Grain boundary strengthening in nanocrystalline Mo_0.2CoNi medium-entropy alloys produced via high-pressure torsion

In this study, we produced nanocrystalline (NC) MoCoNi-based medium-entropy alloys (MEAs) with a single face-centered cubic (FCC) phase, which exhibit exceptional hardness through combining compositional and microstructural features of alloys. The alloys were prepared via powder metallurgy and high-pressure torsion, and the processing condition was optimized to achieve full densification and prevent grain growth in order to maximize the effect of grain boundary strengthening. Consequently, NC Mo_0.2CoNi MEAs revealed the refined microstructures with a grain size of approximately 50 nm and outstanding hardness of 857 HV (corresponding to approximately 2.86 GPa in yield strength). This remarkable property, even compared to the single FCC phase alloys with similar grain size, is attributed to the existence of alloying elemental atoms of Mo with large atomic radius, which leads to severe lattice distortion allows to improve friction stress as well as the Hall-Petch coefficient. This study aims to discuss the potential of developing hard metallic materials by using grain boundary strengthening from alloys with large atomic size difference.