Metals for Various parts.

I am having a little debate about what kind of metals to use for what.

For Boreas i was going to use a steel shaft, since steel is generally cheaper than titanium. But I do not know what would be best for the kind of stress it would be under..

Also for Icarus if i have weight i wanted to use a Steel Armor. For this i was thinking about a Tool Steel that would be hardened. Or maybe a composite of sorts with some Ti and Alum. Or even some kind of hardening coating.

Also for Boreas' drum i found some large enough Steel Round stock. Would it be better if i use Steel or Aluminum. Mind you that the steel would be much much thinner than the aluminum to compensate for the weight.

As for Teeth/Beater i was going to stick with S7, but is there anything else that is better for a weapon? O2? D2? H13?

Anyone care to share ideas on what is best?