CONSTITUTION OF AL RICH ALLOYS OF AL-CU-MG-LI SYSTEM

The influence of the addition of between 1.5 and 2.5 wt-%Li on the phase equilibria of ternary Al-(similar to 13-26)Cu-(similar to 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 degrees 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: a (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 a phase would form in the ternary Al-Cu-Mg system. The presence of Al-Li phase delta was detected as a product of solid state precipitation.