On the Influence of Short R.T. preageing on the formation of the η′-phase in an Al-Zn(4.5 at.%)-Mg(2.5 at.%) alloy

The influence of short R.T. preageing (1 min) after quenching from 480 °C into RT-water on the formation of the η′-phase in an Al-Zn(4.5)-Mg(2.5) alloys has been investigated by means of TEM, XSAS, resistivity, and hardness measurements in the ageing temperature range between T_a = 120 °C comparing samples directly quenched (DQ) to T_a and those quenched into RT-water, kept there for 1 min, and subsequently stored in an oil-bath of T_a (IQ). At T_a ≦ 170 °C both heat treatments result in finely distributed precipitates of the η′-phase accompanied by a fairly high hardness value. But at T_a ≧ 180 °C the DQ leads only to few coarse heterogeneously formed η′- and η-precipitates, whilst the IQ yields qualitatively the same precipitation structure as already described at T_a ≦ 170 °C even till 280 °C. These results are interpreted assuming that. A1) the limitation temperature of homogeneous formation of η′ is located at about T_h = 175°c and A2) during the pre-ageing at RT “nucleation sites” are formed as already suggested by RYUM, which are able to act as sites for heterogeneous formation of η′ even at T_a > T_h. Hints are obtained that the formation of these “nucleation sites” is not so strongly governed by the quenched-in excess vacancies as discussed in the literature.