UC Irvine

Magnesium (Mg) is of interest as a lightweight structural material, given its potential for energy-efficient and eco-friendly applications. However, Mg and its alloys have a hexagonal close packed structure that is characterized by an inherent plastic anisotropy which leads to poor formability and low strength, thereby limiting their use in many important engineering applications. Deformation twinning, an important plastic deformation mode in Mg, critically influences the strength and ductility of Mg. Consequently, a fundamental understanding of twin behavior under different stress and strain conditions is critical to optimize the performance of Mg and its alloys and thereby broaden their applicability in engineering systems.

In this dissertation research, experimental studies, using electron backscatter diffraction (EBSD) and scanning transmission electron microscopy, were used in combination with atomistic simulations using molecular dynamics and the nudged elastic band method to provide insight into twin behavior and investigate how the stress and strain fields modulate deformation twinning in Mg. The results from this dissertation research established multiple correlations between grain-scale microstructural characteristics and twin behavior by collecting and analyzing twin evolution information from large EBSD datasets and established the preferred conditions for twin nucleation, propagation, and growth. We observed room temperature, deformation-induced solute segregation in a Mg-Y alloy at faceted {10-12} twin boundaries. The segregated Y atoms exert a pinning effect and lead to anisotropy on the mobility of twin boundaries. We calculated the stress-strain field modulated energy barrier for coherent twin boundary migration and found that there is a power-law relationship between the rate of CTB migration and the ratio between the twin resolved shear stress and the critical resolved shear stress for CTB migration. We confirmed the presence of a pure-shuffle twin nucleation and early-stage growth mechanism at disconnection-dense sites between stacking faults in a random defect network under deformation. The twin variant selection was found to be correlated with the line direction of the disconnection at the nucleation site. These key findings deepen our understanding of the fundamental mechanisms that govern twin behavior and provide alloy design pathways to control the mechanical response of Mg alloys by engineering the microstructure of deformation twins.