Effect of pre-aging temperature on the behavior in the early stage of aging at high temperature for Al-Mg-Si alloy

Effect of pre-aging temperature on the behavior in the early stage of aging at high temperature for Al-0.81 mass%Si-0.64mass%Mg alloy has been studied by means of measurement of hardness, differential scanning calorimetry, and transmission electron microscopy. Pre-aging temperature influenced on the subsequent aging behavior at high temperature. GP-I zones were formed during the pre-aging below nearly 340K, which remained after the subsequent aging at 448K for 1.8ks. Therefore, GP-I zones coexist with GP-II zones in the alloy aged at 448K for 1.8ks after pre-aged below 340K, and the microstructure is coarse and sparse. This leads to the decrease in hardness. On the other hand, GP-II zones were directly formed during the pre-aging above nearly 340K. And the structure is fine and dense in the alloy aged at 448K for 1.8ks after pre-aged above 340K, which leads to the increase in hardness.