Taming intermittent plasticity at small scales

Abstract: The extreme miniaturization in modern technology calls for deeper insights into the non-conventional, fluctuation dominated mechanics of materials at micro- to nano-scales. Both experiments and simulations show that sub-micron face-centered-cubic (FCC) crystals exhibit high yield strength, which is, however, compromised by intermittent, power law distributed strain fluctuations, typical of wild plasticity. At macro-scales, the same bulk materials show mild plasticity characterized by bounded, uncorrelated fluctuations. Both anomalous strength and intermittency appear therefore as size effects. While the former is highly desirable, the latter is detrimental because stochastic dislocation avalanches interfere with forming processes and endanger structural stability. In this paper we show that defectiveness, which is used in classical metallurgy to harden materials, can be used at sub-micron scales to suppress intermittent fluctuations. We quantify the coexistence of wild and mild fluctuations in compressed Al alloys micro-pillars, demonstrate that the statistical nature of fluctuations is determined by sample size, and propose quantitative strategies allowing one to temper plastic intermittency by artificially tailored disorder. Our experimental results are rationalized using a theoretical framework which quantifies the competition between external (size related) and internal (disorder related) length scales.