Supersonic Titanium

At Mach numbers greater than 2.5, aluminum is no longer an option. The SR-71 was made largely of titanium, which has good high temperature characteristics, but is still light enough for aircraft structures. To allow for the expansion, slip joints are used in many places on the SR-71. On the ground, the SR-71 fuel tanks leak, and they do not seal until the aircraft heats up during flight.

References: BoldMethod, NASA