Plasticity of metallic nanostructures : molecular dynamics simulations

Abstract

During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the faces of the pillars and the means by which strain is implemented in the simulations can affect the simulation results. We find that the twinning anti-twinning asymmetry in bcc materials causes nanopillars to deform by dislocation glide in compression and by twinning in tension. This explains the compression tension asymmetry reported experimentally in bcc micropillars. We find that deformation is mediated by glide of shockley partials in fcc pillars for compressive and tensile strains. Simulations of pure shear of nanocrystalline Fe are also presented. We find a change in deformation mechanisms for this system when at high temperatures. At low temperatures, plasticity is mediated in part by dislocation glide and twinning. However, at temperatures above 1200K the deformation is dominated by grain boundary sliding, recrystallization, and amorphization.