Development of non-linear continuous cooling precipitation diagram for Al-Zn-Mg-Cu alloy

Microstructure evolution for an Al-Zn-Mg-Cu alloy has been investigated under different non-linear cooling processes from the solution temperature. Combined with in situ electrical resistivity measurements, transmission electron microscopy, hardness tests and Newtonian cooling equations, a full continuous cooling precipitation diagram for the experimental alloy has been developed. The critical cooling rate is between 23 and 46 degrees C s(-1) at the transformation temperature and the average cooling rate, (nu) over bar ([400,275]), is between 20 and 35 degrees C s(-1). The phase transformation during the non-linear cooling process can be concluded as: S phase precipitated in the high temperature transformation stage, eta phase heterogeneously precipitated on Al3Zr dispersoids and grain boundaries during the medium temperature transformation stage and homogeneously nucleated eta'/eta phase precipitated at the low temperature transformation stage.