A strain gradient crystal plasticity model was used to explore the grain size effect on the fields of plastic strain and of the dislocation density tensors in two-dimensional polycrystals. Finite-element simulations were performed for several aggregates of 24 and 52 grains with a detailed description of the intragranular fields. The micro-curl model was chosen for its ability to produce a size-dependent linear kinematic hardening coming from the dislocation density tensor or, equivalently, from the geometrically necessary dislocations. The overall response of the polycrystals was strongly linked to the size of the microstructure; especially in a size-dependent zone where the grain sizes ranged from 0.4 to 20lm. A strong dependence upon the size was also observed for the plastic strain and dislocation density tensor fields. It was noted that the increasing energy cost associated with the development of geometrically necessary dislocations led to the formation of a network of intense slip bands accommodating the imposed deformation in ultra-fine grain polycrystals. A network of strain-localisation bands was progressively built up, and lattice curvature spread over the grains when the microstructure size was smaller. This slip-band network in ultra-fine grains was a new feature of generalised crystal plasticity. The problem remained as to whether such a phenomenon would still occurred in the case of three-dimensional simulations having more than 2 slip systems. On the other hand, the size of the intragranular domains affected by lattice curvature found in the simulations could be used as input to calibrate mean field models with internal length scales.

Grain Size Effects on Plastic Strain and Dislocation Density Tensor Fields in Metal Polycrystals. N.M.Cordero, S.Forest, E.P.Busso, S.Berbenni, M.Cherkaoui: Computational Materials Science, 2012, 52[1], 7-13