Precipitation behavior of a two-step aged Al-Mg-Si alloy with excess silicon

The behavior of two step aging in an Al-1.0 mass%Mg₂Si-0.4 mass%Si alloy was investigated by high resolution transmission electron microscopy (HRTEM) together with hardness, electrical resistivity measurements. A number of primary G.P. zones with the diameter of about 1nm were observed in the specimen aged at 293 K for 60 ks. Both the mono- and multi-layer G.P. zones were detected in the specimen aged at 343 K for 60 ks. On the contrary, almost no G.P. zones were formed in the specimen aged at 423 K for 60 ks. Instead, a number of random type precipitates were observed. The aging at 473 K produced precipitates in the following sequences: the random type→parallelogram type→β″ phase→TYPE-B→TYPE-A (→partially TYPE-C). The maximum hardness (HV_max) of the specimen aged at 473 K after pre-aging at 293 K for 60 ks indicates a negative effect, i.e. the HV_max of the two step aged specimen is lower than that of the specimen only aged at 473 K. The HV_max of the specimen aged at 473 K after pre-aging at higher temperatures than 343 K shows a positive effect, i.e. the HV_max of the two step aged specimen is higher than that of the specimen only aged at 473 K. In the specimen pre-aged at 423 K which exhibits a positive effect, a number of the random type precipitates are formed. On the other hand, in the specimen pre-aged at 293 K, which exhibits a negative effect, a number of the β″ phase are formed. It is concluded that the random type precipitates produced at 423 K pre-aging are stable at the 473 K final aging, while the primary G.P. zones produced after pre-aging at 293 K become unstable at the 473 K final aging due to dissolution into the matrix, resulting in the heterogeneous precipitation of the β″ phase.