Microstructure characterization of phase transformations in a Zn-22 wt%Al-2 wt%Cu alloy by XRD, SEM, TEM and FIM

The microstructure evolution in a Zn-22 wt%Al-2 wt%Cu alloy during aging was followed, using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and field ion microscopy. Samples were solution treated at 623 K for 1 week and then some of them annealed and the others quenched. Both annealed and quenched samples were aged at 373, 423 and 473 K for times from 3.6 (0.04 days) to 1800 ks (20.8 days). The X-ray patterns showed that the η phase decomposed according to the following reaction η→η+α during aging. This was also confirmed by TEM and SEM results, which showed first the spinodal decomposition of the η phase and then the formation of a coarse lamellar structure, composed of the α and η phases. The X-ray analysis of aged samples also showed the occurrence of a four-phase reaction α+ε→τ+η. The τ phase was widely recognized in SEM, TEM and FIM results. Additionally, the XRD and TEM results indicated that the formation of the equilibrium ordered τ′ phase is preceded by that of the disordered τ phase.