PC CPU case materials?
It's called the IHS, or integrated heat spreader. It's apparently made from copper or a copper alloy (and nickel plated). There's another (organic) material between the die itself and the IHS. Intel reportedly will not tell you the exact composition of the IHS without a signed NDA in hand (and, presumably, a moving story as to why you need to know and why Intel should tell you). 

Edit: If the description "nickel plated copper" is good enough for you, that appears in some Intel datasheets: 



To verify this, you should be able to detect slight ferromagnetism with a rare-earth magnet from the nickel plating, and scratching it (someplace unimportant if it's a working chip) should let the color of the underlying alloy show. In the past, aluminum alloy has been used.