Extruded Al-8wt.% Zn-1-2.5wt.% Mg-1.2wt.% Cu-4wt.% La alloys with high tensile strength and good stress corrosion cracking resistance were developed and their fracture toughness was investigated. The effect of metallurgical parameter on the fracture toughness value was examined. The main results are as follows;
(1) The fracture toughness values of the extruded Al-8wt.% Zn-1-2.5wt.% Mg-1.2wt.% Cu-4wt.% La, which were about 20MPa√m without anisotropy, were not influenced by Mg content.
(2) The second phase particles in the extruded alloys, which consisted of Zn and La, were estimated quantitatively. The mean diameter was about 4.7μm and the volume fraction of the particles was 1.6-2.3vol.%.
(3) The second phase particles consisting of Zn and La in the extruded alloys cracked easily, and became the nucleus of voids when the matrix fractured. The fracture toughness of the extruded alloys was affected by the particles. The fracture toughness value, KIc, was related to the volume fraction of the second phase particles, fv, and the mean diameter of the particles, D, as follows:
KIc≈[ζσ0.2E(π/6)1/3D]1/2·fv-1/6∝(DEσ0.2)1/2·fv-1/6
where ζ is constant, σ0.2 is 0.2% proof stress and E is Young's modulus.
