Ununpentium

Once a dangerous and unstable metal, it was made into an alloy for shielding engine cores. Once refined into a fine powder, it's seeded into titanium and then further refined until the Ununpentium itself grows through the titanium. The alloy is then cut into shapes used in engine cores for very large ships. The refinement process has been lost to time, but pieces of Ununpentium can still be found in out of the way places