Unraveling the recrystallization mechanisms in refractory multi-principal element alloys via mesoscale modeling and in situ nanoscale characterization

NON-TECHNICAL SUMMARY Steels, copper, aluminum, and nickel alloys are and have been the main backbone of the current world economy for over a century. However, as humankind explores the edge of what is known and what is possible, modern applications are pushing beyond the tolerable limits that these alloys can withstand. Advanced metal alloys based on new compositional paradigms are called to play a critical role in the economy of the future, and –within them– the so called ‘high-entropy’ alloys are emerging as attractive alternatives to current materials with potentially far superior properties. Among these properties is the potential for higher temperature operation than is currently attainable, which can help enable significant technological breakthroughs such as ultra-tough metals, irradiation-resistant coatings, and hypersonic flight. This research project is investigating the fundamental mechanisms and exploring the limits of operation at maximum temperatures possible for a specific type of high-entropy alloy made of refractory metals. One of the main objectives of this research is understanding the mechanisms behind the loss of structural integrity due to recrystallization by combining advanced computational modeling with in-situ characterization. This project is developing tools to optimize microstructure and composition to push the high temperature envelope of these alloys, and broadly sharing the results through national data initiatives and publications in high-impact scientific journals. This project is also involving and exposing high school, undergraduate and Armed Forces veteran students to the topics of material science through several UCLA outreach programs. The knowledge generated through this project is enhancing existing course curricula in engineering by integrating current research discoveries into course syllabi, and accelerating the design, optimization and application of refractory high-entropy alloys for extreme-environment applications. These advances support U.S. leadership in advanced manufacturing and energy technologies. TECHNICAL SUMMARY Refractory multi-principal element alloys are a class of advanced structural materials that retain exceptional strength at temperatures beyond the operating limits of conventional metallic alloys, making them promising candidates for next-generation structural, energy, and nuclear applications. However, deformation at elevated temperatures can induce a phenomenon known as dynamic recrystallization, by which the alloy’s microstructure may be fully altered, leading to mechanical property loss and material failure. Moreover, in body-centered-cubic refractory multi-principal element alloys, dynamic recrystallization is seen to produce anomalous necklace microstructures through localized grain boundary bulging. However, the nucleation of this necklace structure at the atomic scale is not yet understood. This project is combining in-situ experimental techniques and state-of-the-art theoretical/computational tools to understand the imitation and onset of recrystallization in refractory multi-principal element alloys. The model is connecting microstructural evolution, local chemical composition and fundamental deformation mechanisms with dislocation motion and grain boundary evolution. In-situ electron microscopy and micropillar compression experiments containing isolated grain-boundary triple junctions, (where necklace microstructures preferentially nucleate) are being used to validate the theoretical model. This combined computational and experimental approach is providing new insights into the role of dislocations in high-temperature deformation and is determining whether grain boundary sliding is an active mechanism during dynamic recrystallization. The knowledge gained from this research is providing fundamental insight into the deformation of refractory multi-principal element alloys at high temperatures thereby, enabling the design of materials with enhanced high-temperature performance. To maximize the impact of this research project, tools and findings are being broadly disseminated through existing NSF-sponsored data initiatives, as well as via publications in high-impact, peer-reviewed scientific journals. The project is also involving students in science and engineering activities via undergraduate research opportunities and outreach to middle school students, high school students as well as military veteran students. The knowledge being generated through this project is being incorporated into existing course curricula and enriching classroom instruction with current research advances. In this way, the project is exposing students to modern experimental and computational techniques while contributing to the preparation of the next generation of scientists and engineers. 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.