Decomposition process in a Zn-22wt.%Al-2wt.%Cu alloy

The decomposition process in a Zn-22wt.%Al-2wt.%Cu alloy was investigated during aging at 373, 423 and 473 K, using XRD, SEM and TEM techniques. The results of X-ray diffraction of aged samples showed that two kinds of phase transformation occurred. One was the decomposition of metastable η_FC phase, as revealed by progressive shifting of the (0002) X-ray diffraction peak. The activation energy for the thermally activated process was determined to be ~ 71.5 kJ mol -1. The other transformation was detected by the disappearance of the ε phase diffraction peaks, accompanied by the appearance of the (110) and (110) X-ray diffraction peaks of the τ and τ′ phases, respectively. These two events seem to be related to the four-phase reaction: α + ε → η + τ′.