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Microwave-induced plasma was used to anneal precursor powders containing five metal oxides with carbon and boron carbide as reducing agents, resulting in high entropy boride ceramics. Measurements of hardness, phase structure, and oxidation resistance were investigated. Plasma annealing for 45 min in the range of 1500–2000 °C led to the formation of predominantly single-phase (Hf, Zr, Ti, Ta, Mo)B2 or (Hf, Zr, Nb, Ta, Mo)B2 hexagonal structures characteristic of high entropy borides. Oxidation resistance for these borides was improved by as much as a factor of ten when compared to conventional commercial diborides. Vickers and nanoindentation hardness measurements show the indentation size effect and were found to be as much as 50% higher than that reported for the same high entropy boride configuration made by other methods, with average values reaching up to 38 GPa (for the highest Vickers load of 200 gf). Density functional theory calculations with a partial occupation method showed that (Hf, Zr, Ti, Ta, Mo)B2 has a higher hardness but a lower entropy forming ability compared to (Hf, Zr, Nb, Ta, Mo)B2, which agrees with the experiments. Overall, these results indicate the strong potential of using microwave-induced plasma as a novel approach for synthesizing high entropy borides.
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High entropy alloy MoNbTaVW synthesized by metal-oxide reduction in a microwave plasma
A unique approach was used to synthesize the high entropy alloy MoNbTaVW via reduction of metal-oxide precursors in a microwave plasma. The metal-oxides underwent ball milling and consolidation before plasma annealing at 1800 °C for 1 h with hydrogen as feedgas. X-ray diffraction, scanning electron microscopy/energy dispersive x-ray analysis, and Vickers hardness testing reveal characteristics of the high-entropy alloy. This includes a predominantly single-phase body-centered cubic structure, homogeneous distribution of all five metals, and 6.8 ± 0.9 GPa hardness, comparable with other reports for the same five-metal high entropy alloy configuration. Localized microwave plasma particle sintering is evident from the microstructure. These results highlight the promising potential of microwave plasma as a fast, economical, and flexible processing tool for high entropy alloys.
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- PAR ID:
- 10594942
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 124
- Issue:
- 10
- ISSN:
- 0003-6951
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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