Transition from laminar to rotational motion in plasticity

Abstract
On the basis of observations made by electron microscopy of the distribution of dislocations around undeformable particles embedded in a matrix of plastic crystalline metal, it is possible to see a clear transition from ‘laminar plastic flow’ to ‘rotational flow’ at a critical strain which depends mostly upon the particle size, larger particles displaying smaller critical strains. These patterns are produced by plastic relaxation of the internal stress caused by deformation. Although there seems to be no secure way of predicting the patterns on the basis of continuum mechanics, nor on an atomistic basis of dislocation mechanics, it is possible by a combination of dislocation theory and continuum patterns of flow to devise simple models which go some way to explaining the transition and to predicting the distribution of misorientation in the rotational structures. The results have important consequences for understanding recrystallization phenomena, as well as overall work–hardening behaviour. They suggest that indentation plasticity might behave similarly.