Al-Li

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Al-Li alloys 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% lithium.^[1]

Alloying with lithium reduces structural mass by two effects:

Displacement- a lithium atom is lighter than an aluminum atom; each lithium atom then displaces one aluminum atom from the crystal lattice, while maintaining the lattice structure. Every 1% by weight of lithium added to aluminium reduces the density of the resulting alloy by 3%.^[1] This effect works up to the solubility limit of lithium in aluminum.

Strain hardening- The lithium atom is larger than an aluminum one. Introducing a larger atom into the crystal strains the lattice, which helps block dislocations. The resulting material is thus stronger, which allows less of it to be used.

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, and the AgustaWestland EH101 helicopter.^[2]

As of 2006, the third, and current version of the U.S. Space Shuttle's external tank is principally made of Al-Li.^[3] In addition, Al-Li alloys are also used on both the Atlas V and Delta IV EELV rockets, and will be used by NASA for Project Constellation, primarily on its Ares I and Ares V rockets, as well as the Orion spacecraft.

Al-Li alloy swarf, scrap and other recycled material must be kept separate from other aluminium streams during recycling, as mixing Al-Li alloy with conventional aluminium recovery streams will cause extreme fire and explosion hazards.^[4]