Stepwise stabilization of low-melting constituents enables grain refinement and overburning suppression in highly alloyed Al–Zn–Mg–Cu alloys

Abstract

High-strength Al–Zn–Mg–Cu alloys require high-temperature solution treatment to increase solute supersaturation, but severe Zn/Mg/Cu segregation and low-melting eutectic constituents make them highly vulnerable to overburning. Here, a multi-stage solution treatment (MST: 450/2h + 460/2h + 470/2 h) strategy was proposed to widen the safe solution-treatment window of Al–Zn–Mg–Cu alloys with different alloying levels. Compared with direct 470°C solution treatment, MST produced a stronger age-hardening response while suppressing surface blistering and microstructural overburning. After T6 aging, the high-, medium-, and low-alloyed alloys achieved UTS/YS values of 816/796, 725/706, and 707/686 MPa, respectively. For the medium-alloyed alloy, MST refined the average grain size to 6.38 μm, compared with 7.12 and 7.96 μm after direct 470 °C/4 h and 470 °C/6 h treatments, respectively. The underlying mechanism is not prolonged thermal exposure, but stepwise stabilization of the inherited heterogeneous microstructure. The initial low-temperature stage promotes local solid-state dissolution of Zn/Mg-rich multiphase eutectics and enhances fine-grain retention by restricting rapid grain-boundary migration. The intermediate stage further drives Zn and Mg diffusion from residual constituents into the Al matrix, reducing their low-melting character.