The effect of grain size upon the tensile plastic deformation of ultrafine-grained polycrystals was investigated by using a 2-dimensional simulation of the dislocation dynamics. Emphasis was put on the elementary mechanisms which governed the yield stress under multi-slip conditions. Regardless of the grain size, the yield stress was found to obey a Hall-Petch law. However, the elementary mechanism which controlled the slip transmission through the grain boundaries at yield was observed to change with the grain size. At larger grain sizes, the stress concentrations due to dislocations piled-up at grain boundaries were responsible for the activation of plastic activity in the poorly stressed grains. At smaller grain sizes, the pile-ups contained fewer dislocations and were less numerous, but the strain incompatibilities between grains became significant. They induced high internal stresses and favoured multi-slip conditions in all grains. Based upon these results, simple interpretations were proposed for the strengthening of the yield stress in ultrafine grained metals.

Yield Stress Strengthening in Ultrafine-Grained Metals - a Two-Dimensional Simulation of Dislocation Dynamics. S.Lefebvre, B.Devincre, T.Hoc: Journal of the Mechanics and Physics of Solids, 2007, 55[4], 788-802