Mistwraith13

4 minutes ago, asterion137 said:

Even ignoring investiture though a steel sword should cut through an aluminum sheath,

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especially when a radiant is swinging it.

Edit: minor OB spoiler.

For that matter Nightblood Rams itself, sheath and all through its victims chests, so it has to be enhanced somehow. 

2 hours ago, asterion137 said:

why can't nightblood cut through its sheath if it's made out of steel and the sheath is aluminum?

Aluminum is Allomantically inert, maybe it is Investiture inert as well providing extra protection from Nightblood.