Résumé
Most solids like metals and ceramics are polycrystals, i.e. aggregates of crystalline grains separated by 2D defects, the grain boundaries (GBs). Although GB sliding is believed to be involved in the irreversible deformation (plasticity) of polycrystalline solids, the microscopic mechanisms at play are still poorly understood, partly because common techniques for atomic polycrystals visualization preclude any information on their dynamics under stress. We propose a colloidal analog of atomic polycrystals obtained by doping a copolymer micellar crystal with nanoparticles (NPs) and for which the grain size is tuned by changing the crystallization rate. During the crystallization NPs are partially expelled from the lattice and segregated into the GBs allowing one to probe their structure with confocal microscopy (Fig.1) or light scattering (Fig.2). Moreover, the NPs structure into the GBs is probed with neutron scattering (Fig.2). In order to investigate the plasticity of the colloidal polycrystals, dynamic light scattering measurements on the sample submitted to cyclic deformation are performed using a novel light scattering apparatus specifically designed to access the dynamics of the GBs. Plasticity is quantified by analyzing the correlation of the scattered intensity measured after a given number of deformation cycles. We demonstrate that the shear-induced GB dynamics at the origin of plasticity is aging in a manner that depends on the probed length-scale. We extract a characteristic length above which the GB dynamics is stationary. This characteristic length is found to increase with the shear amplitude. Our data suggest a hierarchical organization of the GB network under shear and provides a novel framework to understand the plasticity of polycrystals.