Statistics of dislocation avalanches in FCC and BCC metals: dislocation mechanisms and mean swept distances across microsample sizes and temperatures

Jorge Alcalá,Jaafar A El-Awady,Jeffrey M Wheeler,Jan Očenášek,Javier Varillas,Johann Michler

doi:10.1038/s41598-020-75934-5

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 exhibit a gradual transition from an incipient power-law slip regime (spanning approx 2.5 decades of slip sizes) to a large avalanche domain (spanning approx 4 decades of emission probability) at a cut-off slip magnitude {s}_{mathrm{c}}. This cut-off slip provides a statistical measure to the characteristic mean dislocation swept distance, which allows for the scaling of the avalanche distributions vis-à-vis the archetypal dislocation mechanisms in face-centered cubic (FCC) and body-centered cubic (BCC) metals. Our statistical findings provide a new pathway to characterizing metal plasticity and towards comprehension of the sample size effects that limit the mechanical reliability in small-scale structures.

Highlights

Plastic deformation in crystalline materials consists of an ensemble of collective dislocation glide processes, which lead to strain burst emissions in micro-scale samples
The sample is rapidly strained after the onset a plastic instability, the magnitude of which is controlled by the activation the feedback loop and mechanical dynamics of the testing system
Since the Molecular dynamics (MD) simulations indicate that the propagation time frame is ≈ 20 ns for a medium-sized avalanche event sweeping across a microcrystal with D = 2 μm, it is argued that a larger, dynamically-driven, strain burst ( ε ≈ 0.1) occurring over a time frame several decades greater would be comprised of more than a hundred of individual avalanches

Summary

Introduction

Plastic deformation in crystalline materials consists of an ensemble of collective dislocation glide processes, which lead to strain burst emissions in micro-scale samples. The slip distribution becomes independent of the applied stress, leading to the hypothesis of self-organized critically (SOC)[12,15,16,17,18] Along these lines, it may be argued that the self-similar scaling of the dislocation cells arising at large shear strains during single crystal deformation may tentatively result in SOC. While the applied stress and dislocation density both fluctuate during subsequent avalanche emissions, a net increase in dislocation density prevails due to the storage of the mobile dislocations within the steady network, yielding a net positive slope θ in the stress–strain curve that quantifies onset of strain h ardening[19] This conception is markedly affected by the sample size. Experimental evidence is still needed in support of these conceptions, including the influence of the characteristic face-centered cubic (FCC) and body-centered cubic (BCC) dislocation glide mechanisms in micro-scale crystals deforming under confined and bulk-like plasticity

Methods

Results

Conclusion