Extended dislocation multipoles in low stacking-fault energy alloys

Abstract

The diffraction contrast shown by dislocation multipoles in which the individual dipoles are closely spaced is analysed. It is shown that such multipoles observed in low stacking-fault energy alloys do not in general follow the predicted contrast behaviour (Hazzledine 1967) in that well-developed fringe contrast is not observed for all low-index diffracting vectors. In particular only very weak or zero fringe contrast is observed for those 220 or 311 diffracting vectors that would give zero contrast for a stacking fault on the same plane as the multipole. Further, the contrast behaviour at the edges of the multipoles closely approximates that of a single Shockley partial dislocation bordering an intrinsic fault.

A comparison of experimental and theoretical electron micrographs computed for various dislocation arrangements within the multipole shows that the multipoles in low stacking-fault energy alloys are commonly in an extended form rather than the configuration predicted from isotropic elasticity calculations. The extended multipole is characterized by the arrangement of the dissociated dislocations into a succession of individual Shockley dipoles linked by an almost continuous ribbon of stacking fault. In the absence of an applied stress the extended multipole is stabilized by the high lattice friction in the low stacking-fault energy alloys.