The Control Of Recry Stallization Of Al-Zn-Mg-Cu Alloys And Effects Of Sub-grain Boundaries On The Precipitation Of MgZn₂Phase Particles

Because coarse recrystallized grains can deteriorate their strength, toughness, and stress corrosion resistance, we need to avoid recrystallization during processing Al-Zn-Mg-Cu alloys. Besides, dispersed and micron-sized η’-MgZn2particles are necessary for optimizing mechanical properties of Al-Zn-Mg-Cu alloys. However, coarse MgZn2particles also intrigue quench sensitivity. In the present study, the effect of sub-grain boundaries on the precipitation behavior of MgZn2and effects of the two factors on mechanical properties, corrosive resistance, and quench sensitivity were investigated. The main conclusions are as follows:(1) Nano/submicron scale η’-MgZn2and η-MgZn2particles which were yielded during thermomechanical processing could impede the migration of (sub-) grain boundaries and not induce PSN. Under the experimental conditions of the present study, appropriate thermomechanical processing could reduce the recrystallization volume fraction to4%. Coherent Al3(Sc, Zr) reduced the PFZ of Al3Zr particles and inhibit the migration of grain boundaries, leading to lower recrystallization. Staged homogenization improved uniformity of Al3Zr particles, which reduced recrystallization volume fraction.(2) With an increase in the degree of sub-grain boundaries, precipitation of GP zones and η’-MgZn2is promoted during heating, i.e. precipitated temperature declined and active energy reduced. According to calculating results, the reduced active energy was similar to surface energy offered by sub-grain boundaries. Like heading process, the precipitation of MgZn2, especially η’-MgZn2is promoted, which may have complicated effects on quench sensitivity.(3) Sub-grains and sub-grain boundaries could produce notable sub-structure strength and promote the precipitation of MgZn2phase, leading to higher tensile strength and hardness. Compared with high-angle boundary, small-angle boundary had lower energy, which made it have lower possibility to be the pass of corrosion. Besides, sub-grain boundaries improved nucleation rate of second phase particles, resulting in the discontinuation of second phase particles. The two factors improved the resistance of corrosion. However, due to the promotion of the precipitation of MgZn2phase, the quench sensitivity improved.