I've brought this up before...
Once upon a time Captain America has an Adamantium Plus shield, an accidental merging of metals that was stronger than adamantium. It was in trying to recreate this accident that resulted in adamantium.
In more recent years, same shield, it's been retconned to be a vibranium-steel alloy mix. No idea why. Especially because in the case of the pic above, if that were the case Wolverine would've shredded it no problem.