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

非技术摘要材料开发是并将继续是促进和保持美国经济竞争优势的基本研究领域。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材料类别之一。尽管冶金和材料科学取得了几十年的进步，但金属材料在变形过程中发生的一些过程尚不清楚，因为它们发生在原子尺度上，即使是最先进的实验也不能提供确凿的证据。这项工作是在这种由原子水平的过程控制的材料的机械行为的背景下进行的。PI将结合原子分辨率实验的计算机模拟来研究金属的不连续变形，这是一种不受欢迎的效应，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。这份提案为学生提供了一个独特的机会，让他们能够以综合的方式学习高级计算机建模和实验技术。我们相信，这种专业知识对于进入美国工程工作队伍的下一代材料科学家来说是必不可少的。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来吸引和与反映南加州在种族、性别和社会经济背景方面的多样性的学生合作。技术摘要金属合金中的锯齿状流动(也称为Portevin-Le Chatelier效应，或PLC)是动态应变时效的一种特殊情况，这种情况源于位错和溶质原子的相互作用和共同进化。当两种物质在相似的时间尺度上运动时，溶质拖曳和溶质钉扎的共同作用会引起应力-应变曲线的振荡，从而可能导致不均匀变形和延性损失。本项目的目标是了解和模拟体心立方(BCC)稀疏间隙固溶体中PLC效应的微观机制，并预测一些具有重要技术意义的BCC合金的应变率-温度关系图。所提出的方法是将微结构演变与本构响应以物理方式联系在一起。PI建议对节位错滑动和溶质扩散进行动力学蒙特卡罗模拟，其中两者之间的联系是通过应力场耦合自洽完成的。模型的参数化完全由第一性原理计算完成，没有可调的参数。这种方法将通过特别定制的声发射实验和适当大小的样品的原位透射电子显微镜纳米机械测试相结合来验证。将获得一些技术上相关的体心立方合金(Fe-N、Fe-C、Mo-O、V-O)的应变率-温度关系图，以预测(并最终避免)发生锯齿状流动的操作区域。

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.