Al-Li

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 AlLi (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, and the AgustaWestland EH101 helicopter.

The third and final version of the US Space Shuttle's external tank was principally made of Al–Li. In addition, Al–Li alloys are also used on both the Atlas V and Delta IV EELV rockets (citation needed), and before its cancellation were to be used by NASA for Constellation program, primarily, on its Ares I and Ares V rockets, as well as the Orion spacecraft.

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