Inert gas atomization is the preferred technique for manu-facturing feedstock for TS. It has gained popularity in the HEA coating community and is the most used synthesis method. Gas atomization involves forcing the liquid alloythrough a nozzle under high pressure within an inert gas environment. The liquid stream fragments into spherical droplets that rapidly solidify. Faster cooling allowsphase separation to occur only at very fine scales, as illus-trated in Fig.7, and can usually be mitigated during spraying or annealing after coating. Gas atomization is appealing because it results in spherical particles with good flowability and homogeneous alloy formation. The wide size ranges of particles can be sieved to desired size cuts.
Fig.7 Typical gas-atomized high-entropy alloy particles with (a) spherical morphology and satellite particles, and(b) dendritic growth discernible at higher magnifications
In summary: GA is tailor made for synthesizing feed-stock for thermal spray processes and is suitable for HEAs.Mechanical alloying presents a strong alternate method, although requiring milling parameter optimization and being time intensive. Arc melting followed by mechanical milling is suitable for laboratory-scale studies, while blending can only be used as a post-alloying technique to further enhance feedstock properties.
It is also imperative to mention that while most of thestudies synthesized their own feedstock in a laboratory setup, there are reports of two commercially available sources for HEA powders:(1)F.J. Broadmann & Co., L.L.C., USA, for mechanically alloyed powders, and (2) Vilory Advanced Materials Technology Ltd., China, for gas-atomized powders. While this points toward a growing future for thermal sprayed HEA coatings, it must be borne in mind that the quality of the feedstock defines the coating quality. Thus, care must be exercised when selecting either laboratory or commercially produced HEAs.
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