Effect of mean stress (stress ratio) and aging on fatigue-crack growth in a metastable beta titanium alloy, Ti-10V-2Fe-3Al

The effect of mean stress, or the stress ratio (R), on the fatigue-crack growth (FCG) behavior of alpha-aged and crt-aged microstructures of the beta titanium alloy Ti-10V-2Fe-3Al was investigated. While the mean stress had a negligible effect on the FCG behavior of the alpha-aged microstructure, a strong effect was observed in the alpha-aged microstructure. In particular, the values of the threshold stress-intensity range (Delta K-th) exhibited a strong dependence on R in the omega-aged microstructure, while this dependence was weak in the ru-aged microstructure. These effects seem to arise primarily from fracture-surface roughness-induced crack closure. The crack closure levels for the alpha-aged microstructure were found to be very low compared to those for the omega-aged microstructure. Transmission electron microscopy and scanning electron microscopy studies of microstructures and fracture surfaces were performed to gain insight into the deformation characteristics and crack propagation mechanisms, respectively, in these microstructures. The microstructure-induced differences in FCG behavior are rationalized in terms of the effect of aging on slip and crack closure.