Discrete Dislocations Dynamics with annihilation as the limit of the Peierls-Nabarro model in one dimension

TL;DR

This paper proves that the evolution of phase-field functions modeling dislocations in metals converges to a discrete dislocation dynamics model with annihilation, valid for any number of dislocations and without initial restrictions.

Contribution

It establishes the rigorous connection between the Peierls-Nabarro model and discrete dislocation dynamics with annihilation for arbitrary dislocation configurations.

Findings

Convergence of phase-field functions to a piecewise constant limit.

Limit dislocation positions follow a specific ODE system with annihilation.

The result holds for any number of dislocations and arbitrary initial conditions.

Abstract

Plasticity of metals is the emergent phenomenon of many crystal defects (dislocations) which interact and move on microscopic time and length scales. Two of the commonly used models to describe such dislocation dynamics are the Peierls-Nabarro model and the so-called discrete dislocation dynamics model. However, the consistency between these two models is known only for a few number of dislocations or up to the first time at which two dislocations collide. In this paper we resolve these restrictions, and establish the consistency for any number of dislocations and without any restriction on their initial position or orientation. In more detail, the evolutive Peierls-Nabarro model which we consider describes the evolution of a phase-field function $v_\e(t,x)$ which represents the atom deformation in a crystal. The model is a reaction-diffusion equation of Allen-Cahn type with the half Laplacian. The small parameter $\ep$ is the ratio between the atomic distance and the typical distance between phase transitions in $v_\e$. The position of a phase transition determines the position of a dislocation, and the sign of the transition (up or down) determines the orientation. The goal of this paper is to derive the asymptotic behavior of the function $v_\e$ as $\ep\to0$ up to arbitrary end time $T$; in particular beyond collisions. We prove that $v_\e$ converges to a piecewise constant function $v$, whose jump points in the spatial variable satisfy the ODE system which represents discrete dislocation dynamics with annihilation. Our proof method is to explicitly construct and patch together several sub- and supersolutions of $v_\e$, and to show that they converge to the same limit $v$.