It was recalled that experiment had shown that the presence of free surfaces could induce harder, as well as softer, deformation behaviors in crystalline solids. In order to resolve these contradictory findings, 2-dimensional discrete dislocation dynamics simulations were used to investigate surface-induced size effects. The simulations indicated that, depending upon the surface density of dislocation sources, a free surface could act either as a dislocation sink or as a net dislocation source, and could accordingly exert opposite effects on dislocation density over a boundary layer thickness of up to 500nm into the bulk. This finding provided a possible explanation for the apparent contradictions in experimental observations.

Discrete Dislocation Dynamics Simulations of Surface Induced Size Effects in Plasticity. C.S.Han, A.Hartmaier, H.Gao, Y.Huang: Materials Science and Engineering A, 2006, 415[1-2], 225-33