On the influence of impurity additions and quenching temperature on the course of the microhardness (HV) during a two‐step ageing treatment of an Al‐2.0 at.%Zn‐1.3 at.%Mg Alloy

A pure as well as the analogous technical alloy were directly quenched from T q = 400 °C or 490 °C to T a , preaged at T a (ranging between 50 °C and 160 °C) various times ( t a ), and subsequently postaged at R.T. up to t post = 42 d. — Ageing the samples below the upper limit temperature of the homogeneous formation of G.P. zones ( T hn ) no essential difference in the course of the HV‐number attained after one‐ as well as two‐step ageing in dependence on the addition of impurity atoms and T q could be found. — Contrary to this behaviour after pre‐ageing at T a > T hn and postageing at R.T. a variation of both T q and the content of impurity atoms shows a considerable effect. The reason is the difference in the density of nucleation sites for heterogeneous formation of precipitates of the η′‐ and/or η‐phase in dependence of T q and impurity content.