Originally Posted by Raiden
I also think that it will be Vibranium, and it makes more sense than Adamantium. Cap's shield has already displayed durability against Thor's Majolnir's strike, and probably will withstand Hulk's fist as well as other forms of attack. It will make Ultron virtually indestructible. Also, since Cap got his shield from SHIELD, it is reasonable to assume that they'd have more somewhere, and Ultron can build itself a new body so it can have what it takes to destroy the Avengers.
Makes sense. Except I think they'd have to get more from Wakanda.