The aluminium copper alloy 2024 is competitive on a strength-to-weight ratio with the higher-strength but heavier titanium and steel alloys and thus has traditionally been the dominant structural material in both commercial and military aircraft. In addition, aluminium alloys are not embrittled at low temperatures and become even stronger as the temperature is decreased, without significant ductility losses, making them ideal for cryogenic fuel tanks for rockets and launch vehicles. The wrought heat treatable 2xxx alloys generally contain magnesium in addition to copper as an alloying element.