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Over the past decade, the study of nanostructured high-entropy alloys (HEAs) has attracted great attention due to their high strength (grain boundary hardening) and improved thermal stability over conventional nanostructured metals and alloys. However, a lingering issue yet to be resolved by these studies is that of commercial viability; the synthesis processes used to date are costly, not easily scalable, and energy intensive. This work examines the thermal stability and mechanical properties of electrodeposited nanostructured HEAs, towards establishing a cost-effective and versatile synthesis route to commercialize this underutilized material class. Nanocrystalline, amorphous, and nanoglass alloys of NiFeCoW, NiFeCoMo, and NiFeCoMoW were systematically characterized to assess the stability of their structure and mechanical properties, compared against conventional nanocrystalline materials and HEA counterparts. The structural stability of these electrodeposited HEAs was shown to improve with increasing number of elements, from a peak temperature for grain growth of ∼270 °C in pure nanocrystalline Ni, to ∼500 °C in electrodeposited HEAs, corresponding to a near doubling of the activation energy for grain growth from 1.3 eV (Ni) to a maximum of 3.1 eV (NiFeCoW). This improved stability was matched with a 60 % increase in hardness after annealing to the point of nanograin nucleation (∼500 °C), to a maximum hardness of ∼8.4 GPa. Such hardening trends followed an extrinsic inverse-to-regular Hall-Petch relationship, where the inverse regime was largely dominated by grain boundary relaxation and nanoglass structural breakdown. With these results, we have established electrodeposition as a promising candidate route for fabricating nanostructured HEAs for applications at elevated temperatures. [Display omitted] • First comprehensive investigation of mechanical properties and thermal stability of electrodeposited nanostructured high-entropy alloys. • The family of NiFeCoMoW alloys is structurally stable up to ∼500 °C. • Annealing brings out an extrinsic inverse-to-regular Hall-Petch hardening transition. [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]

In this study, a detailed investigation is conducted on cold sprayed copper coatings in their as-deposited state as well as after annealing at 350 °C for 1 h. The local variations of microstructure and hardness are revealed through a combination of electron backscattered diffraction and high-speed nanoindentation mapping techniques. While a highly heterogeneous microstructure is present in the as-deposited material, annealing results in a more uniform grain size, an increase in the fractions of twin and twin-related boundaries and a decrease in the density of geometrically necessary dislocations and low angle grain boundaries. The relative contributions of grain boundary hardening and work hardening are evaluated via hardness line profile analyses. The identification of oxide films along splat-splat interfaces suggests a potential additional factor contributing to local hardness. • Cold sprayed Cu materials for used nuclear fuel containers were studied using EBSD and nanoindentation. • Both microstructural and mechanical heterogeneity decreased after annealing. • The relative contributions of grain boundary hardening and work hardening were evaluated. • Oxide films along the interfaces of adjacent splats were identified. [ABSTRACT FROM AUTHOR]

The effect of annealing up to 538 °C (1000 °F) on the hardness and ductility of two electrodeposited bulk nanocrystalline cobalt-phosphorus alloys (L-nCoP, 0.14 at%P and H-nCoP, 2.17 at%P) was investigated. Through a combination of electron microscopy and X-ray/synchrotron diffraction characterization, hardness and bend ductility measurements as well as density functional theory calculations, it is shown that hardness is controlled by four additive contributions. At 0.14 at%P, grain size strengthening and likely grain boundary relaxation are the dominant strengthening mechanisms. In the 2.17 at%P alloy, the contributions from solute strengthening and cobalt phosphide precursors and precipitates are much more significant and typical age strengthening behavior is observed with a peak hardness temperature of 371 °C (700 °F). The bend ductility at 0.14 at%P was in the 12–17 % range up to 482 °C (900 °F). In contrast, the ductility at 2.17 at%P decreased rapidly with increasing annealing temperatures approaching values less than 1 % at 427 °C (800 °F) due to excessive cobalt phosphide precursor and precipitate formation. The dominant deformation mechanism in both alloys was found to be basal dislocation slip. [ABSTRACT FROM AUTHOR]

Copper coatings on steel, manufactured by a novel combination of two different manufacturing technologies, electrodeposition and cold spraying, are employed as a corrosion barrier in the current design of used fuel containers (UFCs) for the long-term storage of Canada's used nuclear fuel. This study examines such copper coatings on a prototype container section, with a particular focus on the interface region where copper materials fabricated by the two processes converge. Distinct microstructures for the electrodeposited and cold sprayed copper layers were observed. An unexpected recrystallized layer on top of electrodeposited copper was identified for the first time. Local annealing at 350 °C and 600 °C for one hour resulted in more homogeneous microstructures and Vickers hardness profiles among the copper layers, which is favorable for this application by reducing processing related heterogeneity in the interface region. More homogenized structures with high fractions of special grain boundaries in this region are expected to provide enhanced mechanical and corrosion properties. • Microstructural analysis was performed on Cu coatings for used fuel containers. • A recrystallized layer between electrodeposited and cold sprayed Cu was identified. • Annealing reduces microstructural and mechanical heterogeneity. • Nanotwinned electrodeposited Cu experiences texture evolution during annealing. [ABSTRACT FROM AUTHOR]

The coefficient of thermal expansion (CTE) as measured by dilatometry of electroformed nanocrystalline Ni-32at%Co macro-defect free sheet metal up to 500 °C can be separated into three regions. Below 185 °C, the nanocrystalline material's CTE is similar to its polycrystalline counterpart. Between 185 and 310 °C, the CTE of the nanocrystalline material plateaus, then drops at temperatures above 310 °C due to volume shrinkage induced by different grain growth stages. The grain boundary excess volume (BEV) was determined to be between 0.20–0.24 nm. In addition, it is shown that simple digital caliper measurements are a reliable technique to estimate BEV for specimens with grain boundaries being the main defect. [Display omitted] [ABSTRACT FROM AUTHOR]

In the design of used nuclear fuel containers for deep geological repositories, copper is considered to be a suitable and long-lived barrier for corrosion resistance. The microstructures of state-of-the-art copper materials used in this application produced through extrusion, a grain boundary engineered electrodeposition technique and cold spraying were studied via electron backscattered diffraction. Desirable microstructural characteristics for localized corrosion resistance of pure copper were compiled from the literature considering grain size, grain boundary character distribution, and crystallographic texture. The subject copper materials were found to have favourable microstructures for localized corrosion resistance, in particular, a high fraction of special grain boundaries, especially Σ3 twins, rendering them suitable for the given application. • Cu coating materials for used nuclear fuel containers were characterized with EBSD. • Both electrodeposited and cold-sprayed Cu contain many special grain boundaries. • The microstructural features are favourable for localized corrosion resistance. [ABSTRACT FROM AUTHOR]