Role of Pre-Existing Stacking Faults in the Mechanical Response of Metallic Materials: A Molecular Dynamics Study

Abstract

Nanostructured metallic materials have gained significant interest for their superior performance in the fields of electronics, energy, defense and structural applications. They owe their outstanding mechanical properties to abundant interfaces such as grain boundaries and phase boundaries as well as nanoscale features such as grain size. The role of grain boundaries, twin boundaries, and other interfaces on mechanical response of materials has been richly investigated over decades. In contrast, stacking faults, which are also planar defects, have primarily been studied as defects that arise during deformation. Recent experimental work on hexagonal-closed packed (HCP) and face centered cubic (FCC) Co with stacking faults enables the investigation of how stacking faults may govern the deformation of metallic materials. This dissertation presents an atomistic study of the deformation mechanisms in HCP, FCC metals and nanostructured metallic multilayers dominated by pre-existing stacking faults and other planar defects. First, nanopillar compression is used to investigate the deformation mechanisms in HCP Co with high density stacking faults. Molecular dynamics (MD) simulations reveal high yield strength and significant plasticity owing to stacking fault induced deformation mechanisms that activate phase transformation. Second, we use MD simulations to show that FCC (111)Cu/HCP (0002)Co multilayers with an incoherent layer interface have dramatically better mechanical performance than FCC/FCC (100) and (110) Cu/Co multilayers with a coherent layer interface. Our study reveals a unique interplay among twin boundaries in Cu, stacking faults in HCP Co, and incoherent layer interfaces, which leads to partial dislocation dominated high strength and outstanding plasticity. We further perform MD simulations to investigate the distinct role of each of these pre-existing planar defects on the mechanical behavior of FCC Cu/ HCP Co multilayers. Finally, we perform MD simulations on FCC Co nanopillars. They reveal that the stacking faults and partial dislocations activity dominate the hardening and softening observed during plastic deformation of FCC Co where abundant dislocation junctions favor strengthening. Our simulations, together with experimental data, provide compelling evidence that pre-existing stacking faults can enhance strength and plasticity and suggest future avenues for the design of nanostructured materials with optimized properties using defect networks.