Improving ageing kinetics and precipitation hardening in an Al-Mg-Si alloy by minor Cd addition

This study reports significant improvements in both the age hardening kinetics and peak hardness of an Al-Mg-Si alloy by microalloying with 0.06 at% Cd. A detailed high-angle annular dark-field scanning transmission electron microscopy study shows that most of the needle-shaped hardening precipitates beta (similar to 82%) formed in the Cd-containing alloy are attached to or around Cd-rich precipitates, indicating that the Cd-rich precipitates can act as effective nucleation sites for needle-shaped precipitates, and therefore substantially increase the number density and decrease the size of needle-shaped precipitates. It also shows that most of the beta precipitates in the Cd-containing alloy have a disordered structure, even at the peak-aged state. Additionally, Cu and/or Cd atoms are found to be incorporated into beta and some precipitates with Q'/C-like triangular sub-units are formed, which are very rare in Al-Mg-Si alloys with a trace Cu content of 0.01 wt%.