Originally Posted by TheVileOne
...it's not vibranium since vibranium already existed and it is the material that makes up Cap's shield.
At the same time, would vibranium necessarily have been one of the elements that Tony would have been able to try in IM2 if it was "the rarest metal on earth" and the only vibranium that the Allied Forces had in the 1940s was stuck in a frozen HYDRA aircraft in the arctic circle? Assuming that Wakanda is still fiercely isolationist at this point and probably very secretive about their own stash of the metal, the idea of vibranium 70 years later could have become the stuff of legends/rumors, since no one would have had access to it. I'm just saying it's possible to explain Tony's ignorance of vibranium as an existing element due to the fact that no one had any.
The fact that Howard Stark had apparently lost the only vibranium he ever knew of could have been another reason why he was so desperate to find Cap in the Arctic.