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

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Nontechnical Abstract Materials 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 Abstract Serrated 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.