Microstructural Evolution During Materials Processing

9417326 Thompson本研究计划创建分析模型，以描述在快速连续加热过程中经历陡峭温度梯度的金属合金的微观结构演变，例如在焊接件的热影响区。这项研究开发了在共晶温度及以上的组织演变的解析和数值解。微观组织演变模型预测了沿晶界形成的液膜的晶粒度、沉淀溶解、加热熔化和冷却凝固。实验研究补充了分析和数值模型。研究了四个实验合金系统：高纯金属(溶质对界面迁移的最小阻力)、二元固溶体合金(溶质对界面的迁移)、无固溶度的两相二元合金(最小溶质阻力的齐纳钉扎)和具有固溶度的两相二元合金(溶质拖动的齐纳钉扎)。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。在实验系统上进行了精心准备的钨极气体保护焊焊接。测量了颗粒尺寸、析出物溶解和液膜厚度作为热影响区位置的函数。分析性的透射电子显微镜分析是对特别准备的样品进行的，以突出沉淀溶解的微观结构。分析的一个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与焊缝相邻的焊缝金属和母材的强度(称为焊接热影响区)。这项研究提供了一种预测与微结构相关的缺陷的方法，并为避免它们的发生提供了设计信息。***

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. ***