system
Given the following post, respond as if you're commenting on Reddit:
human
[The boys] What was Soldier Boy's shield made of?
gpt
We don't have a definitive answer (military secrets and all that) however, I'd guess inconel (or another similar nickel superalloy).  It's very heavy, incredibly strong and with outstanding resistance to repeated thermal cycling (say from a user with a habit of setting off routine massive explosions).  Unlike depleted uranium it's also very tough and not especially toxic even when aerosolized (from high energy impacts).

It also would have been potentially easier to procure during WW2 (although both would have been available).