Effects of the exposure to moist air on crack-tip deformation and striation marking in a 7175 T7351 aluminum alloy

This paper investigates the effects of the exposure to moist air on the mechanisms of fatigue crack growth in 7175 T7351 aluminum alloy. A special attention is paid to the crack-tip deformation mechanisms, controlling the fatigue crack growth rate as well as the fatigue striation markings, firstly under moist air and at low loading frequency, corresponding to a high exposure rate, and then as compared to a low exposure rate condition obtained in an inert atmosphere. It is shown that a high exposure rate induces the development at the crack-tip of a 800 nm-thick surface layer, divided into subdomains characterized with a pronounced and highly localized lattice rotation. In contrast, under very low exposure rate, a significantly lower lattice rotation associated with a smoother gradient is observed. This suggests that the formation of subdomains at high exposure rate is actually induced by some environmental effect, presumably in relation with an assistance of hydrogen highly accumulated in a thin subsurface layer in the intense localization of crack-tip deformation resulting into lattice rotation. Finally, it is clearly established that fatigue striation spacing in moist air exactly coincides with the distance separating two successive subdomains exhibiting the highest lattice disorientation.