Graphene steel
From Halo: Daybreak
Graphene steel, also known as grapheel, was an alloy blend that was used for armor plating on human warships. Introduced in 2317, it was eventually phased out upon the introduction of titanium-A battleplate. Although weaker and heavier, graphene steel remained in use as second-line armor plating because it relied on less-advanced forging processes and was made from more abundant materials.