Measurement and Prediction of Texture Evolution During Processing: Application to Rolled α-Uranium and Extruded Magnesium Alloys 1528 views

The goal of this thesis research was to enhance the understanding of the property and texture evolution of non-cubic metals and alloys during industrial deformation processes: cold-rolling and hot extrusion. Low-enriched uranium foil is a candidate target material for production of the medical isotope Tc-99m. To explore the effects of thermo-mechanical processing on these foils, surrogate depleted uranium foils were prepared for study by cold-rolling a cast plate to the target thickness with intermediate annealing steps. The post-rolling texture of the samples was measured by x-ray diffraction and their microstructure was examined by optical microscopy. Continuum finite element (FE) analysis, with an isotropic J2 (von Mises) plasticity constitutive law, was used to simulate the strain history experienced by the material during rolling. This strain history was used as an input for viscoplastic self-consistent (VPSC) polycrystal plasticity simulations of texture evolution. The ultimate goal of the combined FE-VPSC framework is to allow the effect of variations in rolling process parameters on the texture to be predicted a priori. Measured texture data of uranium samples reveals that (020) poles will align with the rolling direction of the foil. An additional feature of the texture has the (002) poles aligning close to the normal direction of the foil, but tilted towards the transverse direction. Predicted textures via the FE-VPSC system capture the alignment of the (020) poles along the rolling direction, and predict that the (002) poles will align with the normal direction with a tilt towards the rolling direction. Comparison of predicted and measured textures with other texture data in the literature reveal that the effect of recrystallization must be considered. The elastic and thermal properties were predicted based upon the functional forms of single crystal thermo-elastic properties and self-consistent polycrystal averaging. Plastic properties were predicted via VPSC’s capability to generate yield surfaces and stress-strain curves based upon inputs consisting of the single crystal plastic behavior a texture input. The property predictions reveal that the resulting foil will exhibit near elastic isotropy, moderate thermal expansion anisotropy, and strong plastic anisotropy. Magnesium alloy tubes offer an attractive combination of strength and density. However, their poor crushing behavior under axial loads has prevented their use in crash-sensitive automotive structures such as crush rails. Previous investigations suggest that it is possible to achieve dramatic modifications to both strength and ductility of magnesium alloys through a combination of alloying, grain refinement, and texture control. The texture evolution was predicted using the viscoplastic self-consistent (VPSC) crystal plasticity model, with strain path input from continuum-based FE simulations of extrusion. Magnesium alloy tube samples were created and post-extrusion textures were measured both by x-ray diffraction and electron backscatter diffraction. The measured textures had a combination of features commonly observed in extruded and rolled magnesium textures. Pole figure plots of these textures show the (00.2) poles aligning with the radial direction of the tube as well as the (10.0) poles sweeping from the extrusion direction to the hoop direction of the tubes. Crush testing of the tubes reveal that the WE43 magnesium alloy exhibits superior energy absorption. The crush behavior of the Mg alloy tubes is discussed in terms of their crystallographic texture. Textures from the WE43 samples contain additional features, with some (00.2) poles aligning 45° from the extrusion direction, as well as along the extrusion direction. These additional features orient favorably for slip deformation and impede c-type extension twinning, respectively. This situation in which the texture causes the sample to accommodate strain by slip rather than twinning mechanisms helps to explain the absence of a twinning-type plateau in the force v. displacement curves generated via the axial crush tests performed on the WE43 samples.

MS (Master of Science)

texture; magnesium; uranium; abaqus

English

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2016-08-01