Statistics and Mechanisms of Intermittent Plasticity in FCC and BCC Microcrystals

Abstract

Plastic deformation in crystalline materials consists of an ensemble of collective dislocation glide processes, which lead to strain burst emissions in micro-scale samples. To unravel the combined role of crystalline structure, sample size and temperature on these processes, we performed a comprehensive set of strict displacement-controlled micropillar compression experiments in conjunction with large-scale molecular dynamics and physics-based discrete dislocation dynamics simulations. The results indicate that plastic strain bursts consist of numerous individual dislocation glide events, which span over minuscule time intervals. The size distributions of these events follow a power-law function which bifurcates from an incipient slip regime of uncorrelated glide (spanning ≈ 2.5 decades of slip sizes) to a large avalanche domain of collective glide (spanning ≈ 4 decades of emission probability) at a critical slip magnitude s_c. This critical slip size characterizes the transition between bulk-like and localized plasticity. In face-centered cubic (FCC) metals, s_c is essentially governed by the interplay between dislocation annihilation, cross-slip and junction formation processes developing as a function of microcrystal size and stacking fault width in Al, Ni and Cu. Dislocation starvation then rules the avalanche statistics in smaller microcrystals. In body-centered cubic (BCC) metals, s_c evaluates the combined role of temperature and the applied stress level upon the glide of the sluggish screw dislocations via cross-kinking mechanisms. Different s_c values result in BCC Ta and W due to the distinctive thermal and stress-dependent activation of cross-kinking. These FCC and BCC dislocation glide mechanisms determine the evolution from self-organized to stress-tuned avalanching processes.