Determining the dependence of grain boundary mobility on misorientation in high purity aluminum with zirconium additions

David J. Shields

doi:10.17918/etd-4304

Aluminum alloys are used frequently for applications within the aerospace industry, creating a demand for finely tuned aluminum alloys that maximize a material property of interest (strength, toughness, etc.) and minimize weight. In order to formulate more complex alloys, it is important to understand how alloying elements affect the kinetics of grain growth in the solvent system. This study analyzes the effect of small concentrations of zirconium in high purity aluminum on grain growth during primary recrystallization by empirically determining the grain boundary mobility via measuring grain boundary velocity as well as stored energy within a sample and correlating grain boundary misorientation to mobility data. Grain boundary velocity is measured by annealing single crystal samples of deformed high purity aluminum with Zr additions in a box furnace and completing orientation imaging microscopy (OIM) scans that use electron back scattered diffraction (EBSD) patterns to index a lattice and create an inverse pole figure (IPF). This inverse pole figure assigns colors to orientations of crystalline grains and allows for the tracking of grain boundaries after subsequent heat treatments as well as for the acquisition of the misorientation at any given grain interface. TSL software allows for analysis of the EBSD data which can calculate the average subgrain size and average misorientation within a region to provide a stored energy term. This value is used in tandem with micro hardness measurements to estimate stored energy. With a measured grain boundary velocity and stored energy, it is possible to calculate grain boundary mobility and correlate mobility with grain boundary misorientation. Grain boundary Mobility is a useful parameter to metallurgists as a predictor of grain size after deformation and heat treatments. However, grain boundary mobility has a variety of variables that are subject to change with composition, and is thus difficult to calculate. As such, it is necessary to experimentally determine the grain boundary mobility in unexplored alloy compositions for modeling as well as processing. Alloying metals for use in industry requires knowledge of how alloying elements will alter the processing parameters used to generate a desired set of properties. Thus by determining the bulk grain boundary mobility of high purity aluminum samples with Zirconium additions via heat treatment, this work validates the combined use of EBSD and microhardness as a useful means of collecting data that replicates previous results obtained in the literature. Stored energy results obtained in this work align well with values in the literature obtained by Huang and Humphreys as well as values obtained from microhardness by Taheri as well as Huang, Tao and Lu. The dependence of grain boundary mobility on misorientation is also seen to correspond well with results in the past by Taheri, Gottstein and Rollett with boundaries near 40°<111> having higher mobility than random HAGBs. Additionally, data found in this study aligns with results predicted by the effect of preferential Zr segregation observed by Taheri. However, the less prominent shift of peak mobility from 40° to higher misorientation as anneal temperature increases is in contrast with previous results, calling into question if there is a true difference in observed mobility peaks between 38° and 40°.

Determining the dependence of grain boundary mobility on misorientation in high purity aluminum with zirconium additions

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Determining the dependence of grain boundary mobility on misorientation in high purity aluminum with zirconium additions

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