Development of the microhardness of an al-2.0 at.% zn-1.3 at.% mg alloy with additions of technical impurities during a two-step ageing treatment (ii)

The alloy was preaged in the temperature range between T_pre = 50 °C and 160 °C for various times and afterwards postaged at R.T. The course of the microhardness and the related structure changes (TEM and SAXS investigations) were investigated. The main results are the following: – Preageing the samples times less than the incubation time no essential structure changes take place at least not in the sense that the further decomposition happening during the postageing at R.T. is remarkably influenced. – The samples preaged at T_a 110 °C for times somewhat longer than the incubation time show a distinct minimum after the end of the postageing period of 42 d at R.T. Most probably at T_a 110 °C in the starting period of the decomposition nuclei of the α-phase or of spherical hexagonal zones were formed. These nuclei lessen considerably the amount of G.P. zones able to be nucleated during the R.T. postageing.