Microstructural aspects of crack nucleation during cyclic loading of AA7075-T651

A series of fatigue test samples made of 7075-T651 aluminum were interrupted at various life fractions and the number of debonded, cracked particles and cracks in the metal matrix was determined quantitatively as a function of load cycles. It was found that only cracked constituent particles nucleate a matrix crack. The crystallography of one individual crack and its three-dimensional shape was determined by serial sectioning in a scanning electron microscope by applying focused ion beam (FIB) milling in combination with orientation imaging microscopy (OIM). The limited data suggest that the initial growth direction of a crack is influenced by the crystallographic orientation of the matrix into which the crack is growing.