Al-Li alloys are a series of alloys of aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 and lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

, often also including copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 and zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

. Since lithium is the least dense
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 these alloys are significantly less dense than aluminium. Commercial Al-Li alloys contain up to 2.45 wt% lithium.

Alloying with lithium reduces structural mass by three effects:
  • Displacement—a lithium atom is lighter than an aluminium atom; each lithium atom then displaces one aluminium 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% and increases the stiffness
    Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...

     by 5%. This effect works up to the solubility
    Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

     limit of lithium in aluminium, which is 4.2%.

  • Strain hardening—Introducing another type of 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.

  • Precipitation hardening—When properly aged, lithium forms a metastable Al3Li phase (δ') with a coherent crystal structure. These precipitates strengthen the metal by impeding dislocation motion during deformation. The precipitates are not stable however and care must be taken to prevent overaging with the formation of the stable AlLi (β) phase. This also produces precipitate free zones (PFZs) typically at grain boundaries and can reduce the corrosion resistance of the alloy.

The crystal structure for Al3Li and AlLi, while based on the FCC crystal system
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....

, are very different. Al3Li shows almost the same size lattice structure as pure aluminum except lithium atoms are present in the corners of the unit cell. The Al3Li structure is known as the AuCu3, L12, or Pm m and has a lattice parameter of 4.01 Å. The AlLi structure is known as the NaTl, B32, or Fdm structure which is made of both lithium and aluminum assuming diamond structures and has a lattice parameter of 6.37 Å. Interestingly, the interatomic spacing for AlLi (3.19 Å) is smaller than either pure lithium or aluminum.

Al-Li alloys are primarily of interest to the aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...

 industry due to the weight advantage they provide. They are currently used in a few commercial jetliner
Jetliner are an alternative rock band from the USA. They focus on melodic piano based rock and have been compared to Queen and early Elton John, being heavily influenced by early 70s rock. Composed of Adam Paskowitz of The Flys, son of Doc Paskowitz, on piano and lead vocals, Jeff Kluesner on...

The airframe of an aircraft is its mechanical structure. It is typically considered to include fuselage, wings and undercarriage and exclude the propulsion system...

s, and the AgustaWestland EH101
AgustaWestland EH101
The AgustaWestland AW101 is a medium-lift helicopter for military applications but also marketed for civil use. The helicopter was developed as a joint venture between Westland Helicopters in the UK and Agusta in Italy...

A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...


The third and final version of the U.S. Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

's external tank
Space Shuttle external tank
A Space Shuttle External Tank is the component of the Space Shuttle launch vehicle that contains the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent it supplies the fuel and oxidizer under pressure to the three Space Shuttle Main Engines in the orbiter...

 was principally made of Al-Li. In addition, Al-Li alloys are also used on both the Atlas V
Atlas V
Atlas V is an active expendable launch system in the Atlas rocket family. Atlas V was formerly operated by Lockheed Martin, and is now operated by the Lockheed Martin-Boeing joint venture United Launch Alliance...

 and Delta IV
Delta IV rocket
Delta IV is an active expendable launch system in the Delta rocket family. Delta IV uses rockets designed by Boeing's Integrated Defense Systems division and built in the United Launch Alliance facility in Decatur, Alabama. Final assembly is completed at the launch site by ULA...

Evolved Expendable Launch Vehicle is an expendable launch system program of the United States Air Force , intended to assure access to space for Department of Defense and other United States government payloads...

 rockets, and before its cancellation were to be used by NASA for Project Constellation
Project Constellation
Constellation Program is a human spaceflight program within NASA, the space agency of the United States. The stated goals of the program were to gain significant experience in operating away from Earth's environment, develop technologies needed for opening the space frontier, and conduct...

, primarily, on its Ares I
Ares I
Ares I was the crew launch vehicle that was being developed by NASA as part of the Constellation Program. The name "Ares" refers to the Greek deity Ares, who is identified with the Roman god Mars...

 and Ares V
Ares V
The Ares V was the planned cargo launch component of the Constellation program, which was to have replaced the Space Shuttle after its retirement in 2011. Ares V was also planned to carry supplies for a human presence on Mars...

 rockets, as well as the Orion spacecraft.

Some Al-Li alloys, such as Weldalite 049, are weldable; however, this property comes at the price of density; Weldalite 049 has about the same density as 2024 aluminium and 5% higher elastic modulus
Elastic modulus
An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it...

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