Healing the high-temperature-retrogression-caused wide precipitation-free zones in Al-Zn-Mg-Cu alloy via strain-aging induced precipitates

Abstract

Retrogression and re-aging (RRA) treatments have been demonstrated to effectively increase the corrosion resistance of high-strength Al-Zn-Mg-Cu alloys. However, RRA-treated Al-Zn-Mg-Cu alloys still suffer from poor ductility and intergranular corrosion because the wide precipitation-free zones (PFZs) caused by high-temperature retrogression treatment cannot be eliminated during the following re-aging process. Herein, inspired by the fact that retrogression dissolves part of the existing precipitates to obtain supersaturation in the matrix and dislocations as heterogeneous nucleation sites for precipitation are more prone to accumulate into the soft PFZs, a strategy of introducing pre-deformation after retrogression to trigger the secondary precipitation in the retrogression-caused PFZs during the subsequent re-aging is developed to heal this inherent weakness. Furthermore, the dislocations aggregation around the precipitated phases within grains accelerate the transformation of their morphology from platelet-like to circle-like after re-aging. In particular, the precipitates in the dislocation-dense band structures become coarser than those in the out-of-band regions. The synergy of the secondary precipitation at retrogression-caused PFZs and the circle-like and band-distributed precipitates within grains weakens the strength and electrochemical difference between the grain interior and the grain boundary. Compared with the traditional RRA treatment, this innovative technique further enhances the corrosion resistance and ductility of the fully recrystallized Al-8.99Zn-2.12Mg-1.98Cu-0.092Hf-0.12Zr alloy without sacrificing strength. This finding offers practical and straightforward implications for overcoming the current limitation in RRA-treated Al-Zn-Mg-Cu alloys and has the potential to expand to other aging-hardening alloys.