Unstable dislocations in face-centred-cubic metals

Abstract

Elastic instabilities in dislocations containing a range of characters over which their line tension is negative, and the resulting zig-zag shapes, are described with a model that unifies the Wulff method and the self-stress method of Brown (1967). All the dislocation locks, in their undissociated form, are found to be elastically unstable in Cu, Cu-15at.% Al and Ag. In agreement with the theory, experimental observations show that in Cu the Bl lock forms a zig-zag in which the dislocation alternates between the undissociated Lomer form on {001} and the Lomer-Cottrell form dissociated on two {111} planes. The two forms differ in energy by 1·3 + 0·7%. In a dissociated 60° glide dislocation the unstable edge partial forms a zig-zag which may have any small wavelength up to an upper limit depending on the particular material. The amplitude of the zig-zag is too small to be observable except when the stacking fault energy is low. In Cu-15 at.% Al the parameters of the zig-zag, as measured by weak-beam electron microscopy, are in good agreement with the theory.