Reversion Phenomena in an Aluminium-4% Copper Alloy

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Abstract
Reversion characteristics due to the dissolution of G. P. zones in an Al-4wt% Cu alloy has been investigated by means of the electrical resistivity measurement and tensile tests. When the state of the aged alloy shows the electrical resistivity higher than that in the as-quenched state, the reversion heat treatment results in a decrease in electrical resistivity. On the other hand, when the state of the aged alloy shows the resistivity lower than that in the as-quenched state, the electrical resistivity increases in the initial stages of reversion and the electrical resistivity versus reversion time curve shows a peak. In all cases, the tensile strength decreases by the reversion treatment. It has been concluded that only the dissolution of the G. P. zones is responsible for the reversion. A complete reversion can only be observed above the minimum temperature, TGP, which depends sensitively upon the ageing temperature and the time corresponding with the stability of the G. P. zones formed during the ageing. Rather complicated reversion characteristics of this alloy reported in the literatures can be fully explained taking into account the stability of the G. P. zones as a function of ageing temperature and time. In Al-4wt% Cu alloy, the highest possible temperature for the formation of the G. P. zones is estimated to be 275°C. The reversion kinetics shows that the reversion is rate-controlled by the diffusion of copper atoms in the matrix and the activation energy for the reversion is sensitively affected by the excess vacancies introduced in the alloy by quenching before ageing.

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