A method for the construction of initial structures for molecular dynamics simulations of nanocrystals with nonequilibrium grain boundaries containing extrinsic dislocations

A A Nazarov,R T Murzaev

doi:10.22226/2410-3535-2018-1-5-10

Abstract

A method for the construction of initial atomic models of nanocrystals with extrinsic grain boundary dislocations (EGBDs) in grain boundaries (GBs) for molecular dynamics (MD) simulations is developed. The method is realized for f.c.c. nanocrystals with columnar grains having common crystallographic axis [112] parallel to the column axis and thus divided only by [112] tilt GBs. This system is convenient for studies of interactions between GBs and lattice dislocations, since each grain can be deformed by edge dislocations of only one slip system, which have lines parallel to the [112] axis. In order to introduce extrinsic dislocations to the boundaries of a selected grain, its contour is assumed to be strained by a given shear strain  so that a contour of a freely sheared grain is formed. This contour is filled in by atoms of a f.c.c. lattice with [112] direction parallel to the column axis and then the grain thus formed is subjected to an elastic shear strain -. This results in a deformed grain having the original shape, on the boundaries of which precursors of EGBDs are formed. In order to prevent these precursors from spontaneous annihilation during MD relaxation, one can temporarily fix GB atoms, or apply a proper external stress, or do both. A case study is carried out using two different protocols of MD relaxation to determine atomic structures and energies of nonequilibrium GBs.

Highlights

During low-temperature plastic deformation of polycrystalline materials their grain boundaries (GBs) act as strong barriers for slip and accumulate extrinsic grain boundary dislocations (EGBDs) [1,2]
Nonequilibrium GBs induce long-range internal stress fields and possess an enhanced specific energy that results in their different properties and effect on the properties of polycrystals as compared to equilibrium GBs typical for well annealed materials [2,4]
The nonequilibrium GB structure is typical for the structure of bulk nanostructured materials processed by severe plastic deformation (SPD) methods [10,11]

Summary

Introduction

During low-temperature plastic deformation of polycrystalline materials their grain boundaries (GBs) act as strong barriers for slip and accumulate extrinsic grain boundary dislocations (EGBDs) [1,2]. Starting from the most general concepts concerning the interaction of GBs with lattice dislocations during plastic flow, Rybin and co-authors [14] developed a theory of fragmentation of materials, i.e., grain subdivision, which is a basis of SPD-processing of metallic materials Basing on this theory, dislocation and disclination models of nonequilibrium GB structure in nanostructured materials were proposed [15 – 17]. Several atomistic studies of nonequilibrium GB structures in bicrystals and nanocrystals have been reported recently [18 – 20] In these studies, the nonequilibrium structure was either introduced into GBs by pseudo-random local shifts of GB atoms from their equilibrium positions [18] and creating free volumes [19], or was assumed to occur due to the Voronoi construction [18,20]. A case study of the structures of nanocrystals using two different protocols of MD relaxation is carried out and the excess energy of GBs for these structures is calculated

Plastic strain of a grain and formation of EGBDs

Simulation results and discussion

Conclusions