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A novel electroformed nanocrystalline nickel cobalt alloy (n-NiCo) developed for sheet metal applications was investigated in terms of microstructure, as well as mechanical, thermal stability, and corrosion properties. The as-received material with a grain size of 18 ± 5 nm exhibited enhanced hardness compared with conventional polycrystalline NiCo mainly due to the Hall–Petch strengthening effect. Differential scanning calorimetry and annealing treatment results revealed that the n-NiCo was stable up to 200 °C for at least 1 h, and the onset of abnormal grain growth was observed when the material was annealed to 250 °C. While low-temperature annealing treatment was shown to increase the hardness of the material slightly, annealing at temperatures of 300 °C and above resulted in a reduction in hardness due to rapid normal grain growth. The n-NiCo sheet exhibited mixed Ni/Co anodic polarization behavior in environments of varying pH. [ABSTRACT FROM AUTHOR]

High-entropy alloy (HEA) design is an emerging methodology that stabilizes nanocrystalline materials, which themselves are highly sought after due to their high strength and hardness resulting from grain boundary hardening. In the past decade, studies have examined the synthesis and performance of such materials made primarily from vapour- and solid-state processing methods, which offer little flexibility in scalability. By contrast, electrodeposition is a relatively low-cost, highly scalable fabrication route for nanocrystalline materials that can be applied to almost any substrate shape or material, which also has industrial-scale infrastructure already in place. Here, we report the first comprehensive study on the synthesis and characterization of electrodeposited HEAs made from aqueous electrolytes, building on the groundwork of past work on electrodeposited medium-entropy alloys. High-quality alloys of NiFeCo-W, NiFeCo-Mo, and NiFeCo-MoW were deposited onto Cu substrates at thicknesses in the range 31–44 µm. W-containing alloys took on banded structures with underlaying growth-oriented features that are postulated to be nanoglass domains. NiFeCo-Mo exhibited the highest hardness-to-density ratio among these alloys, owing to its laminated nanocrystalline-amorphous structure. We also revealed an inverse Hall-Petch relationship in NiFeCo-W, as well as a linear dependence of hardness on Ni content in the alloys. Our study continues to build a framework for electrodeposition of HEAs, paving the way for developing new high-strength, thermally stable, and scalable nanostructured materials. [Display omitted] • First comprehensive investigation of the use of electrodeposition to fabricate nanostructured HEAs. • Alloys containing W exhibit what is postulated to be nanoglass character, with boundaries elongated in the growth direction. • Post-process annealing can be used to drive nanocrystal formation in amorphous precursor alloys. • Inverse Hall-Petch hardening exists in NiFeCo-W alloys at grain sizes below 10 nm. [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]

The Hall-Petch to inverse Hall-Petch (HP-IHP) transition of an electrodeposited nanocrystalline Ni-32at%Co alloy was examined by annealing the as-plated alloy to obtain grain sizes ranging between 18 – 239 nm. We describe this transition as an extrinsic HP-IHP transition (extrinsic to the fabrication process) as opposed to the intrinsic HP-IHP transition observed in materials in their as-plated state (intrinsic to the fabrication process). The extrinsic transition of the Ni-32at%Co alloy took place in the mid-stage of abnormal grain growth, at an average grain size of ∼30 nm. Grain boundary relaxation governs the IHP region and maximum hardness was achieved at initial development of bimodal grain size; meanwhile late-stage abnormal grain growth and further normal grain growth led to the reduction in hardness in the HP region. Segregation of sulfur impurities to grain boundaries, crystallographic orientation and annealed twin development did not have significant contributions to the observed change in hardness. [Display omitted] [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]