The strengthening effect of grain boundaries is well established and observed experimentally as the Hall-Petch relationship. In this paper different mechanisms proposed in the literature to explain the observed Hall-Petch effect are reviewed critically. The fundamental implications of the different approaches are discussed with reference to experimental data for two different classes of materials;
-Materials with locked dislocations, i.e. with a sharp yield point behaviour.
-Materials without locked dislocations, i.e. with a smooth yielding behaviour.
It is shown that a simple model (Bergström) can be used to understand the grain size strengthening in the latter class of materials while more work is needed to quantitatively understand the behaviour of materials showing a sharp yield point.