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

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础研究领域。用于结构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。尽管冶金学和材料科学取得了几十年的进步，但在金属材料变形过程中发生的一些过程仍未被理解，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。这项工作是在由原子水平过程控制的材料的机械行为的背景下进行的。PI将在原子分辨率实验中结合计算机建模来研究金属的不连续变形，这是一种不良影响，但在冶金中非常普遍。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。这个建议为学生提供了一个独特的机会，使他们能够综合运用先进的计算机建模和实验技术。我们相信，这种专业知识对于进入美国工程劳动力的下一代材料科学家至关重要。此外，拟议中的工作将利用加州大学洛杉矶分校的项目和基础设施，吸引并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：金属合金中的锯齿流（也称为Portevin-Le Chatelier效应或PLC）是由位错和溶质原子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运动时，溶质拖拽和溶质钉住的共同作用会引起应力-应变曲线的振荡，从而导致不均匀变形和延性丧失。该项目的目标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，并预测许多技术上重要的bcc合金的应变率与反温度图。提出的方法是制定这样的微观结构演变是连接到本构响应的物理方式。PI建议对关节错位滑动和溶质扩散进行动力学蒙特卡罗模拟，其中两者之间的连接通过应力场耦合自一致地完成。模型的参数化完全由第一性原理计算完成，没有可调参数。这种方法将通过特别定制的声发射实验和适当尺寸的原位透射电子显微镜纳米力学测试来验证。将获得许多技术相关的bcc合金（Fe-N, Fe-C, Mo-O, V-O）的应变率与逆温度图，以预测（并最终避免）发生锯齿状流动的操作制度。

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.