Intermittent dislocation flow in viscoplastic deformation

Abstract

The viscoplastic deformation (creep) of crystalline materials under constant stress involves the motion of a large number of interacting dislocations [1]. Analytical methods and sophisticated 'dislocation-dynamics' simulations have proved very effective in the study of dislocation patterning, and have led to macroscopic constitutive laws of plastic deformation [2][3][4][5][6][7][8][9]. Yet, a statistical analysis of the dynamics of an assembly of interacting dislocations has not hitherto been performed. Here we report acoustic emission measurements on stressed ice single crystals, the results of which indicate that dislocations move in a scale-free intermittent fashion. This result is confirmed by numerical simulations of a model of interacting dislocations that successfully reproduces the main features of the experiment. We find that dislocations generate a slowly evolving configuration landscape which coexists with rapid collective rearrangements. These rearrangements involve a comparatively small fraction of the dislocations and lead to an intermittent behavior of the net plastic response. This basic dynamical picture appears to be a generic feature in the deformation of many other materials [10][11][12]. Moreover, it should provide a framework for discussing fundamental aspects of plasticity, that goes beyond standard mean-field approaches that see plastic deformation as a smooth laminar flow.Whenever dislocation glide is the dominant plastic deformation mechanism in a crystalline material, we observe a constant strain-rate regime usually described by Orowan's relationγ ∼ ρ m bv. Here, the plastic strain-rate of the materialγ is simply related to average quantities such as ρ m , the density of mobile dislocations, and v, their average velocity along the slip direction (parallel to the Burgers vector b) [1]. Transmission electron micrographs of plastically deformed materials display, on the other hand, complex features such as cellular structures and fractal patterns [2,3], which are the fingerprint of a complex multiscale dynamics not appropriately accounted for by the mean-field character of Orowan's relation. In addition, rapid slip events [10] have been observed in the plastic deformation of various metals and alloys [11,12], and in the Portevin-LeChatelier effect [13]. We believe that formulating plastic deformation as a nonequilibrium statistical mechanics problem [14] requires a substantial understanding of basic collective dislocation dynamics.Experimentally, the complex character of collective dislocation dynamics can be revealed by acoustic emission measurements. The acoustic waves recorded in a piezoelectric transducer disclose the pulse-like changes of the local displacements in the material during plastic deformation, whereas a smooth plastic flow would not be detected [15].Thus, this method is particularly useful for inspecting possible fluctuations in the dislocation velocities and densities.Ice single crystals can be used as a model material to study glide dislocation dynamics by acoustic emis...

Fabric development with nearest‐neighbor interaction and dynamic recrystallization

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“…As a result of these interactions, and of the spatial dislocation structures they give rise to, self-induced constraints build up in the system and the motion of dislocations may eventually cease. Nevertheless, small variations of the external loading, the density, the dislocation distribution or the temperature can enhance dislocation motion in a discontinuous and intermittent manner [120].…”

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