Crystallographic orientation-dependent growth mode of microstructurally fatigue small crack in a laminated Ti-6Al-4V alloy

The fatigue life of Ti-6Al-4V alloy exhibits significant scatter, which is derived from the statistical scatter of uncontrollable material factors. In particular, crack propagation behaviors in microstructurally small cracks strongly depend on material factors. It is necessary to understand the mechanisms underlying the scatter of fatigue properties. In this study, the dominant material/mechanical factors of microstructurally small fatigue crack growth behaviors are extracted, and the mechanisms of fatigue crack propagation life scatter derived from them are clarified. A Ti-6Al-4V billet was studied in which the microstructure was fully laminated with α and β phases. The major constituent phase was α. Microstructurally small artificial defects were introduced at a center of prior β grains by a focused ion beam (FIB). A fatigue test was carried out at a stress ratio R = 0, stress amplitude σa = 400 MPa, and frequency 1.3 × 10 ^-2 Hz in a vacuum using scanning electron microscopy (SEM). In situ SEM observation was carried out around each FIB notch. After the fatigue test, the dependence of the crack growth path on crystallographic orientation was investigated by using electron backscattered diffraction. It was found that microstructurally small crack growth behaviors depended on crystallographic orientation. The dominant crack growth mechanism was the mode II crack growth mechanism by shear stress along the basal plane as a driving force. The dominant factor that causes large scatter of fatigue life in a α- Ti-6Al-4V alloy is the difference in resolved shear stress along the basal plane. Specifically, the difference in the angle between the basal plane and loading direction, the low reproducibility of the mode II crack growth mechanism, and the random position of pre-existing damage cause the scatter of fatigue crack growth rate.