Microstructure and properties of an Al-Zn-Mg-Cu alloy pre-stretched plate under various ageing conditions

The microstructures after various ageing treatments and their relation to the strength, fracture toughness, and corrosion behavior of an Al-Zn-Mg-Cu alloy pre-stretched plate were investigated. The results show that retrogression and reaging (RRA) treatment led to a combination of high strength and stress corrosion cracking (SCC) resistance of the alloy. The TEM microstructure of the RRA-treated alloy is a distribution of very tine precipitates in the aluminum matrix grains, similar to that obtained under T6 condition, and the distribution of coarse eta MgZn2, precipitates on the grain boundaries similar to that obtained by T7 temper. SEM observations revealed that most of the intergranular fracture characteristics were present on the fracture surface of both the T6 and RRA-treated specimens. On the contrary, the fractographs of the T7 treated specimens mainly consisted of dimple-type ductile transgranular fracture with minor intergranular cracking.