The Tough Alloy Ingot is a key material used in crafting many of the items and machines in NuclearCraft.
As of version 2.3 of the mod, it is made in the Alloy Smelter by this series of craftings:
Iron + 2 Graphite = Steel
Steel + Boron = Ferroboron
Ferroboron + Lithium = 2 Tough Alloy.