Microstructural Evolution During Materials Processing

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9417326 Thompson This research program creates analytical models to describe the evolution of microstructure in metal alloys experiencing steep thermal gradients during rapid continuous heating, such as in the heat-affected zone of weldments. The research develops analytical and numerical solutions to microstructural evolution at and above the eutectic temperature. The models for microstructural evolution predict grain size, precipitate dissolution, liquation during heating, and solidification during cooling of liquid films that form along the grain boundaries. Experimental studies compliment the analytical and numerical models. Four experimental alloy systems are studied: a high purity metal (minimal solute drag on interface migration); a binary solid solution alloy (solute drag on interface migration); a two-phase binary alloy with no solid solubility (Zener pinning with minimal solute drag); and a two-phase binary alloy with solubility (Zener pinning with solute drag). A finite element heat flow model of the weld heat-affected zone thermal cycle is tested by the data analysis. Carefully prepared autogenous gas tungsten arc welds are made on the experimental systems. The grain size, precipitate dissolution, and liquid film thickness are measured as a function of position in the heat-affected zone. Analytical transmission electron microscopy analysis is performed on samples specially prepared to accentuate precipitate dissolution microstructures. A goal of the analysis is to generate data on interface and matrix chemical composition gradients for comparison to the analytical models of precipitate dissolution. %%% The strength of welded assemblies depends on the strength of the weld metal and base metal adjacent to the weld (referred to as the weld heat-affected zone). This research provides a means of predicting microstructure-related defects and provides design information to avoid their occurrence. ***