Influence of loading control on strain bursts and dislocation avalanches at the nanometer and micrometer scale

Abstract

Through three-dimensional discrete dislocation dynamics simulations, we show that by tuning the mode of external loading, the collective dynamics of dislocations undergo a transition from driven avalanches under stress control to quasi-periodic oscillations under strain control. We directly correlate measured intermittent plastic events with internal dislocation activities and collective dynamics. Under different loading modes, the role of the weakest dislocation source and the defect population trend are significantly different. This finding raises new possibilities of controlling correlated dislocation activities, and obtaining low defect density in nano-structured devices by tuning external constraints. In addition, the effect of machine stiffness comes to light. The statistical analysis on the burst magnitude is revisited and carefully discussed. Self-organized criticality and scale-free statistics of strain bursts are obeyed under stress control. However, this behavior is shown to break down under strain control. Rapid stress drops under pure strain control force truncation of dislocations avalanches, leading to a dynamical transition to quasi-periodic oscillations.2

Avalanches and plastic flow in crystal plasticity: an overview

“…Strain bursts were found to be dominated by the intermittent operation of the weakest sources [41,43]. However, power law statistics were found to be the result from correlated activation of dislocation sources, and that such correlations are strongly influenced by the loading mode [35,197].…”

“…For 1 ∼ 20 µm LiF crystals, a larger power law exponent of 1.8 ∼ 2.9 is found [34]. A detailed summary of the recorded burst quantity and the power law exponent in micro/nano pillars is given in Table 3 of reference [35], and in [23]. It is generally believed that dislocation avalanches result from the collective and correlated dynamics of dislocations [36,37].…”

Plate-like precipitate effects on plasticity of Al-Cu alloys at micrometer to sub-micrometer scales

“…Instead of mean-field theory arguing for a universal landscape of intermittent plasticity [25], it has been revealed that the associated statistical properties are material-and size-dependent, with varying scaling exponent and wildness [17,32,77]. Note that the loading method in these experiments remain the same, and the varying should not be an effect of machine stiffness analyzed in [78]. On the other hand, we found that the relationship between these two variables, ( ), in pure Al and nano-sized disorder reinforced Al alloys, is "universal" (material-independent) [17].…”

Modeling Plastic Deformation of Nano/Submicron-Sized Tungsten Pillars Under Compression: A Coarse-Grained Atomistic Approach

“…Plastic deformation of pillars has been extensively studied using experimental (Brinckmann et al, 2008) and modeling (Weinberger and Cai, 2008) approaches, with a focus on the yield/flow stress as a function of the pillar diameter, height-to-diameter aspect ratio, cross-sectional shape, strain rate, temperature, lattice orientations, boundary conditions, etc., in a wide range of metallic materials. For example, the discrete dislocation dynamics (DDD) method, which assumes the motion of dislocations as the only carrier of plasticity, has been applied to simulating nanopillars (Papanikolaou et al, 2017), submicropillars (Cui et al, 2017), and micropillars (El-Awady, 2015). However, besides dislocation slip, processes such as twinning and phase transformation have also been identified as possible plastic deformation mechanisms in nano-/micropillars (Greer and De Hosson, 2011) in both face-centered cubic (FCC) and body-centered cubic (BCC) metallic systems.…”