Thermal stability of electrodeposited nanostructured high-entropy alloys

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.