Effects of Si Addition with Natural Aging Time on the Two-Step and Multi-Step Aging Behavior in Al-Mg-Si Alloys

The amount of Si and the heat treatment history strongly affect the age-hardening behavior of alloy. The formation of nanoclusters and precipitates is accelerated in 9M10S (high Si) relative to that in 9M4S (low Si). There is a period where the increase of hardness is retarded during two-step aging at 170 degrees C for the natural aging (NA) samples. On the other hand, hardness directly increased during multi-step aging at 170 degrees C for the NA samples after pre-aging (PA) treatment at 100 degrees C for 3.6 ks. Cluster (1) formed during NA for 2 months is thermally unstable at the early stage of two-step aging at 170 degrees C regardless of chemical composition. The thermal stability of nanoclusters during two-step aging at 170 degrees C is changed with the NA time based on the hardness results. Changes of microstructure with the heat treatment design are understood based on transmission electron microscope (TEM) observations. Higher number density of precipitates for 9M10S compared to 9M4S is confirmed. Larger precipitates are identified for the NA samples relative to PA. The formation behavior of precipitates observed by TEM is consistent with the hardness results.