Understanding the Fundamental Mechanisms of Serrated Flow in BCC Alloys and their Impact on Mechanical Response: A Validated Mesoscopic Computational Study

材料的发展是并将继续保持一个基本的研究领域，以促进和保持美国经济的竞争优势。结构中使用的金属材料仍然是工业，基础设施和技术中最重要的一类材料。尽管在冶金学和材料科学方面取得了数十年的进步，但在金属材料的变形过程中发生的一些过程尚未被理解，因为它们发生在原子尺度上，即使是最先进的实验也无法提供确凿的证据。这项工作的框架内，由原子级过程控制的材料的力学行为的背景下。PI将在原子分辨率实验中使用计算机建模的组合来研究金属的不连续变形，这是一种不受欢迎的效果，但在冶金学中非常普遍。这种方法将使PI能够了解控制这些过程的因素，以便为下一代金属结构材料提出解决方案。该提案为学生提供了一个独特的机会，以综合的方式研究先进的计算机建模和实验技术。我们相信，这种专业知识对于下一代材料科学家进入美国工程劳动力市场至关重要。此外，拟议的工作将利用加州大学洛杉矶分校的计划和基础设施，以吸引和与学生的工作，反映了在南加州发现的多样性，在种族，性别和社会经济背景方面。（也称为波特万-勒夏特列效应，或PLC）是由位错和溶质原子的相互作用和共同演化引起的动态应变时效的一种特殊情况。当两种物质在相似的时间尺度上移动时，溶质拖曳和溶质钉扎的综合效应引起应力-应变曲线的振荡，这可能导致不均匀的变形和延展性的损失。本项目的目的是了解和模拟体心立方（bcc）稀间隙固溶体中PLC效应的微观机制，并预测一些技术上重要的bcc合金的应变率与逆温度图。所提出的方法是制定这样的微观结构的演变是联系在一起的本构反应的物理方式。PI建议对关节位错滑移和溶质扩散进行动力学Monte Carlo模拟，其中两者之间的连接通过应力场耦合自洽地完成。模型的参数化完全是用第一原理计算完成的，没有可调参数。这种方法将使用专门定制的声发射实验和原位透射电子显微镜纳米力学测试的适当大小的标本相结合进行验证。将获得许多技术上相关的体心立方合金（Fe-N、Fe-C、Mo-O、V-O）的应变率-逆温度图，以预测（并最终避免）出现锯齿形流动的操作状态。

Nontechnical AbstractMaterials development is and will continue to remain a fundamental area of research to advance and preserve the competitive advantage of the US economy. Metallic materials used in structures still remain one of the most important class of materials in industry, infrastructure, and technology. Despite decades of advances in metallurgy and materials science, there are some processes that occur during the deformation of metallic materials that are not yet understood because they occur at the atomic scale, where even the most advanced experiments cannot provide conclusive evidence. This work is framed within this context of mechanical behavior of materials controlled by atomic-level processes. The PI will use a combination of computer modeling at atomic-resolution experiments to study discontinuous deformation of metals, which is an undesirable effect but highly prevalent in metallurgy. This approach will allow the PI to gain an understanding of the factors controlling these processes so that solutions to it can be proposed for the next generation of metallic structural materials. This proposal provides a unique opportunity for students to work on both advanced computer modeling and experimental techniques in an integrated fashion. We believe that this expertise is essential in the next generation of materials scientists entering the US engineering workforce. Further, the proposed work will take advantage of UCLA's programs and infrastructure to attract and work with students that reflect the diversity found in Southern California, in terms of ethnicity, gender, and socio-economic background.Technical AbstractSerrated flow (also known as Portevin-Le Chatelier effect, or PLC) in metallic alloys is a particular case of dynamic strain aging that arises from the interactions and coevolution of dislocations and solute atoms. When both species move on similar time scales, the combined effects of solute dragging and solute pinning give rise to oscillations in the stress-strain curve that may lead to non-uniform deformation and a loss of ductility. The objective of this project is to understand and model the microscopic mechanisms responsible for the PLC effect in body-centered cubic (bcc) dilute interstitial solid solutions and predict the strain-rate versus inverse-temperature diagrams for a number of technologically important bcc alloys. The proposed approach is formulated such that microstructural evolution is linked to constitutive response in a physical way. The PI proposes to perform kinetic Monte Carlo simulations of joint dislocation glide and solute diffusion, where the connection between both is done self-consistently via stress field coupling. The parameterization of the model is done entirely with first-principles calculations, with no adjustable parameters. This approach will be validated using a combination of specially tailored acoustic emission experiments and in-situ transmission electron microscopy nano-mechanical tests of suitably sized specimens. strain-rate versus inverse-temperature diagrams will be obtained for a number of technologically relevant bcc alloys (Fe-N, Fe-C, Mo-O, V-O) to predict (and, ultimately, avoid) the operating regimes within which serrated flow occurs.