On the influence of dislocations on precipitation in an Al-Zn-Mg alloy

The influence of predeformation on the precipitation and on the resulting mechanical properties in an Al-Zn-Mg alloy was studied using a combination of complementary experimental techniques, namely transmission electron microscopy, differential scanning calorimetry, small-angle X-ray scattering and microhardness. The thermomechanical treatment was designed to show a significant effect of deformation on the precipitation process. It was observed that the presence of dislocations during precipitation promoted fast nucleation of the equilibrium eta phase in the eta(2) orientation instead of nucleation of the eta' phase with subsequent transformation to eta(1) and eta(4) outside dislocations. The large growth rate of precipitates lying on dislocations provoked a growing precipitate-free zone around dislocations. This resulted in a lower value of peak hardness and a faster overall coarsening kinetics.