Captain America's only weapon was his shield, a concave disk 2.5 feet in diameter, weighing 12 pounds. It is made of a unique Vibranium-metal alloy that has never been duplicated. The shield was cast by American metallurgist Dr. Myron MacLain, who was contracted by the U.S. government to create an impenetrable substance to use for tanks during World War II. MacLain was never able to duplicate the process due to his inability to identify a still unknown factor that played a role in it. The shield was awarded to Captain America by the government several months after the beginning of his career.

Source: Marvel