Atomic-scale analysis of the segregation and precipitation mechanisms in a severely deformed Al-Mg alloy

Due to their interaction with crystalline defects, solute atoms play a critical role in the microstructure evolution of aluminum alloys during deformation. In addition, deformed structures often exhibit a modified aging response. For a better understanding of these mechanisms, we provide here a thorough study of deformation-induced segregation and precipitation mechanisms in an aluminum alloy containing 5.8 wt.% Mg subjected to severe plastic deformation (SPD). The solutionized alloy was processed by high-pressure torsion at room temperature and at 200 °C. The investigation of the microstructure and of the distribution of Mg after deformation by scanning transmission electron microscopy and atom probe tomography revealed that clustering and segregations occurred during severe deformation. Mg atoms agglomerate on grain boundaries (GBs), forming mostly nanoscaled clusters at room temperature and more uniform segregation along GBs at 200 °C. In any case, however, the equilibrium Al3Mg2 phase does not nucleate. Using post-deformation annealing treatments, it was found that it can proceed only through a very specific orientation relationship with the face-centered-cubic Al matrix. Both the contribution of dislocations and deformation-induced vacancies were considered to account for the enhanced mobility of Mg atoms. From theoretical estimations it is, however, concluded that Mg atoms are dragged by the vacancy flux toward GBs while dislocations should not play a significant role. These data provide new insights about mechanisms controlling dynamic precipitation and segregation during SPD of aluminum alloys. The segregation and formation of clusters that is revealed can additionally contribute to the strengthening of these alloys, leading to a new understanding of dynamic ageing in non-age-hardenable alloys.