Influence of cooling rate on the precipitation microstructure in a medium strength Al–Zn–Mg alloy

Medium strength Al–Zn–Mg age hardening alloys are widely used when a low quench sensitivity is required, such as in welding applications. In this work we present a detailed characterization of the precipitate microstructures resulting from different quench rates from the solution treatment, and from the subsequent artificial ageing to the T6 state, in an Al–4.5Zn–1Mg (wt%) alloy. This work is carried out using differential scanning calorimetry, transmission electron microscopy and in situ small-angle X-ray scattering. It is shown that for quench rate between 5 and 200 °C/min substantial heterogeneous precipitation is observed, nucleated on dispersoids and on grain boundaries, the former being of much larger size than the latter. During subsequent ageing, it is shown that the precipitation kinetics in the material unaffected by the quench-induced precipitates is independent on the quench rate used.