A study of the structure of Lomer and 60° dislocations in aluminium using high-resolution transmission electron microscopy

Abstract

The structure of asymmetric [110] tilt boundaries developed during creep of pure aluminium single crystals is studied using high-resolution electron microscopy. The small- and moderate-misorientation boundaries are composed of Lomer and 60° dislocations. Extensive image simulation is used to deduce the detailed core structures of these two dislocations. Comparisons with calculated elastic displacements indicate that the Lomer dislocation is not detectably dissociated in its (001) glide plane, or into the sessile Lomer-Cottrell configuration. The agreement between the observed atomic column positions and continuum elasticity is excellent, except for the innermost positions near the core of the Lomer dislocation. The core structure of these discrete Lomer dislocations also correlates well with previous atomistic calculations for a larger-misorientation Σ = 19 boundary. A similar analysis of the displacements around 60° dislocations using isotropic elasticity indicates a slight dissociation of about 0.55 ± 0.15nm on the {111}. The lateral migration of boundaries in the thin transmission electron microscopy foils is also observed to occur by the motion of 60° dislocations within the boundary plane via a reversible Lomer reaction. The observation of an alternate core structure for the Lomer dislocation during this migration process is explained by the presence of a kink along the dislocation line. Multislice image simulations in which the structure varies with depth are used to study the effects of kinks and dislocation inclination on core structure images in thin foils. The implications of these observations in terms of dislocation glide on (001) in aluminium, are discussed.