A novel strategy to enhance the mechanical properties and corrosion resistance of ultra-high strength Al-Zn-Mg-Cu alloy: Pre-recovery Multi-Stage Solution Treatment (P-MST)

Abstract

This study proposed a novel solution treatment method: Pre-recovery Multi-Stage Solution Treatment (P-MST), which involved a pre-recovery treatment before the conventional MST. Hardness testing results indicated that the recrystallization initiation temperature of the alloy is around 240 °C. Through in-depth analysis of the microstructure, mechanical properties, and corrosion resistance of samples subjected to conventional MST (450 °C/2h + 460 °C/2h + 470 °C/2h, water quenched), P-MST-350 °C (350 °C/12h + MST), and P-MST-250 °C (250 °C/12h + 350 °C/12h + MST), it was found that under the treatment of 350 °C/12 h and 250 °C/12 h + 350 °C/12 h, some MgZn2 phases were dissolved, resulting in a decrease in the area fraction of residual secondary phases and refinement in their size, leading to a higher degree of solid solution for Zn and Mg in the alloy. Simultaneously, pre-recovery treatment not only suppressed recrystallization phenomena and decreased intragranular dislocation density, but also caused the intragranular aging precipitates to transition from η' + η to GP zones + η′, with smaller precipitate sizes. Consequently, the alloy's tensile strength, elongation, and corrosion resistance showed significant improvement. Compared to the MST sample, the P-MST-250 °C sample exhibited enhancements in yield strength, ultimate tensile strength, and elongation from 725 MPa, 741 MPa, and 4.1 % to 778 MPa, 802 MPa, and 6.5 %, respectively. Quantitative calculations demonstrated that solid solution strengthening, low-angle grain boundary strengthening, and aging precipitation strengthening contributed approximately 9.6 MPa, 26.3 MPa, and 43.3 MPa, respectively, while the dislocation strengthening decreased 27.3 MPa to the yield strength of the alloy.