Morphological Effects of Precipitates on Fracture Mechanism of High Purity Al-Zn-Mg Alloys.

Microstructures of high purity Al-Zn-Mg alloys were controlled by Cr and Zr additions and different conditions of aging. The properly Zr-added alloys mainly fracture from the crack initiated at grain boundaries. The fracture toughness is determined by the size and distribution of grain boundary particles. This is attributed to that the fracture toughness is directly proportional to the area fraction of grain boundary particles, but not to the particles in the matrix and PFZ width. Cracking of the Cr-added alloys initiates at Cr dispersoids in the matrix. The fracture toughness of these alloys is proportional to the volume fraction of particles in the matrix, but not to the grain boundary particles and PFZ width. The PFZ width in the Cr-added and Zr-added alloys has no effect on the fracture toughness. The PFZ width may have an influence to crack propagation. Cr addition is superior to Zr addition in fracture toughness. The fracture mode of Cr-added alloys is transgranular, while that of the Zr-added alloys is predominantly intergranular.