Abstract
Ageing and reversion in an Al-4 mass%Zn alloy under various conditions have been studied mainly by measurements of the electrical resistivity.
(1) When the alloy is quenched from temperatures lower than 823 K and aged at 233 K or when it is quenched from temperatures lower than 723 K and aged at 263 K, the maximum value of resistivity (ρM) is independent of the quenching temperature. When the alloy is quenched from 873 K and aged at 233 K or when it is quenched from a temperature higher than 773 K and aged at 263 K, the resistivity reaches a value a little larger than ρM in the ageing temperature after a relatively short time and does not change since then. This is considered to correspond to substantial cessation of changes in the number and size of G.P. zones.
(2) Results of the isothermal and isochronal annealing experiments are able to be well explained by considering that there exist two species of G.P. zones, G.P. (s) and G.P. (n), differing from each other in the thermal stability and that G.P. (s) zones formed at the initial stage of ageing may change into G.P. (n) zones as the ageing advances. It is considered that the change from G.P. (s) zones to G.P. (n) zones is facilitated with higher vacancy concentration.
