Orientation dependence of dislocation structure evolution of aluminum alloys in 2-D and 3-D

by 1979- Merriman, Colin Clarke

by Colin Clarke Merriman, M. S. Washington State University August 2007 Chair: David Field A proper understanding of the relationships that connect deformation, microstructural evolution and dislocation structure evolution is required to extend service lifetime of components, reduce the manufacturing costs, and improve product quality. This requires significant efforts in performing accurate analysis of undeformed and deformed microstructure and identifying the microstructural response to an applied stress, be it in compression tension, or fatigue. Current models are based on observed phenomenology of the process and therefore fail to predict microstructural response of a material beyond a given set of known parameters. Current research is aimed towards making contribution in the areas of (i) microstructural characterization, (ii) understanding the influence of various microstructural parameters on the evolution of dislocation structures and (iii) on relating the physically measurable microstructural parameters to stress response. In a continuing effort to improve characterization of the dislocation structures of materials the local orientation gradient in deformed polycrystalline samples is examined by the collection of electron back-scatter patterns. Along with the lower bound calculation of the excess dislocation content (planar dataset), a 3-D excess dislocation density calculation is introduced, for serial section datasets, to better understand the bulk microstructural response. In addition, the iv excess dislocation density dependence on step size is examine to determine if there is proper step size to be used to for the excess dislocation density calculation. Microstructural evolution during small and large strain channel die deformation of aluminum alloy (AA) 1050 and AA 7050 T7541 was investigated using SEM techniques. From this the orientation dependence of dislocation structures was examined through the initial texture of the material and the plotting of excess dislocation content and Taylor factor in orientation space. It was observed that the Taylor factor and the initial texture has an influence on the deformation behavior and dislocation evolution of aluminum. Neighboring grains (including lattice orientation and dislocation content) and precipitate morphologies also were observed to play a significant role in the microstructural and dislocation response. The observed difference in the evolution of dislocation structures of AA 1050 and AA 7050 T7541 were attributed to their varying manufacturing parameters and differing alloy content. v