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

Abstract

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°C s^–1 at the transformation temperature and the average cooling rate, , is between 20 and 35°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, η phase heterogeneously precipitated on Al₃Zr dispersoids and grain boundaries during the medium temperature transformation stage and homogeneously nucleated η′/η phase precipitated at the low temperature transformation stage.