I agree with John - melting is a whole science in and of itself, and there are many variables, not only purity of the raw getting melted but what type of furnace, refractory, slag, pouring method, chronological order of alloying additions, etc. He is correct that deoxidation is typically performed via addition of some sort of "getter" element such as aluminum, titanium, etc., which is mostly removed in the slag but you will have some amount of inclusions left over.
That said, here are two melting processes you might consider to achieve the level of purity you're after:
A vacuum melting process such as VIM or VAR. The idea here is that, by pulling a vacuum on the melt, you can remove some of the gases that way. Often this is done two or three times until a point of diminishing returns is approached. This is a commercially available and well-established process, although if you're ordering a heat from a mill, there will be a minimum buy of at least several thousand pounds (vs. finding something off the shelf at a distributor that was melted this way to buy in the small quantity you're after).
Electrolytic refining. This is not my area of expertise so I will defer to anyone else in the forum who may know more about it but often precious metals such as gold are purified via electrolysis; essentially you have them in solution and then electroplate them onto a cathode. I don't believe that you could electroplate out this specific 80/20 alloy you're looking for but you could use it as a method to get very pure nickel and chromium independently (or buy said pure metals from someone else who has used this process) and then melt them together in a vacuum furnace to get your alloy.
I'm not aware of what companies out there are performing these processes on a small scale but the big name manufacturers of nickel alloys that come to mind are SMC, Carpenter, Haynes, and ATI; you could contact representatives of those companies to see whether they have divisions that do small jobs like this but I'm sure that the price per pound will be exorbitant.
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Sean Piper
Product / Process Metallurgist
Ellwood Texas Forge Houston
Houston TX
(713) 434-5138
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