Microstructural Modelling of Fracture Toughness of Al Alloys

The influence of nonsoluble precipitates on the fracture toughness of 6000 and 7000 series alloys is investigated experimentally and modeled. Two different extruded AA6082 alloys having different Fe, Mg, and Si content are tested in peak aged temper. Hot rolled 7475 and 7075 alloys are also tested in T3, T6, and T7 tempers. The mechanical properties of these materials are measured along various directions (L, T, S) using conventional tensile specimens and notched specimens. The latter geometries are used to calibrate ductile fracture models. Quantitative metallography is used to measure the distribution of nonsoluble precipitates (size, distance, volume fraction) on various planes. Special attention is given to the analysis of the clustering of these phases. Conventional fracture toughness and Charpy tests are carried out on specimens cut in different directions to measure the anisotropy of K(1c) or J(1c). Two models developed in the framework of the local approach to fracture are used to explain the variations of fracture toughness as a function of specimen direction and material temper. The applicability of these models is discussed. It is shown that the modeling of ductility and fracture toughness of these materials often requires that nonsoluble particle clusters be taken into consideration. (Author)