Foley, David Christopher (2014-08). Texture Control by Selective Deformation Mechanism Activation in Magnesium Alloy. Doctoral Dissertation.
The need for high strength, light weight structures in automotive and aerospace applications has driven a resurgence of interest in magnesium and its alloys. Unlike aluminum, wrought magnesium typically has a high degree of mechanical anisotropy because of its hexagonal close packed structure. Our objectives were to develop high strength (>350MPa yield) in a bulk magnesium alloy using grain refinement and to control the mechanical anisotropy by controlling crystallographic texture. This dissertation covers the development of thermomechanical processing methods used to tailor the strength and anisotropy of a magnesium alloy with 3%Zn and 1%Zr. The areas of focus in this study were as follows. First, we developed severe deformation processing strategies that increase strength in single-phase Mg alloy via grain refinement to submicron average grain size. We also established the achievable crystallographic textures in Mg alloy using 90o equal channel angular extrusion. In support of these first two goals, we determined the deformation mechanisms activated by differing strain paths and temperatures in single phase Mg alloy. Then, we established the effectiveness of these severe deformation processing strategies on both bar and plate workpiece geometries. We generated a wide range of crystallographic textures using thermomechanical processing. Using this knowledge, we established the effect of grain sizes down to submicron levels on room temperature deformation mechanism activity in single phase Mg alloy. We accomplished these goals through the use of equal channel angular extrusion, rolling, and heat treatment coupled with microscopy, diffraction, and mechanical testing. Notable achievements include demonstration of tensile twin suppression by grain refinement, the development of quasi-single-crystal textures, and the capacity to generate material with nearly identical texture but a range of grain sizes spanning almost two orders of magnitude. The experiments also supported (and were supported by) the development of visco-plastic self-consistent crystal plasticity modeling predictions thanks to the efforts of my colleagues. This work will further the development of advanced manufacturing and design using wrought Mg alloys.