Effects of Cu and (Cr + Fe) additions on age-hardening of Al-Mg-Si alloys

Age-hardening behavior of the Al-Mg-Si alloys added with (chromium + iron) and copper were investigated by hardness measurements and high-resolution transmission electron microscopy (HRTEM). Age-hardenabilities of the alloys aged at 423 K increased with excess Si or Mg₂Si contents. Cu addition increased their age-hardenability, whereas the addition of (Cr + Fe) decreased peak hardness. At the peak hardness in Cu-bearing alloys, the mean size of precipitates was smaller and the number density of precipitates was higher than those in Cu-free alloys. (Cr + Fe)-bearing alloys included AlSi(Cr, Fe) and AlSiFe dispersoids, which had depleted zones around them, and the number density of precipitates decreased in those alloys. Decrease in the number density of precipitates might be explained by consumption of solute atoms by the formation of dispersoids. It was confirmed that the Random-type precipitates existed in alloys aged at 423 K up to the peak hardness by HRTEM observation.