Analyzing Si Precipitation on Age Hardening of an Al-Si-Mg Cast Alloy

The correlation of age hardening behavior and Si precipitation in alpha(Al) of Al-8wt%Si-0.35wt%Mg alloy has been investigated by micro hardness measurement, electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) analysis. The EPMA results show that Si concentration in the center of alpha (Al) dendrites is higher than that in the edge and the main concentration is about 1.5wt% for Al-8wt%Si-0.35wt%Mg alloy in as cast condition. After solution treatment at 530 degrees C for 8 h followed by water quenching (T4 treatment), hardness value decreases 9 HV, which is accompanied by the decrease of Si concentration in alpha (Al). Aging the as-cast sample and T4 treatment sample at 150 C for 20 h, the main concentration of Mg and Si in alpha (Al) changes little. Hardness value after as-cast aging is only 3 HV lower than that after T6 treatment. Nanometer Si particles and beta" and/or beta' phases are found in aged samples. The higher hardness value for as-cast aging samples should contribute to the nanometer Si particles in alpha (Al).