>>14788185found the rule book short version.
Materials For Armor
Stainless steel is allowed at the present but it minimal use is encouraged since it is not known to have existed within the Middle Ages.
Titanium is allowed hidden protection but not for exposed armor, i.e. coat of plates yes, breast plate no.
Aluminum is not allowed for use in armor.
Materials For Shields
When constructing shields the first and foremost rule is, of course, its correspondence to the historical analogue as with all things the reenactors (combatants) use. If you can provide visual or written source it will work. And, surely, the material should be natural: for shields its mostly wood, although very few steel shields were found.