Aluminium–scandium alloys (AlSc) are aluminum alloys that consist largely of aluminium (Al) and traces of scandium (Sc) as the main alloying elements. In principle, aluminium alloys strengthened with additions of scandium are very similar to traditional nickel-base superalloys in that both are strengthened by coherent, coarsening resistant precipitates with an ordered L12 structure. But Al–Sc alloys contain a much lower volume fraction of precipitates, and the inter-precipitate distance is much smaller than in their nickel-base counterparts. In both cases however, the coarsening resistant precipitates allow the alloys to retain their strength at high temperatures. Composition The addition of scandium to aluminium limits grain growth in the heat-affected zone of welded aluminium components. This has two beneficial effects: the precipitated forms smaller crystals than in other aluminium alloys, and the volume of precipitate-free zones at the grain boundaries of age-hardening aluminium alloys is reduced. Scandium is also a potent grain refiner in cast aluminium alloys, and atom for atom, the most potent strengthener in aluminium, both as a result of grain refinement and precipitation strengthening. The precipitate is a coherent precipitate that strengthens the aluminum matrix by applying elastic strain fields that inhibit dislocation movement (i.e., plastic deformation). An added benefit of scandium additions to aluminium is that the nanoscale Al3Sc precipitates that give the alloy its strength are coarsening resistant at relatively high temperatures (~350 °C). This is in contrast to typical commercial 2xxx and 6xxx alloys, which quickly lose their strength at temperatures above 250 °C due to rapid coarsening of their strengthening precipitates. The effect of Al3Sc precipitates also increase the alloy yield strength by . has an equilibrium L12 superlattice structure exclusive to this system. A fine dispersion of nano scale precipitate can be achieved via heat treatment that can also strengthen the alloys through order hardening. Recent developments include the additions of transition metals such as Zr and rare earth metals like Er produce shells surrounding the spherical precipitate that reduce coarsening. These shells are dictated by the diffusivity of the alloying element and lower the cost of the alloy due to less Sc being substituted in part by Zr while maintaining stability and less Sc being needed to form the precipitate. These have made somewhat competitive with titanium alloys along with a wide array of applications. The alloy is as strong as titanium, light as aluminium, and hard as some ceramics. However, titanium alloys, which