Microstructural Influence on Deformation, Degradation and Fracture of Ductile Materials

Abstract

The overarching goal of this dissertation is to quantify the microstructural influence on deformation, degradation, and fracture of some of the most technologically sought after advanced structural materials. In the first part of this work, the mechanical response of a variety of advanced high strength steels with complex microstructure is characterized by in-situ mechanical tests. The experimental results show that even though the uniaxial tensile response of several of these steels is the same, their fracture response differs significantly. This contradicts classical analysis that predicts the same fracture response for materials with the same uniaxial tensile response. The high-resolution in-situ tests coupled with microscale digital image correlation (µDIC) and microstructure-based finite element analysis are then used to elucidate how the interlacing of imposed heterogeneous deformation field and the material microstructure affects the fracture response of these materials. The implications of these findings extend to both manufacturing issues and issues of materials development (or selection) based upon a specific need. The second part of this work focuses on understanding the influence of deformation-induced twinning on fracture response of single crystal specimens of an austenitic manganese steel. The crystallographic orientations of single crystal specimens are chosen to selectively activate crystallographic slip or twinning. The high-resolution in-situ tests coupled with µDIC and EBSD analysis are then used to elucidate how the interlacing of imposed heterogeneous deformation field and the evolving material microstructure affects the fracture response of the material. Finally, the synergistic effects of mechanical loading and corrosive environment on fracture response of an aluminum alloy are investigated. In general, environmental assisted fracture of a material is analyzed by simply characterizing mechanical response in a corrosive environment, while the emerging electrochemical characteristics are ignored. To overcome this, a 3-electrode setup for electrochemical impedance spectroscopy is built to characterize the electrochemical response of the material while being subjected to mechanical loading and corrosive environment. The results of these experiments coupled with microscopic characterization provide an unprecedented view of the synergistic effects of mechanical loading and corrosive environment on the environmental assisted fracture process.