Abstract
The effect of artificial aging on the microstructure and hardness of an ultrafine-grained (UFG) Al–4.8%Zn–1.2%Mg–0.14%Zr (wt%) alloy was studied. The UFG microstructure with an average grain size of about 260 nm was obtained by severe plastic deformation applying four passes of equal-channel angular pressing (ECAP) at room temperature. Then, artificial aging was performed on the ECAP-processed samples at 120 °C and 170 °C for 2 h. In the ECAP-processed sample Guinier–Preston (GP) zones, MgZn2 precipitates and a high dislocation density were observed. After aging at 120 °C, coarse MgZn2 precipitates were formed in the grain boundaries, leading to softening, while the dislocation density did not decrease. Annealing at 170 °C yielded a growth of the matrix grains to ~ 530 nm with a significant decrease in the dislocation density. In addition, GP zones disappeared and MgZn2 precipitates were formed in both the grain interiors and the boundaries. This overaging of the precipitate structure and the decrease in the dislocation density resulted in a lower hardness than after annealing at 120 °C. It was found that the hardness reduction due to the change of the precipitate structure at 170 °C was higher than that caused by the decrease in the dislocation density.
Highlights
There is a large interest in age-hardenable Al–Zn–Mg alloys (7xxx series) due to their technological and practical importance, as these alloys can be treated to have a preferable combination of ductility and strength, as well as reasonable weldability and corrosion resistance
The UFG microstructure was obtained by four passes of equal-channel angular pressing (ECAP) at room temperature (RT)
The present study revealed that the microstructure of the matrix in the ECAP-processed Al–4.8%Zn– 1.2%Mg–0.14%Zr alloy exhibited a good stability during aging at 120 and 170 °C for 2 h
Summary
There is a large interest in age-hardenable Al–Zn–Mg alloys (7xxx series) due to their technological and practical importance, as these alloys can be treated to have a preferable combination of ductility and strength, as well as reasonable weldability and corrosion resistance. If a supersaturated Al–Zn–Mg alloy is aged under different conditions, various metastable and stable precipitates may form [1,2,3,4,5,6,7,8]. Aging can be used for tailoring the mechanical behavior of these alloys. For higher aging temperatures (typically between 80 and 150 °C), metastable intermediate g0 particles are formed directly from the SSSS and/or mediately from the GP zones. The formation of stable g precipitates with a composition of MgZn2 is expected. For an effective application of aging on tailoring the mechanical properties of Al– Zn–Mg alloys, it is necessary to know the influence of the chemical composition and the history of the sample (e.g., pre-deformation) on the precipitation during aging
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