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• 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. [ABSTRACT FROM AUTHOR]

The formation of "mud-cracking" networks in coatings and thin films is a widely observed phenomenon, particularly in Cr-containing electrodeposited materials. Here we have documented and described aspects of mud-cracking specific to Cr-containing electrodeposited alloys made from trivalent Cr electrolytes. In addition to describing the different crack patterns observed in these materials and their relationship to the electroplating parameters, thickness, and composition of the coatings, we also investigated a never-before-reported phenomenon of post-deposition time-dependent cracking. Our analysis shows that cracks continue to form after the electroplating procedure has been completed, generally saturating 2 h after synthesis, but can continue for days afterwards. We theorize that the behaviour is related to the state of oxygen in these samples, which can sometimes reach beyond 30 at.%. It is believed that the breakdown of Cr-hydroxides and Cr-hydrides drives the increase in internal stresses that result in continued cracking. As a result, this time-dependent cracking phenomenon may impact the suitability of certain Cr-alloy electrodeposits in some applications. Unlabelled Image • Crack networks form in Cr-alloy electrodeposits during and after their synthesis. • Crack morphology is dependent on synthesis parameters, thickness, and composition. • Cr(III)-based alloys contain large amounts of oxygen. • Volume changes from Cr(OH) 3 impurity decomposition to Cr 2 O 3 drives crack growth. [ABSTRACT FROM AUTHOR]

• Similar overall polarization behaviour for conventional poly- and electrodeposited nanocrystalline Ni-Fe alloys. • Differences in anodic polarization behaviour in neutral environment attributed to sulfur impurities more so than grain size. • Negligible difference in corrosion behaviour in alkaline environment irrespective of grain size and composition. Potentiodynamic polarization behaviour of electrodeposited nanocrystalline and conventional polycrystalline Ni-Fe alloys of similar compositions was investigated in neutral (pH 7) and alkaline (pH 10) 0.5 M Na 2 SO 4 environments. In neutral solution, elevated current densities and delayed passivation were observed for all nanocrystalline materials, independent of composition. This behaviour was attributed to sulfur, present in the nanocrystalline materials as an impurity, more so than an effect of grain refinement. In alkaline solution, the effect of sulfur was exceeded by oxide film formation, and no significant difference in corrosion behaviour was observed as a function of grain size i.e. intercrystalline volume fraction. [ABSTRACT FROM AUTHOR]

Electrodeposited nanocrystalline and conventional polycrystalline NiFe alloys of similar iron content were investigated to understand the effect of grain size on the corrosion behaviour of these materials. Potentiodynamic polarization testing in 1 N H 2 SO 4 revealed similar active-passive-transpassive behaviour with negligible differences in the Tafel and transpassive regions regardless of grain size. It is shown that differences in the region of active dissolution are the result of strong contributions of sulfur impurities in the nanocrystalline electrodeposits rather than being due to grain size alone. [ABSTRACT FROM AUTHOR]

• Using PARXPS, passive films on NiFe alloys of varying grain size were investigated. • Poly- and nanocrystalline NiFe alloys exhibited similar passive film compositions. • Grain size reduction increases the anodic activity of the alloy, affecting passivity. • Current transients suggested increased passivation kinetics for nano NiFe alloys. • Passive currents of poly- and nanocrystalline NiFe were similar (order-of-magnitude). Electrodeposited nanocrystalline NiFe alloys are compared to their polycrystalline, coarse-grained counterparts to understand the effects of grain size reduction on the passivation kinetics and the chemical structure of the passive film. The passive film formed in acidic sulfate environment was characterized using parallel angle-resolved XPS (PARXPS) to obtain non-destructive compositional depth profiles. The composition (Ni:Fe) of the passive film was insensitive to grain size; however, grain size reduction resulted in increased anodic activity producing thicker films that contained increased concentrations of higher oxide species. [ABSTRACT FROM AUTHOR]