What metal is Captain America's shield primarily composed of?

Captain America’s iconic shield is primarily composed of vibranium.

In the classic Marvel comics origin, metallurgist Myron MacLain accidentally creates the shield’s nearly indestructible alloy while experimenting with vibranium and an experimental steel mixture. The exact formula cannot be reliably reproduced, which is why the shield is often described as a unique vibranium-based alloy rather than simply a slab of pure vibranium.

Vibranium gives the shield its distinctive ability to absorb and redirect kinetic energy and vibrations. That property helps explain why Captain America can survive powerful impacts and use the shield as both protection and a ricocheting weapon. The circular shield first appeared in Captain America Comics #2, published in 1941.

This answer can vary slightly by continuity. Some stories give Rogers a pure-vibranium replacement shield, while others introduce shields made from adamantium, uru, photonic energy, or other materials. Carbonadium is another fictional metal, and uru is associated particularly with Asgardian weapons such as Thor’s hammer. For the standard shield in mainstream Marvel lore, vibranium is the accepted answer.