Hulk vs. Thor vs. Avengers: Robot Smashoff

There is no adamantium in the Marvel movieverse.

but there is vibranium

According to Stark Sr. the entire stockpile of Vibranium went to making Cap's shield.

Perhaps that's all America had at the time. Considering they are making a Black Panther there likely will be far more.

Considering that one scene was the only time the metal has even been mentioned in all the movies to date, it'd be kind of weird to suddenly have a major villain be made of the stuff.

Originally posted by Genesis-Soldier
yes, we see the hulks arms acting like a blenders and smashing the bots to pieces... however i have a theory that the bots we see on screen are inferior mentally and physically to the ultron we see as a singularity. ultron has a tendency to upgrade alot

Yeah, that's why I mentioned just drones. I imagine that the main Ultron is going to be much tougher. That being said, unless he gets multiple upgrades throughout the movie, his shell isn't as tough as vibranium. At about 1:29 in the 3rd trailer Cap nails the main Ultron with his shield and it probably goes a good 4-5 inches into Ultron's chest. But then rumours have it that Cap is getting beefed up even more in this film. I swear with every new film they don't just make him more skilled, but inflate his stats a bit as well.