[rotorwrench]

Most of the old stuff had copper plated steel. The copper plating was excellent to prevent rapid corrosion. This is what FoMoCo used back in the day. Copper fatigues faster than steel under vibration stresses. It will still last a long time but it isn't used at all in aviation for the fatigue factor. 5052 aluminum tube and stainless steel tube are the common types used there.

Copper tubing generally always has some service loops turned into it to absorb vibrations a bit better and lessen it's chances of fracturing as Tom Walker previously mentioned.