Effect of different aging microstructures on the fracture toughness of high strength Al-Zn-Mg-Cu alloys.

Tinsile, impact, fracture mechanics and dynamic tear tests were carried out on 7075, 7175 and 7475 alloy extrusions. Fracture toughness of Al-Zn-Mg-Cu alloys is considerably influenced by aging condition. Under-aging results in high fracture toughness and resistances to crack propagation and unstable fracture. The fracture toughness decreases with increasing yield strength. The fact is related to localized plastic deformation due to finely distributed precipitate particles and to the resulting strengthened matrix. Over-aged meterials have lower fracture toughness than the other aged materials. Precipitate particles coagulate at the grain boundary and transgranular fracture at under- and peak-aging conditions transits to intergranular fracture at over-aged condition as the aging proceeds.