Ageing processes of (α+T) type Al-Zn-Mg alloys at relatively high temperatures

Abstract

Effects of the composition on the ageing processes of (α+T) two-phase Al-Zn-Mg alloys were investigated by electric resistivity measurement, hardness measurement, and transmission electron microscopy mainly in the temperature range of 80150°C. Specimens were solution-treated at 470°C, quenched, and finally subjected to isochronal and isothermal ageing.The results were summarized as follows:For example, even the total amount of (Zn+Mg) was a definite value in at%, the ageing processes of the alloy system were confirmed to vary in the following way with the increase in Zn/Mg ratios in the specimen.The rate of increase in resistivity and the maximum value of resistivity obtained became higher when TA (where TA: ageing temperature and T : temperature of solubility limit in G. P. zone of each specimen). But, the rate of decrease became higher when TA On the other hand, the rate of hardening and the maximum value of hardness obtained became higher at any of TA. However, there was a tendency to overageing due to η phase precipitation. The distribution of η' phase in grains was finer in the state of the maximum hardness of specimens. The width of PFZ, which was considered to be due to the grain boundary precipitation, became smaller.It was suggested that Zn-vacancy pairs and free vacancies played an important role in the ageing processes even in such a high temperature ageing as being expected from the results of low temperature ageing or calorimetric studies previously reported.