Exothermic reaction in eutectoid Zn–Al based alloys

Both solution treated and quench-aged eutectoid Zn–Al based alloys (ZnAl20Cu3Si2 and ZnAl22Cu2 in wt.%) were studied by differential scanning calorimeter (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. An exothermic phenomenon was observed at the early stage of ageing. Similar behavior was also found for the cast eutectoid Zn–Al alloy. The activation energy for the exothermic phenomenon was calculated, which was in a good agreement with that derived from X-ray diffraction examination for the decomposition of the supersaturated β′s phase in the eutectoid Zn–Al based alloy. It is concluded that the decomposition of the β′s phase is an exothermic solid state reaction in the eutectoid Zn–Al based alloys.