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 mass of lithium added to aluminium reduces the density of the resulting alloy by 3% and increases the
stiffness by 5%. This effect works up to the
solubility 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 Al–Li, while based on the
FCC crystal system, are very different. Al3Li shows almost the same-size lattice structure as pure aluminium, except that lithium atoms are present in the corners of the unit cell. The Al3Li structure is known as the AuCu3, L12, or Pmm and has a lattice parameter of 4.01 Å. The Al–Li structure is known as the NaTl, B32, or Fdm 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. ==Usage==