Aluminum-lithium
Aluminium–lithium alloys (Al-Li) are a series of alloys of aluminium and lithium, often also including copper and zirconium. Since lithium is the least dense elemental metal, these alloys are significantly less dense than aluminium. Commercial Al–Li alloys contain up to 2.45% by weight of lithium.
Alloying with lithium reduces structural mass by three effects:
The crystal structure for Al3Li and Al–Li, while based on the FCC crystal system, are very different. Al3Li shows almost the same size lattice structure as pure aluminium except lithium atoms are present in the corners of the unit cell. The Al3Li structure is known as the AuCu3, L12, or Pm3m and has a lattice parameter of 4.01 Å. The Al–Li structure is known as the NaTl, B32, or Fd3m structure which is made of both lithium and aluminium assuming diamond structures and has a lattice parameter of 6.37 Å. The interatomic spacing for Al-Li (3.19 Å) is smaller than either pure lithium or aluminium.
Al–Li alloys are primarily of interest to the aerospace industry due to the weight advantage they provide. They are currently used in a few commercial jetliner airframes, the fuel and oxidizer tanks in the SpaceX Falcon 9 launch vehicle, Formula One brake calipers, and the AgustaWestland EH101 helicopter.
The third and final version of the US Space Shuttle's external tank was principally made of Al-Li 2195 alloy. In addition, Al–Li alloys are also used in the Centaur Forward Adapter in the Atlas V rocket, in the Orion Spacecraft, and were to be used in the planned Ares I and Ares V rockets (part of the cancelled Constellation program).
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