Scale-Free Intermittent Flow in Crystal Plasticity

Abstract

Under stress, crystals irreversibly deform through complex dislocation processes that intermittently change the microscopic material shape through isolated slip events. These underlying processes can be revealed in the statistics of the discrete changes. Through ultraprecise nanoscale measurements on nickel microcrystals, we directly determined the size of discrete slip events. The sizes ranged over nearly three orders of magnitude and exhibited a shock-and-aftershock, earthquake-like behavior over time. Analysis of the events reveals power-law scaling between the number of events and their magnitude, or scale-free flow. We show that dislocated crystals are a model system for studying scale-free behavior as observed in many macroscopic systems. In analogy to plate tectonics, smooth macroscopic-scale crystalline glide arises from the spatial and time averages of disruptive earthquake-like events at the nanometer scale.

Alternating starvation of dislocations during plastic yielding in metallic nanowires

“…Recently, Uchic et al [15,16] and Greer et al [17,18] reported that the plastic deformation behavior of single-crystalline sub-micropillars is dependent on the size of the pillar, even without a deformation gradient. More recently, a ''mechanical annealing" test was used to demonstrate that dislocations can be swept out of the samples through the progressive activation and exhaustion of dislocation sources [19].…”

“…Due to the unique mechanical, thermal, electrical and optical properties, materials with nanometer-sized structure have attracted a great deal of interest as potential building blocks in nanoelectronic and nanoelectromechanical devices [1]. Many researchers have demonstrated, through both experiments and analysis, that the structure and properties of nanowires can be quite different from those of bulk materials due to the effect of the large surface to volume ratio [2][3][4][5][6][7][8][9][10][11][12][13][14].Recently, Uchic et al [15,16] and Greer et al [17,18] reported that the plastic deformation behavior of single-crystalline sub-micropillars is dependent on the size of the pillar, even without a deformation gradient. More recently, a ''mechanical annealing" test was used to demonstrate that dislocations can be swept out of the samples through the progressive activation and exhaustion of dislocation sources [19].…”

A Novel Approach to Modelling Nanoindentation Instabilities

“…There has been a spurt of activity in size dependent studies on plastic deformation of small volume systems in the last three decades due to technological importance as well as the scientific challenges it offers. For instance, intermittent flow is observed when the diameter of micrometer rods are below a certain value while it is smooth when it is large implying the instability manifests when the aspect ratio is reduced [4][5][6]. Similar intermittent plastic flow in the form of load fluctuations or displacement jumps is reported when the indentation depth is less than 100 nm both in thin and bulk samples [7][8][9][10][11][12].…”

Micron-Scale Deformation: A Coupled In Situ Study of Strain Bursts and Acoustic Emission

“…al. found that instabilities in the form of strain jumps dominate micrometer scale crystal plasticity [15][16][17] raising the questions (i) what is the limit between microscopic and macroscopic deformation, and (ii) how one can define material strength parameters, like flow stress, for micron-scale objects [18][19][20] .…”

“…The new fast micro-pillar fabrication procedure developed gives us the chance to perform a statistical analysis of the deformation processes. This is essential because during an individual compression experiment the stress varies intermittently (see below), 3 meaning that the dislocation system gives stochastic response to the acting force 11,14,16,27,28 . As a result of this defining material parameters from a unique measurement is impossible.…”