A HIGH-RESOLUTION ELECTRON-MICROSCOPIC STUDY OF THE MICROSTRUCTURE INRAPIDLY SOLIDIFIED AL-LI-CU-MG-ZR ALLOYS

The microstructure in rapidly solidified Al-Li-Cu-Mg-Zr alloys has bee n reexamined by transmission electron microscopy (TEM). The results sh ow that the weakening intermetallic phases, e.g. hexagonal, Z, face ce ntred cubic (f.c.c), T, and tetragonal, H, (new phase) phases, exist i n 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 param eters 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. H igh resolution images reveal that there are planar defects and 90 degr ees domains in the Z phase, multiple twins in the T phase, and two wel l defined crystallographic orientation relationships between the Z and T phases.