Extended jogs in dislocations in face-centred cubic metals

Abstract

The nature of the minimum energy configuration of jogs of various lengths in dislocations in face-centred cubic metals is studied. It is concluded that long jogs of various types are partially dissociated into intrinsic stacking-fault ribbons on close-packed planes, and that elementary jogs in dislocations in a range of orientations around that of the screw are extended partially to form linear defects which, following Seeger and Bross (unpublished), can be described as lines of one-third vacancies. Many of these jog configurations are sessile in the sense that the jogs must first be constricted before conservative movement can take place. Elementary jogs can move conservatively by an alternative mechanism in which the jog line is displaced by one atomic distance, by formation and movement of a partial vacancy. Tentative approximate estimates of the activation energies for conservative movement of elementary jogs are of the order of several tenths of an electron volt. It is shown that the activation energy for constriction can only be lowered by stress if the dislocation is advancing in a direction in which interstitials would be produced if the dislocation moved non-conservatively. For movement in the opposite direction (producing vacancies) the constriction activation energy has to be supplied entirely by thermal activation. Somewhat similar conclusions apply to the alternative mechanism of conservative motion. It is concluded, therefore, that at low temperatures elementary vacancy (producing) jogs must move non-conservatively with the aid of stress, forming lines of vacancies, but that interstitial (producing) jogs move conservatively. Sessile vacancy jogs are considered to contribute to τ_g, interstitial jogs to τ_s. Jogs are thought to contribute to hardening in stages II and III of the stress strain curves of single crystals, and cross-slip is thought to be initiated at jogs. Above a critical temperature jogs can move non-conservatively with the aid of thermal activation, and this mechanism explains the observed decrease in τ in Al and Cu at high temperatures (Hirsch and Warrington 1961).