Nitrogen-induced hardening of refractory high entropy alloys containing laminar ordered phases

Most attempts to improve the properties of high entropy alloys (HEAs) involve the exploration of nonequimolar compositions, addition of alloying elements, and/or manipulation of the microstructure. Alternatively, this work reports on intentionally doping HEAs with non-metallic species to precipitate coherent, ordered phases in order to maximize hardness. A refractory MoNbTaW HEA was synthesized via cryogenic mechanical alloying and doped with nitrogen by using liquid nitrogen as the cryogen. Overall, the doping strategy was successful as multiple nitrogen-rich secondary phases were observed. In particular, a unique ordered laminar phase was developed and identified as tetragonal (Mo,W)(Nb,Ta)N nitride via aberration-corrected scanning transmission electron microscopy (STEM). Another complex (Nb,Ta)(2)CN carbonitride was also identified. The growth behaviors of the ceramic phases were studied using long term aging treatments of up to 100 hours at 1200 degrees C. A second MoNbTaW alloy was also prepared via high-energy ball milling, without nitrogen, by using liquid Ar as the cryogen. A comparison of the two alloys' microstructures and properties confirm that intentional formation of complex nitride phases greatly enhanced the hardness of mechanically alloyed MoNbTaW by up to 3-4 GPa. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study explores intentionally doping high entropy alloys with non-metallic species to precipitate ordered phases and enhance hardness. The successful doping of nitrogen resulted in multiple nitrogen-rich secondary phases, including a unique ordered laminar phase and a complex carbonitride. Long term aging treatments and comparison with undoped alloys confirmed that intentional formation of complex nitride phases greatly increased the hardness by up to 3-4 GPa.