Strain hardening and dislocation avalanches in micrometer-sized dimensions

Present experiments and computational simulations furnish a fundamental background to the understanding of plastic flow across sample sizes. It is shown that self-organized criticality (SOC) governs the size distribution of dislocation avalanches in micrometer-sized sample dimensions. Onset of SOC denotes inception of a dislocation network so that dislocation avalanches occur at constant criticality level irrespectively of the applied stress. In these microcrystals, we find that the ratio between the characteristic sample dimension and the mean free path traveled by the mobile dislocations, D/LeffD/Leff, rules the onset of strain hardening. This index simultaneously accounts for the role of loading orientation and dislocation density upon microscale plasticity. It is shown that strain-hardening emerges for D/Leff>2D/Leff>2, where surface dislocation annihilations are inconsequential to network development and the flow stress scales with dislocation density. This regime naturally evolves toward bulk plasticity at increasing sample sizes. Conversely, strain hardening is suppressed when confining sample dimensions dominate plastic flow for D/Leff<1.5D/Leff<1.5. Confining microscale plasticity is characterized by a significant increase in the size of dislocation avalanches under a stagnant dislocation network.