UC Irvine

Being the lightest structural metal, Mg is a promising candidate in numerous energy-saving and environmental-friendly applications. The research interest in Mg and its alloys continues to rise in past decades, however, their commercial applications were held back by some longstanding intrinsic difficulties. Belonging to the hexagonal close packed family of metals, Mg has anisotropic slip and twinning systems, leading to strong textures invariably associated with wrought Mg and small strains-to-failure compared to those of current structural metals. Further enhancement of the performance in Mg requires mechanistic understanding of the deformation and innovative design strategies.

Microstructure is known to have tremendous influence on the mechanical and functional properties of materials. To shed light on the microstructural design for low density, high strength and formable Mg alloys, the objective of this dissertation is to advance the fundamental understanding of how microstructure affects their mechanical properties. Particular focus is on examining the influence of grain morphology (equiaxed or laminated), grain size (micro- or nano-crystalline), twin mesh features (with or without a high density of intersecting twins) and dilute solute addition of Y in polycrystalline Mg. For this purpose, we designed the experimental strategies leveraging powder metallurgy or surface mechanical treatment approaches to achieve samples with various internal microstructures. Bulk form and in-situ mechanical testing together with multiscale microstructure characterization were carried out on the Mg samples being synthesized.

In this dissertation, we have demonstrated strong grain morphology and grain size effects on the active deformation modes in polycrystalline Mg, resulting in a reversed yield strength anisotropy and a large compressive strain >120% at room temperature, in the microlaminated and nanocrystalline Mg, respectively, that have not been observed in conventional Mg. The toughening effect of twin meshes was explored by in-situ TEM nanocompression, and was successfully achieved in a gradient twin meshed Mg polycrystal at the macroscale. Lastly, the alloying effect of Y was analyzed experimentally in regard to the slip and twinning behavior. The difference in activation barriers among the slip modes was reduced by Y addition, and the mobility of twin boundaries may be affected by the segregation of Y atoms on such boundaries.