Copper nickel alloy, actually...

http://www.brakequip.com/pdf/ezibend_tube_booklet.pdf

FYI, copper has a number of really cool metallurgical properties that allow it to be many things to many different people depending on how it is worked/heat treated, etc.

My uncle-in-law creates refinery process valve innards (technical term) from quench-hardened copper allow. SS was/is the traditional material until people got tired of it failing "catastrophically" instead of gracefully becoming less valve-like and being replaced on a regular maint schedule. When little SS bits end up downstream in your process seconds after an alarm goes off and minutes before the stream can be throttled...bad juju.

Mike