Al-Li alloys are a series of alloys of aluminium and lithium, often also including copper and zirconium. Since lithium's the least dense elemental metal, these alloys are significantly less dense than aluminium. Commercial Al-Li alloys contain up to 2.45% lithium. This effect works up to the solubility limit of lithium in aluminium. Strain hardening- The lithium atom's larger than an aluminium one. Introducing a larger atom into the crystal strains the lattice, which helps block dislocations.… (More on Al-li)