Pre-precipitation study in the 7012 Al–Zn–Mg–Cu alloy by electrical resistivity

The early stages of pre-aging at near room temperature in an Al–Zn–Mg–Cu based commercial alloy were studied by electrical resistivity. The resistivity changes can be adequately described in the same terms of a Johnson–Mehl–Avrami (JMA) type function for the volume fraction growth of the Guinier–Preston zones or pre-precipitate solute clusters formed. For one specific case, resistivity results were correlated with those obtained using synchrotron-radiation small-angle X-ray scattering (SR-SAXS). Based on the analysis of the parameters of the JMA equation, the existence of a complex diffusion process during pre-precipitation is deduced. An apparent activation energy of 0.69±0.04 eV for the formation of the above mentioned particles is found. From this energy value, the migration of Mg-vacancy complexes is identified as the main microstructural mechanism responsible for the pre-precipitation.