Dear Dr Graham Straley,

most HEAs have been produced using liquid-phase methods include arc melting, induction melting, and Bridgman solidification. Solid-state processing is generally done by mechanical alloying using a high-energy ball mill.

For more details, please see the source at: https://en.wikipedia.org/wiki/High_entropy_alloys

....and the following, interesting paper:

Processing, alloy composition and phase transition effect on the mechanical and corrosion properties of high entropy alloys: a review

Kenneth Kanayo Alaneme, Michael Oluwatos inBodunrin, Samuel RantiOke

Journal of Materials Research and Technology

Volume 5, Issue 4, October–December 2016, Pages 384-393

Available at: https://www.sciencedirect.com/science/article/pii/S2238785416300126

Best regards, Pierluigi Traverso

Dear Graham Straley, I can join to the previous answers - foundry and additive manufacturing. Our group has performed the laboratory production of HEA using arc-melting, suction casting, and additive manufacturing using blended powders.

if it will be relevant have a look:

Article Selective electron beam melting of Al0.5CrMoNbTa0.5 high ent...

Article Microstructural Evolution and Phase Formation in 2nd-Generat...