Fatigue crack surface crystallography near crack initiating particle clusters in precipitation hardened legacy and modern Al–Zn–Mg–Cu alloys

SEM-based stereology in conjunction with electron backscattered diffraction established fatigue crack surface crystallography within the region from ∼1 to 50 μm of crack initiating particles in aluminum alloys, 7075-T651 and 7050-T7451, stressed in ambient moist-air and −50 °C dry-N2 environments. Fatigue crack facets were parallel to a wide variety of crystallographic planes, with pole orientations distributed broadly across the irreducible stereographic triangle between the 〈0 0 1〉 and 〈1 0 1〉-poles within each environment. The results indicate environmentally-affected fatigue cracking in both cases, given the similarity between the observed morphology and crystallography with that of a variety of aluminum alloys cracked in the moist environment. The deviation from low-index {0 0 1}/{1 0 1}-planes and the occurrence of high-index cracking planes observed are interpreted as trans-subgranular decohesion or inter-subgranular cracking, due to trapped hydrogen. There was no evidence of {1 1 1} slip-plane cracking typical of the stage I crack growth mode observed in single crystals and high-purity polycrystals of face centered cubic metals, and which has been widely assumed for the present materials within fatigue crack initiation and early growth models. Rather, the facets tend to have near-mode I spatial orientation. The results provide necessary observations upon which environment-dependent, physically-based multi-stage fatigue models can be hypothesized and validated.