Effect of Deep Cryogenic Treatment Time on the Microstructure, Mechanical Properties, and Corrosion Resistance of High-Alloyed Al–Zn–Mg–Cu Alloys

Abstract To investigate the regulation mechanism of deep cryogenic treatment (DCT) after artificial aging on the properties of ultra-strong and highly alloyed Al–Zn–Mg–Cu–Zr–Sc aluminum alloy, this study systematically examined the influence of DCT duration on the alloy’s microstructure, mechanical properties, and corrosion resistance. The results show that DCT can significantly increase the dislocation density of the alloy and enhance the dislocation strengthening effect, among which the DCT-9 scheme exhibits the optimal strengthening effect. However, DCT has no significant impact on the morphology, size, quantity of coarse second phases or the average grain size of the alloy. In terms of mechanical properties, DCT can improve the alloy’s hardness, yield strength, and tensile strength, with a slight decrease in plasticity. The DCT-9 alloy achieves the optimal comprehensive mechanical properties, with a yield strength, tensile strength, and elongation after fracture of 808.2, 837.6 MPa, and 2.9%, respectively. Compared with the alloy without deep cryogenic treatment (DCT-0), the yield strength is increased by 31.1 MPa. Regarding corrosion resistance, DCT significantly enhances the alloy’s corrosion resistance, and the DCT-9 alloy performs the best: the maximum intergranular corrosion depth is 25.50 μm, which is 36.13 μm smaller than that of the DCT-0 alloy; the exfoliation corrosion rating is improved from EB grade to PC grade; and the electrochemical corrosion current density is reduced from 9.23 × 10–5 to 2.01 × 10–5 A/cm2. This study confirms that the introduction of DCT after artificial aging can effectively optimize the comprehensive properties of the alloy, and DCT-9 is the optimal process parameter. The research results provide a theoretical basis and technical support for the process optimization and performance improvement of this alloy.