Constitution of Al rich alloys of Al–Cu–Mg–Li system

Abstract

The influence of the addition of between 1·5 and 2·5 wt-%Li on the phase equilibria of ternary Al–(∼13–26)Cu–(∼7–12)Mg (wt-%) alloys has been investigated as a basis for understanding the phase transformations which occur within the Al–Cu–Mg–Li quaternary system. Alloys as cast and also annealed at 500 or 510°C were examined using light microscopy and scanning electron microscopy (including phase compositional analyses), secondary ion mass spectroscopy, X-ray diffraction, and differential scanning calorimetry. Three phases based on the Al–Cu–Mg system were identified as products of solidification: α (Al based solid solution), S (Al₂CuMg), and T (Al₆CuMg₄). A phase designated S* and having a higher Li content than S but of similar stoichiometry based on Al₂Cu(Mg,Li) also formed during solidification. The X-ray diffraction pattern of the S* phase was similar to that of R (Al₅CuLi₃) phase. The addition of Li to Al–Cu–Mg alloys leads to primary S* phase formation during solidification with an associated increase in the liquidus in a certain composition range where α phase would form in the ternary Al–Cu–Mg system. The presence of Al–Li phase δ was detected as a product of solid state precipitation.

MST/1736