Theoretical Understanding of Porosity-Induced Mechanisms during Solidification of Cast Alloys and their Influence on Process-Structure-Property Correlations

铸造是将液态金属倒入模具，然后冷却和凝固的过程。铸造合金被用于许多不同的应用，从汽车部件到航空航天部件。长期以来，铸造合金在凝固过程中产生的气孔一直被认为是限制其机械性能的关键因素，特别是在重复使用会导致“疲劳”行为的情况下。通过铸造设计和凝固过程的优化相结合，有一个强大的驱动力来最小化气孔率。为了实现这一目标，有必要对气孔形成、生长和迁移率的基本物理及其对疲劳的影响有一个基本的了解，并加强预测能力。这项研究通过在多个长度尺度上对过程和行为进行理论和计算建模来满足这些需求。这项研究的结果将导致具有时间和成本效益的设计和优化过程，汽车、航空航天和其他行业可以利用这些过程来获得更好的合金和合金部件。这项研究以材料科学、制造和计算力学为基础的协同方法为特色，将使研究生接触到更广泛的概念和技能。这项研究的目的是对定向凝固过程中气孔形核、生长和迁移的机制有一个基本的了解，并利用这一了解来确定铸造合金中的工艺-组织-性能关系。这一目标将在利用理论和计算工具对现有凝固实验数据进行分析和解释的基础上实现。控制气孔形核(凝固收缩和氢偏析)、生长(氢扩散、凝固前沿生长和枝晶生长)和迁移(浮力和热毛细效应)的竞争和贡献机制将被理论化和模型化。多尺度计算框架将由发展的相场模型和现有的内部状态变量模型组成，结合第一性原理计算、晶体塑性和有限元方法，将能够在多个长度尺度上对凝固过程进行唯象解释。然后，累积的本构描述将被用来建立工艺-结构-性能的相关性。

Casting is a process in which liquid metal is poured into a mold and then allowed to cool and solidify. Cast alloys are used in many diverse applications, ranging from automotive parts to aerospace components. Porosity that arises during the solidification process has long been recognized as a critical factor in limiting the mechanical performance of cast alloys, especially under repetitive use that results in "fatigue" behavior. There is a strong drive towards minimizing porosity through a combination of casting design and optimization of the solidification process. To achieve that, a fundamental understanding of the underlying physics of porosity formation, growth and mobility and its impact on fatigue, and an enhanced predictive capability become necessary. This research addresses these needs through theoretical and computational modeling of processing and behavior at multiple length scales. Results from this research will lead to time- and cost-effective design and optimization processes that automotive, aerospace, and other industries can utilize to obtain better alloys and alloy components. This research features a synergistic approach based on materials science, manufacturing, and computational mechanics, which will expose graduate students to broader concepts and skills. Active focus will be given to ensure participation of underrepresented undergraduate students in research and outreach to high school students will strive to inspire interest in engineering education from an early age.The objective of this research is to acquire a fundamental understanding of the mechanisms that govern porosity nucleation, growth, and migration during directional solidification process and employ that understanding to identify process-structure-property relations in cast alloys. This objective will be accomplished based on analysis and interpretation of existing solidification experimental data using theoretical and computational tools. Competing and contributing mechanisms that control porosity nucleation (solidification shrinkage and hydrogen segregation), growth (hydrogen diffusion, solidification front grow, and dendritic growth), and migration (buoyancy and thermocapillary effects) will be theorized and modeled. The multiscale computational framework, which will consist of a developed phase-field model and an existing internal state variable model combined with first-principles calculations, crystal plasticity, and the finite element method, will enable a phenomenological interpretation of the solidification process at multiple length scales. The accumulated constitutive descriptions will then be used to develop process-structure-property correlations.