Crystallographic selection rule for the propagation mode of microstructurally small fatigue crack in a laminated Ti-6Al-4V alloy: Roles of basal and pyramidal slips

We investigated the microstructurally small fatigue crack growth (FCG) in a fully laminated Ti-6Al-4V alloy. Basal slip only causes Mode II FCG, which is the predominant FCG mechanism in the present alloy. However, when the basal plane was aligned approximately parallel to the notch/crack surface, Mode I or II FCG was activated by pyramidal slip. When a single pyramidal slip system was activated, Mode II FCG on the pyramidal slip also occurred. The shear stress on the basal plane strongly depends on the geometrical relationship between the basal plane and the notch/crack alignment, which caused scatter in the fatigue life.