Complex interactions between precipitation, grain growth and recrystallization in a severely deformed Al-Zn-Mg-Cu alloy and consequences on the mechanical behavior

Combining submicrometer grain size with nanoscaled precipitation is an attractive approach to achieve high yield stress with reasonable ductility in aluminum alloys. The control of super saturated solid solutions decomposition in ultrafine grain structure during precipitation annealing treatments is however a huge challenge. It raises indeed the general question of competition between precipitation, grain growth and recrystallization. All these microstructure changes are driven by the minimization of the free energy of the system but connected to three different factors, the thermodynamic driving force, the grain boundary energy and the dislocation density, respectively. In this work, we have systematically investigated the interconnection of these mechanisms in an Al-Zn-Mg-Cu alloy processed by high pressure torsion at room temperature to achieve a submicrometer grain size from the solutionized state. Ageing heat treatments were carried out to study the precipitation kinetics using complementary characterization techniques such as in-situ small-angle X-ray scattering, differential scanning calorimetry, transmission electron microscopy and atom probe tomography. Experimental data clearly demonstrate that precipitation starts at much lower temperature in the deformed state and that the precipitation sequence is modified as compared to the classical coarse grain alloy. Beyond the detailed understanding of precipitation mechanisms and the competition with recrystallization and recovery processes, the systematic relationship between microstructural features and the mechanical behavior was also studied. A special emphasis was given on the specific contribution of precipitates nucleated heterogeneously along grain boundaries.