Microstructural Evolution During Materials Processing

这个研究项目创建了分析模型来描述金属合金在快速连续加热过程中经历陡峭热梯度的微观结构演变，例如在焊接的热影响区。本研究发展了在共晶温度及以上的微观组织演变的解析解和数值解。微观组织演化模型预测沿晶界形成的液膜的晶粒尺寸、析出物溶解、加热时的液化和冷却时的凝固。实验研究补充了分析模型和数值模型。研究了四种实验合金体系：高纯金属（界面迁移的溶质阻力最小）；二元固溶体合金（界面迁移时溶质拖拽）；无固溶性的两相二元合金（溶质阻力最小的齐纳钉钉）；以及具有溶解度的两相二元合金（具有溶质阻力的齐纳钉钉）。通过数据分析，验证了焊接热影响区热循环的有限元热流模型。在实验系统上制备了精心制备的自燃气体钨极弧焊。晶粒尺寸、析出物溶解和液膜厚度作为热影响区位置的函数进行了测量。分析透射电子显微镜分析是对样品进行特别制备，以强调沉淀溶解微观结构。分析的目标是生成界面和基质化学成分梯度的数据，以便与沉淀溶解的分析模型进行比较。焊接组件的强度取决于焊缝附近的焊缝金属和母材的强度（称为焊缝热影响区）。本研究提供了一种预测微结构相关缺陷的方法，并提供了避免其发生的设计信息。＊＊＊

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. ***