Atomistic modeling of fracture in α-Fe

Abstract

The focus of this thesis has been to use large-scale atomistic simulations to increase the understanding of fracture processes at the atomistic level. The Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) has been run on the supercomputer Vilje, at the NOTUR centre at NTNU. The atomistic models were fully 3D, notched micro-cantilever structures in bcc-Fe, modeled to resemble micro sized fracture mechanics structures tested in literature. The EAM potential "Mendelev II" has been applied, having been shown to be fairly accurate for dislocations while also computationally cheap enough for the systems to be simulated. Simulations on atomistically sharp cracks in bcc-Fe were carried out in Paper I, and a novel crack flange extrapolation method first used to calculate the plastic zone size in order to estimate the stress intensity in cracks in Si was applied. The stress intensity factors found using the crack flange extrapolation method was found to give reasonable estimates in iron, but more investigation would be necessary to establish this method as a robust measure of fracture toughness. Paper II simulates the strain rate effect for sharp and blunted crack tips in pentagonal cross-section cantilevers in bcc-Fe. The rounded crack tip is shown to reproduce experimentally observed crack behavior more accurately than the sharp crack tip. The crack flange extrapolation method is applied, with estimates for fracture toughness using this method showing a more realistic trend than continuum based fracture toughness calculations. A brief postdeformation comparison between simulations and SEM-images of Focused Ion Beam-fabricated micro-cantilevers was also done, showing possible signs of similar deformation mechanisms and dislocation systems between them. Four different orientations for notched cantilevers were studied in simulations, and the crack behavior was compared to existing literature in Paper III. The crack flange extrapolation method was used to calculate fracture toughness, and was found to better reflect the qualitative behavior during crack initiation than estimates from continuum based fracture mechanics. FIBfabricated notched cantilever beams with initial (100)[0¯11] crack system loaded experimentally, and examined using Transmission Electron Microscopy (TEM), showed a local brittle fracture consistent with results from the modeling.