LEDS: Properties and effects of low energy dislocation structures

Abstract

Not only epitaxy and phenomena in connection with solid transformations, but also the static mechanical properties of crystalline materials can be understood via Low Energy Dislocation Structures (LEDS). They are characterized by areas of high dislocation density embedded in, or interspersed with, almost dislocation-free areas. LEDS include misfit boundaries, low angle dislocation boundaries, the Taylor lattice (formed either from interacting pile-ups of alternating sign or from accumulations of dipolar dislocation loops), dipolar dislocation walls (as in “deformation bands” formed in unidirectional deformation, or in the “ladder structure” as well as in the “maze structure” formed in fatigue) and the “mosaic block” structure of old, now recognized to be indentical with the dislocation cell structure. By means of LEDS the empirical rules of (1) the parabolic dependence of shear stress on dislocation density and (2) the decrease of the dislocation cell size with increasing stress are readily understood, as is (3) the three-stage work-hardening curve. The quoted inverse dependence of dislocation cell diameter on flow stress can be used to deduce the stress distribution which caused the cell structure, e.g. after shock loading, about crack tips or underneath wear scars. “Recovery” is the removal of jogs and kinks in LEDS, while the mild stress dependence of flow stress on strain rate is explained through the increase in the fraction of the dislocations which move at any given moment.