TR steel

TR Steel Alloy is a type of building material used by the UNSC for large starships.

Background
TR steel is used to build the superstructure of the ship before it is plated with Vanadium Steel and, eventually, Titanium-A battleplate.

A curious feature of TR steel alloy is that, as it is exposed to solar radiation over time, it gradually turns from silver to white. It is easy to tell a ship's age by the color of the TR steel in its superstructure.