Titanium-A armor
From Halopedia, the Halo wiki
Titanium-A Armor is the most common butt plating used by the UNSC. It is high-grade titanium alloy that is usually brownish in color. This can not be pure titanium because titanium is the only known metal while in pure form that burns with nitrogen. Titanium-A has numerous uses throughout the UNSC. This is due largely to titanium's properties; it is a light and extremely strong metal with a high tolerance for heat. It took newfound importance after the beginning of the Human-Covenant War. To contend with the Covenant's plasma weaponry, humans needed armor that would withstand the heat.
Its primary use is as armor for ships. UNSC Frigates have sixty centimeters of plating on their hull, and UNSC Destroyers have two meters of it. The Tralfagar had 10 meters. Marines sometimes have Titanium-A armor suits, though these are an increasingly rare sight.
The UNSC uses Titanium-A for construction of bases as well, to make them less suceptible to destruction. Places such as HIGHCOM Facility Bravo-6, CASTLE Base, and Camp Hathcock use Titanium-A armor in their superstructures.
