TEM observation for Al-Zn-Mg (-Cu, -Ag) alloys peak-aged at 423 K

Al-Zn-Mg alloy has been known as one of the aluminum alloys with the good age-hardening ability and the high strength among commercial aluminum alloys. The mechanical property of the limited ductility, however, is required to further improvement. Four alloys were prepared with or without alloying elements of Cu and Ag. Precipitation sequences on the alloys were investigated with hardness measurements and TEM observations. Ag or Cu added alloy showed higher maximum hardness than Ag or Cu free alloy. The number densities of the precipitates in Ag or Cu added alloys were higher than those in Ag or Cu free alloys. The GP(II) zone and n′ phase were observed in all alloys peak-aged at 423 K, although GP(I) zone were observed in the Cu added alloy.