Subgrains and Dynamic Grain Growth in BCC Metals

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Non-Technical SummaryMetals and their alloys provide the load-bearing structures needed in sophisticated machines that support our modern society. Useful applications range from aircraft to automobiles to bridges. These structural materials are composed of microscopic crystals called grains. The sizes, shapes, and orientations of grains control a material's mechanical properties, such as strength and ductility. These microscopic features, part of the material microstructure, are determined by the manufacturing processes that produce and shape materials into useful machine components. Manufacturing frequently requires deformation at elevated temperatures, the modern equivalent of hot forging by the ancient blacksmith, to control the material microstructure and final component shape. During hot deformation, grains change size and shape in ways that we cannot yet predict or fully explain. This project addresses the fundamental science behind the changes in grain shape, size, and orientation during hot deformation. The goal of this project is to understand the mechanisms responsible for these changes. Through this scientific understanding, opportunities are sought to: 1. improve material performance and 2. identify manufacturing methods that create improved material microstructures. Both graduate and undergraduate students are educated and trained through experimental research. Primary-school teachers are provided summer research experience, training, and resources to bring to their classrooms.Technical SummaryDynamic grain growth involves changes in grain shape, size, and orientation during deformation at elevated temperatures. It is critically important to the processing of many technologically useful metals and alloys. However, the mechanisms that cause it are not yet understood. This experimental study assesses the leading hypothesis among several proposed for mechanisms that control dynamic grain growth. The hypothesized mechanism of subgrains driving high-angle grain boundary migration is investigated for (mechanically) steady-state plastic deformation of body-centered-cubic metals at elevated temperatures. The goal of this study is to characterize and quantitatively measure the effects of subgrain interactions with high-angle boundaries during both dynamic normal grain growth and dynamic abnormal grain growth. Recent advances in high-resolution analysis of electron back-scattered diffraction data are leveraged to provide quantitative characterization of subgrain interactions with grain boundaries. An improved understanding of dynamic grain growth will provide capabilities for predicting and controlling material microstructures during hot deformation processing. This will benefit manufacturing and domestic competitiveness. Integrated with the proposed research are training and educational activities for both graduate and undergraduate students. Outreach involves primary-school teachers participating in a summer program of research experience and curriculum development to positively impact their own students. This outreach focuses on imparting an accessible understanding of materials through hands-on projects and examples that utilize the aesthetic beauty of microstructure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.