It's probably some combination of steel and titanium, but the exact composition is a secret, as are the ingredients in the wafer of insulating material that preserves the data on the chip even after it's cooked for an hour at over 2,000°C. In their rites of passage, black boxes are shot out of a cannon at a crushing 3,400 times the force of Earth's gravity, squeezed in a hydraulic press at 2,000 kilograms of pressure for five minutes and subjected to water pressure at a simulated depth of 6,000 metres. They are soaked in jet fuel, lubricating oil and hydraulic fluid for 48 hours, and immersed in seawater for 30 days. The housing isn't meant to be watertight -- you don't want 600 atmospheres of pressure differential bearing on the walls, Schmutz says -- but the data has to survive anyway.
Black box flight recorders are miraculous things, able to preserve the data on its memory chips even under the most trying physical and chemical distress (as one comedian, I can't remember which, once joked, shouldn't we make planes out of the same material we make these black boxes from). But wouldn't it just be easier to stream that data to land so we wouldn't have to track down the flight recorders after plane accidents?