Experiments have shown that the presence of free surfaces may induce harder as well as softer deformation behaviors in a crystalline solid. In order to shed some light on these apparently contradictory findings, two-dimensional discrete dislocation dynamics simulations were performed to investigate the surface induced size effects. The simulations indicate that, depending on the surface density of dislocation sources, a free surface may 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 provides 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