Quench sensitivity of novel Al–Zn–Mg–Cu alloys containing different Cu contents

The effect of copper content on quench sensitivity in novel Al–Zn–Mg–Cu alloys containing high zinc content was investigated by Jominy end quench test. Electrical conductivity and hardness test, temperature collecting, and transmission electron microscopy (TEM) technique were adopted for the properties and microstructure characterization of three alloys with different copper contents. The results indicate that the electrical conductivity of all three alloys increases with the increase of distance from the quenched end, while the hardness shows an opposite trend. If the dropping of 10% hardness is defined as the critical evaluation standard of quenching, the depth of quenched layer of Alloys I, II, and III are 70, 55, and 40 mm, respectively. The precipitation behavior on grain boundaries of three alloys is similar except for a little difference in size, while the size of precipitates in grains of Alloy III with higher copper content is larger than those of the other two alloys at the same location. Considering all results, the stability of the supersaturated solid solution of Alloy III is lower than those of the other two alloys, meaning that Alloy III shows the highest quench sensitivity. Higher copper content leads to higher quench sensitivity in novel Al–Zn–Mg–Cu alloys with the same content of magnesium, zinc, and other trace elements.