Influence of macrozones on the fatigue cracking behavior and fracture mechanisms of rolled Ti–6Al–4V alloy

In this work, rotating bending fatigue, scanning electron microscope in-situ fatigue, pre-fatigue (and post-fatigue) electron backscatter diffraction tests were carried out for rolled Ti–6Al–4V alloys. The macrozone sizes and orientations of the materials were classified and the effect of macrozones on fatigue cracking behavior and fracture mechanisms was studied. The results indicated that intra-granular fracture was dominated in the process of crack propagation. Basal and prismatic slip systems were favored for the macrozones whose crack paths were straight and zigzag, respectively. Multiple active slip systems can also induce the fatigue crack deflections inside grains. Moreover, fatigue crack propagation rate, threshold stress intensity factor range (ΔK_th) and fracture toughness (K_IC) were measured for different kinds of macrozones. For the macrozones favorably orientated for prismatic slip, their crack propagation resistance and ΔK_th were excellent. However, the K_IC values for different macrozones were similar. Finally, the effect of macrozone orientations on fracture mechanisms (cleavage and plastic fracture) was discussed through a combination of Schmid factor, active slip systems and the Δθ angle (between -Ti phase (0001) plane normals (c-axes) and cyclic load directions). The Δθ is an adequate parameter to predict the fatigue fracture modes.