β̄ Phase decomposition in Zn-22mass%Al and Zn-22 mass%Al-2 mass%Cu alloys at room temperature

The phase decomposition of the β̄ phase in the Zn-22 mass%Al and Zn-22 mass%Al-2 mass%Cu at room temperature was followed by means of the X-ray diffraction (XRD), transmission electron microscopy (TEM) and Vickers hardness (VH) measurements. Alloys were homogenized at 623 K for 432 ks and then quenched at 275 K. Immediately, they were characterized by XRD and simultaneously other samples were analyzed by hardness Vickers measurements. The XRD results showed that the β̄ phase is unstable at room temperature and its decomposition finished after 1.8 and 18 ks by the following reaction, β̄ ̄ α + η, for the Zn-22 mass%Al and Zn-22 mass%Al-2 mass%Cu alloys, respectively. The TEM analysis was carried out in the Zn-22 mass%Al-2mass%Cu alloy, which showed a slower kinetics than the Zn-22 mass%Al alloy. The TEM results showed in situ that the β̄ phase is in coexistence with the e phase and its decomposition occurs by the formation of colonies composed of nanometric grains of the α and η phases. Such colonies extend to cover completely all the surface of the alloy, followed by the coarsening of grains to the micrometer scale. The Vickers hardness results showed an increase in hardness up to a maximum of 108 and 148 VH, followed by a decreasing in hardness of 50 and 80 VH, for the natural aging of the Zn-22 mass%Al and Zn-22 mass%Al-2mass%Cu alloys, respectively. These results can be attributed to the presence of the nanometric and micrometric grains, respectively.