Effect of Cu content on precipitation and age-hardening behavior in Al-Mg-Si-xCu alloys

Abstract

The effect of varying the Cu content of Al-Mg-Si-xCu (x = 0.5, 1.0, 2.5 and 4.5, in wt.%) alloys on precipitation and their age-hardening behavior was investigated through hardness measurements, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The results indicated that aging at 165 °C, the peak aging hardness increased from 109 to 218 HV with the Cu content increasing from 0.5 to 4.5 wt%. Similarly, there was an increase in the peak aging time with Cu content. However, in over-aging stage, the hardness of 4.5Cu alloy decreased more rapidly than that of 0.5, 1.0 and 2.5Cu alloys. Furthermore, an increase in Cu content brought about an apparent change in the main precipitate of the peak-aged samples from needle-shaped β″ phase in 0.5Cu alloy to: needle-shaped β″ phase and granular Q′ phase in 1.0Cu alloy, granular Q′ phase and θ′ platelets in 2.5Cu alloy, and θ′ platelets with a small amount of Q′ phase in 4.5Cu alloy.