PRECIPITATION BEHAVIOR OF A 2-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 all oy was investigated by high resolution transmission electron microscop y (HRTEM) together with hardness, electrical resistivity measurements. A number of primary G.P. zones with the diameter of about 1nm were ob served in the specimen aged at 293 K for 60 ks. Both the mono- and mul ti-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 precipitate s were observed. The aging at 473 K produced precipitates in the follo wing sequences: the random type-->parallelogram type-->beta '' phase-- >TYPE-B-->TYPE-A (-->partially TYPE-C). The maximum hardness (HVmax) o f the specimen aged at 473 K after pre-aging at 293 K for 60 ks indica tes a negative effect, i.e. the HVmax of the two step aged specimen is lower than that of the specimen only aged at 473 K. The HVmax of the specimen aged at 473 K after pre-aging at higher temperatures than 343 K shows a positive effect, ie. the HVmax of the two step aged specime n is higher than that of the specimen only aged at 473 K. In the speci men pre-aged at 423 K which exhibits a positive effect, a number of th e random type precipitates are formed. On the other hand, in the speci men pre-aged at 293 K, which exhibits a negative effect, a number of t he beta '' phase are formed. It is concluded that the random type prec ipitates produced at 423 K pre-aging are stable at the 473 K final agi ng, while the primary G.P, zones produced after pre-aging at 293 K bec ome unstable at the 473 K final aging due to dissolution into the matr ix, resulting in the heterogeneous precipitation of the beta '' phase.