Discontinuous precipitation in a Zn-1.6 wt.% Al alloy

The decomposition processes in a Zn-1.6 wt.% Al alloy, homogenized at 628K and subsequently water quenched, have been studied by scanning electron microscopy during homogenization and isothermal aging at 296K. The presence of the metastable ZnAl phases was verified during homogenization, and their dissolution has given rise to the discontinuous precipitation reaction. The discontinuous precipitation cell structure consisted of the rodlike precipitates interposed in the matrix of Zn-rich solid solution. The interrod spacing coarsened during aging because of the interaction between the discontinuous precipitation cells and the continuous matrix precipitates. From the calculated volume fractions of matrix and precipitated phases during homogenization and aging, probable mechanisms of dissolution of metastable phases and discontinuous precipitation formation have been proposed.