2, New Technologies Research Centre, Pilsen, , Czechia
3, Johns Hopkins University, Baltimore, Maryland, United States
4, ETH Zürich, Zürich, , Switzerland
5, Empa, Swiss Federal Laboratories for Materials Science and Technology, Thun, , Switzerland
Based upon extensive micropillar compression experiments, molecular dynamics and DDD simulations, here we show that the size and frequency distributions of dislocation avalanches triggered under progressive straining are characterized by transitional cut-off slip sc from incipient to large avalanche domains, each satisfying universal power-law functions with distinctly different exponents. As compared to conventional load-controlled straining, where large bursts result in plastic collapse of the material sample, presently applied progressive straining prevents collapse as the applied stress decreases and the dislocation network evolves during large avalanche propagations. Our results reveal that the critical slip sc is the distinctive length scale that challenges the prevailing conception on the scale invariance of dislocation avalanches irrespectively of underlying dislocation glide mechanisms. Therefore, sc is ruled by crystalline structure, sample size and temperature vis-à-vis onset of specific dislocation glide, dislocation interaction and dislocation annihilation mechanisms.
In FCCs, a reduction in sc is indicative of severe interactions between the mobile dislocations and a heavily entangled dislocation network, as well as of the enhancement in the dislocation pinning capacity in crystals where dislocation cross-slip is inhibited. Increasing sc marks onset of surface dislocation annihilation hindering dislocation network development in micrometer-sized samples, whereas dislocation source starvation reduces sc in submicrometer sizes. Parameter sc is smaller in BCCs than in FCCs. In the latter, sc is governed by the mobility of the screw dislocations present in various arrangements as a function of sample size and temperature. High-temperature mutual annihilation of mobile dislocations is found to effectively increase sc.
Stress binning of the avalanche distributions emerges as a powerful element in the assessment of sample size effects in dislocation-mediated plasticity. In the spirit of self-organized criticality (SOC), large FCC and BCC micropillars show constancy in the slip distribution irrespectively of the stress level. On the other hand, the likelihood for large avalanche emissions in confining FCC sample sizes is associated with the sudden destabilization of the dislocation network at large stresses, leading to substantial stress drops and to stress-tuned (binned) avalanche distributions. The enhancement in the mobility of the screw dislocation population in BCCs at large applied stresses and elevated temperatures leads to large avalanche emissions, promoting onset of stress-binned distributions.