Due to Fox having the rights to X-Men, there are few restrictions to what can be mentioned and eluded to in the Marvel Cinematic Universe (eg: 'Mutants'). Does this restriction stretch to Adamantium? I understand that -in the comics- some incarnations of Ultron were made of the substance.

I'm not asking, in the upcoming movie, Avengers: Age of Ultron, if Ultron will be made of Adamantium. I would simply like to know if it is/was ever an option?