TEM diffraction study of Al2CuMg (S′/S) precipitation in an Al-Li-Cu-Mg(Zr) alloy

Transmission electron microscopy (TEM) was used to study the precipitation in an 8090 (Al-2.7Li-1.4Cu-1Mg-0.11Zr (wt %)) alloy. TEM diffraction patterns revealed reflections from the orthorhombic S'/S (Al2CuMg) phase in addition to those of the fcc alpha matrix, the coherent and ordered delta'(Al3Li) phase of Ll(2) structure and the hexagonal T1 (Al2CuLi) phase. The reflections from the T1 phase were identified and indexed using literature results. Concerning the S'/S phase, a theoretical study, based on its orientation relationships with alpha, allowed the construction of transformation matrixes between planes and directions of their reciprocal lattices. The calculation shows that 12 different S'/S systems can diffract on a given alpha phase selected area diffraction (SAD) pattern. Theoretically, two diagrams ((100)*(alpha) and (112)*(alpha)) are considered, on which the S'/S reflections are indexed. The results obtained for the theoretical patterns are in good agreement with those observed experimentally.