Early decomposition of supersaturated solid solutions of Al–Zn–Mg casting alloys

•Characteristics of GP zones formed in Al–Zn–Mg cast alloys using SAXS technique.•GP zones formation and growth are accompanied with increase in electrical resistance.•Direct quenching–aging process results in significant increase in electrical resistance peak.•The electrical resistance peak of Al–Zn alloys was low compared to Mg-containing alloys.

This study aimed at investigating the structural, electrical resistivity and hardness changes of aged Al–Zn–Mg alloys. It was observed that the isothermal annealing in temperature range of 25–200 °C leads to the following precipitation sequence: Guinier–Preston zones → η′-phase → η-phase. The significant increase of electrical resistivity (ρ) of the considered alloys was associated with the stage of GP zone decomposition. The formation of the intermediate η′-phase was accompanied with a decrease of electrical resistivity and an increase of hardness. Non-continuous quenching was found to suppress the η′-phase formation; while elevated quenching rates led to the formation of uniformly distributed fine precipitates in the alloy bulk.