Electrodeposited nanocrystalline medium-entropy alloys – An effective strategy of producing stronger and more stable nanomaterials

•First systematic approach to use electrodeposition to synthesize nanocrystalline MEAs for mechanical applications.•Electrodeposited nanocrystalline MEAs show superior thermal stability to nanocrystalline binary alloys and pure metals.•Regular and inverse hardening mechanisms exist in electrodeposited MEAs.•Exploitation of electrodeposition to fabricate nanocrystalline HEAs could further improve their strength and stability. [Display omitted] For decades, enhancing both strength and thermal stability in nanocrystalline materials for structural applications has been a significant challenge. Recently, entropy-based stabilization strategies for nanostructured materials have gained traction in the high-entropy alloy (HEA) community, however, such studies typically focus on synthesis techniques that require high energy input, such as severe-plastic-deformation or sputtering-based techniques. By contrast, electrodeposition offers itself as a low-energy and low-cost method of producing nanocrystalline materials, which has seen infrequent investigation in the HEA design space. Here we identify two nanocrystalline medium-entropy alloys (MEAs), NiFeCo (grain size, D = 13–360 nm) and NiFeCr (D ~ 1 nm), to serve as a baseline for the further development of higher-order quaternary and quinary systems. These alloys show enhanced thermal stability when compared to pure metal and binary alloy electrodeposits, and even nanocrystalline CoCrFeNiMn (made by high-pressure torsion) in the case of NiFeCr. Hardness values ranged from 3.4-5.5 GPa in NiFeCo and 4.6–8.5 GPa in NiFeCr, which are comparable with nanocrystalline HEAs made by other techniques. This study provides a framework for the development of nanocrystalline HEAs by electrodeposition, whose further development has the potential to accelerate the commercialization of HEAs, which currently have limited use despite their widespread acclaim in the materials community.