Effects of natural aging on the artificial aging kinetics and responses of Al–5Mg–3Zn–1Cu (wt.%) alloy

Abstract

Effects of natural aging (NA) on artificial aging (AA) behavior of Al–5Mg–3Zn–1Cu alloy were studied from the perspectives of kinetics and microstructure evolution. The results show that short-term NA (24 h) has marginal effects on AA at any temperature, as well as long-term NA (>240 h) on low-temperature AA (120 °C), but long-term NA can accelerate aging kinetics and alleviate severe hardening ability attenuation at high temperatures (>150 °C). Analysis of the precipitates evolution demonstrates that such attenuation is primarily attributable to coarsening and reduction of intragranular precipitates and is also slightly related to the formation of precipitate-free zone (PFZ). Three main factors cause the unfavorable microstructure in NA-free alloy at high temperatures AA: (1) Insufficient nucleation of intragranular precipitates induced by weakened nucleation driving force, larger critical nucleation radius, and rapid vacancy annihilation; (2) The nucleated precipitates are prone to coarsening; (3) The precipitation near grain boundary is obstructed due to the depletion of vacancies and solutes. Clusters formed during NA can inhibit the unfavorable microstructure by delaying the annealing out of excess vacancies, promoting the nucleation of precipitates, and elevating the activation energy for precipitation coarsening, thereby enhancing the hardening potential. This work is instructive for the formulation of heat treatment strategies for Al–Mg–Zn–Cu alloys with high Mg/Zn ratios.