Ignoring HFS:
Bronze is the most lethal overall (?!), Silver is the most incapacitating overall.
Steel is more effective against Iron & Steel than it is against Bronze & Copper, though only marginally so (it's still delightfully lethal).
Bronze & Copper are better at incapacitating Iron armor-wearers than Steel & Iron.
Comparing their weapon stats:
SILVER: 10.49den, 350iy, 595if, 350ie, 100sy, 170sf, 333se
STEEL: 7.85den, 1505iy, 2520if, 940ie, 430sy, 720sf, 215se
BRONZE: 8.25den, 602iy, 843if, 547ie, 172sy, 241sf, 156se
COPPER: 8.93den, 245iy, 770if, 175ie, 70sy, 220sf, 145se
Silver is 1.56 more dense than Copper, 2.24 more dense than Bronze, and 2.64 more dense than Steel. Silver is otherwise only superior to other material grades in its IE (other than Copper), and has the worst SE; this seems to indicate that Density has a rather significant impact on Incapacitation, far moreso than IY or IF. IE might help.
Compared to Steel, Bronze has around 1/3 the IY and IF, but almost half as much IE. It has around 1/3 the SY and SF, but around 1/4 less SE. Bronze's lethality would seem to then depend on its IE and SE values, then, but Copper has more density and better IE & SE values... yet is sub-par against armored targets (other than iron).
Based on this, I'd make a guess that... Density increases incapacitation, Elasticity properties increase lethality, and Yield properties must be of certain thresholds to penetrate armor, which would explain why Copper fares so relatively poorly against any armored target other than Iron, and why materials fare best vs. lesser materials (which all have smaller yield ratings).