A high resolution electron microscopic study of the microstructure in rapidly solidified Al-Li-Cu-Mg-Zr alloys

The microstructure in rapidly solidified Al-Li-Cu-Mg-Zr alloys has been reexamined by transmission electron microscopy (TEM). The results show that the weakening intermetallic phases, e.g. hexagonal, Z, face centred cubic (f.c.c), T, and tetragonal, H, (new phase) phases, exist in alloys with relatively high contents of Mg besides the strengthening phases. The sizes of these weakening intermetallic phases are on the scale of micrometres. The newly found tetragonal H phase, having parameters of a = 2.8 nm and c = 2.4 nm, was observed to coexist with the Z phase, and has a defined orientation relationship with the Z phase. High resolution images reveal that there are planar defects and 90 ° domains in the Z phase, multiple twins in the T phase, and two well defined crystallographic orientation relationships between the Z and T phases.