The common misconception states that the black boxes are "indestructible." No manmade device is indestructible, and no material has ever been developed that cannot be destroyed under severe enough conditions. The black boxes are instead designed to be highly survivable in a crash. In many of the worst aviation accidents, the only devices to survive in working order are the Crash Survivable Memory Units (CSMUs) in the black boxes. The remainder of the recorders, including the external case and other internal components, are often heavily damaged.
The box is composed of three layers to provide different types of protection to the recording medium. The outermost shell is a case made of hardened steel or titanium designed to survive intense impact and pressure damage. The second layer is an insulation box while the third is a thermal block to protect against severe fire and heat. Together, these three layered cases allow the FDR and CVR to survive in all but the most extreme ...more

Sorry bhai,, no idea

Because the plane would become too heavy to take off and fly. the black box is made of a very heavy metal.

The black box is made of a very heavy metla. If the entire plane is made of that then it will not be able to fly.. Unless of course you make it as big as acity.. then probably it will displace a very large volume of air that will give it buoyancy.

Because the aircraft cannot be made of the same material as the black box