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′sphase in the eutectoid Zn-Al based alloy. It is concluded that the decomposition of the β′sphase is an exothermic solid state reaction in the eutectoid Zn-Al based alloys.