It's probably a magnesium alloy
"Magnesium alloys are mixtures of magnesium with other metals, often aluminum, zinc, manganese, silicon, copper, rare earths and zirconium. "
They maintain much of the light weight nature of the magnesium but loose the nasty habit of oxidizing when it gets hot, magnox reactors had a magnesium-aluminum alloy in the components of the reactor hence the name (magnesium non-oxidizing).
