Material Property Enhancement by Controlled Defects

Abstract

Ever since the Bronze Age, defects in materials have been manipulated to enhance their properties. Examples include, dislocation induced strengthening via cold working, solid solution strengthening via alloying and precipitation hardening via second-phase particles. However, limited efforts have been made towards enhancing ductility and fracture properties of materials. This is because fracture of materials is usually analyzed using the weakest link theory, which relates the probability of fracture of the material as a whole to the probability of fracture of each volume element within it. Consequently, design of materials to resist fracture often aim to eliminate weak links, i.e., features of low intrinsic strength, and introduce features of high intrinsic strength. Nevertheless, there are many natural materials that utilize weak links and heterogeneities to enhance their fracture resistance. This and the recent rapid advancements in the advanced and additive manufacturing techniques that promise better control over defect distributions in a material has led to current efforts of exploiting defects to enhance fracture properties. The possibility of defect-induced strengthening and fracture resistance of materials are far from being exhausted, and herein, a variety of defects under diverse loading conditions are explored in aspiration to enhance ductility and fracture resistance of materials using largescale microstructure-based finite element simulations of deformation and fracture. Specifically, in this dissertation the focus is confined to three defect-induced strategies to enhance selected mechanical performance metrics of materials: (1) improving ductility by exploiting grain level heterogeneities, (2) toughening of interface networks by introducing weak links, and (3) mitigating spall fracture by porosity induced wave attenuation. These results suggest that fracture resistance-oriented material development calls for improved understanding of defects and their judicious incorporation into materials, rather than their complete elimination. Hopefully, these results will inspire and instigate future works in this direction.