Introduction

Abstract

The deformation of a crystal is a process that has many stages. At first, it is reversible and elastic; the atoms or ions return to their initial positions when the external force is removed, the displacement being proportional to the stress. Apart from this proportional change (Hooke’s law), real crystals show nonlinear but reversible changes at fairly low stresses. This imperfect elasticity causes delay in the strain corresponding to a changed stress (or conversely): it is associated with migration in the lattice, and the displacements differ essentially from the elastic ones in an ideal lattice, being combinations of various elementary diffusion processes*.