Mesostructural approach of localization
Résumé
In polycrystals, heterogeneities of plastic deformation have a strong impact on technological forming processes resulting in global softening, specific deformation texture and recrystallization. An experimental work was performed in a scanning electron microscope at the scale of the grain, focused on the formation of bands of localization during monotonic and sequential loadings in steel polycrystals. For monotonic loading, such experiments pointed out, an early localization of the strain field in parallel mesobands, few micrometers width, crossing grain boundaries. For sequential loading path, instabilities led to some macrobands composed of a set of mesobands. A crystalline model using a finite element code is proposed. This model gives a description of the microstructure evolution which mainly depends on the nature of the activated slip systems and on their interactions by a hardening matrix expressed in terms of dislocation densities. Due to the particular discretization of the sample based on an actual set of grains, the interactions between adjoining grains were emphasized. The strain localization predicted by this model, during monotonic and sequential loading paths, fits with the experimental observations. These results are compared to those obtained by a mechanical model based on the Asaro's criteria of bifurcation and a discussion on the weight of the different physical parameters acting on localization bands is conducted.