The specifications for keys and keyseats speaks to fit (how tight) and shape (square, rectangle, tapered, rounded ends or square ends etc.) and doesn't involve material selection. (I'm 99% sure of this) Most folks make their keystock from 1018 bar... or whatever material might be available at the best price that day. Corrosive environment use S.S.. Sometimes aluminum for certain shear to protect components down the line...
Guess it boils down to the key material being as tough as the weakest component it is in contact with. Unless you want it to shear for protection...
User defined?
MSUdawg... if you find anything out on this how about posting it. I'd be interested in knowing about it if there is some sort of material spec.
Good Luck,
Bob