A Microwave Plasma Approach for Manufacturing Refractory High Entropy Alloys

Advanced alloys with outstanding properties such as high temperature stability, strength, and ductility are highly sought after for applications serving national defense, space exploration, and power generation. There is a need for developing such alloys that prioritize clean, energy-efficient operations while offering potential for scale-up. Concerns with many conventional manufacturing approaches include energy, time, and financial costs associated with requirements of mechanical milling, melting, sintering, and process scale-up. This award supports fundamental research to develop a particularly promising class of alloys, called high entropy alloys, using the fourth and most ubiquitous state of matter known as “plasma,” activated by microwave energy. Insights from this project will lead to plasma engineering for minimal processing time, temperature, and energy costs with water vapor as primary byproducts and without need for melting. Insights from this study will also provide knowledge about how microwave-induced plasma influences reaction rates and pathways leading to alloy formation, how critical mechanical properties can be measured at the nanoscale and linked to overall macroscopic behavior, and how to translate the process from a small-scale benchtop reactor to a large industrial-scale unit. Additionally, the project will provide opportunities to educate and train graduate and undergraduate students in cross-disciplinary areas of engineering, materials science and physics, as well as provide exciting opportunities for K-12 students through local outreach programs. Concerns with conventional manufacturing methods such as arc melting for high entropy alloys include potential for solidification defects or severe chemical and phase segregation. Powder metallurgy routes such as mechanical alloying or field-assisted sintering are time and/or energy intensive and difficult to scale up. To overcome these limitations, this research aims to take advantage of the extremely high reactivity, focused energy distribution, and strongly non-equilibrium nature of a microwave-induced hydrogen plasma to produce refractory high entropy alloys from metal oxide precursors. Experiments will elucidate the role of reactive (hydrogen radical) vs inert (argon) plasma on thermodynamics and kinetics to push the limits toward lower temperature and/or lower processing time. Spherical nanoindentation will gauge local nanoscale deformation including incipient plasticity and elastoplastic deformation mechanisms to investigate strength/ductility behavior for the alloys studied. Scale-up of the process to a multi-source large area plasma system will enable an understanding of how key experimental parameters (such as microwave pulse frequency/duration) are expected to influence atomic hydrogen production and plasma energetics for high-throughput alloy production. 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.