Decomposition of an Al–Mg–Cu alloy—a high resolution transmission electron microscopy investigation

Decomposition of the solid solution of an Al–3Mg–0.4Cu–0.12Si (wt.%) alloy has been studied by examining samples aged for periods of 1–16 h at a temperature of 180 °C. High resolution transmission electron microscopy (HRTEM) was the primary method of investigation. Within the 4–16 h aging interval, nanometer scale, homogeneously dispersed, coherent, lath-like particles were detected in the matrix. The orientation of the lath particles in the matrix is such that the edges are aligned along the 〈1 0 0〉Al directions. From the Fourier transformation of the HRTEM images obtained on 〈1 0 0〉Al zones, it was possible to confirm that two variants give rise to diffraction spots that were previously associated with the presence of S″-phase in “Cu lean” Al–Mg–Cu alloys. Two other variants give rise to streaked diffraction spots positioned around the (1 1 0)Al. The detected particles are termed GPBII in this study, instead of previously suggested S″-phase notation. The findings of this study are also compared with those of the frequently studied “Cu rich” Al–Cu–Mg alloying system.