thin surface layer having a shear modulus much greater than the matrix. The results indicate that the equilibrium spacing between partials decreases to zero on approaching a free surface, but increases to a value equal to twice the equilibrium spacing at the foil centre as a rigid surface is approached. The equilibrium spacing as a function of depth is symmetrical about the foil centre. Surface effects of both kinds can be neglected in calculating the equilibrium spacing at the foil centre providing , where p is the foil thickness, b ^A and b ^B are the Burgers vectors of the partials, γ is the stacking-fault energy and μ is the shear modulus of the matrix. This condition is generally obtained in practice.