13th International Conference on Fracture June 16–21, 2013, Beijing, China 5 [7]. The difference between the actual crack path direction and the direction of the maximal energy release varies between 2 to 15 degrees, which has been verified by Rubinstein [8]. The high magnification graphs in Figure 7(a) and (b) show that the crack deflection and branching in both samples are severe. This means that it needs more energy to proceed and enables a larger fraction of volume to contribute to stress relief by plastic deformation [9]. Fatigue fracture surface features (Figure 8) on FCG samples show a correlation to the underlying microstructure, where microstructural parameters such as the lamellar spacing and lamellar colony size appear to affect the crack propagation resistance. Chan and Kim [10] have suggested a relatively complex dependence of fatigue fracture toughness on the colony size. The thickening of alpha platelets can increase fatigue fracture toughness by contributing to crack deviation. Such improvement by alpha plate coarsening has been observed in many previous investigations [11]. It is important when the crack tip lies in a colony or the plastic zone size is similar to the thickness of platelets. The colony size can also affect the fatigue crack growth when crack propagates across colony boundaries [12]. In terms of grain boundary α, the increase in thickness of α at β grain boundary is helpful for frequent crack deviations when cracks go through the grain boundaries [13]. Fatigue striations and secondary cracks were found on fatigue fracture surfaces (Figure 8) of both samples, which indictate some ductillity of the materials. However the fatigue fracture surfaces also show that both sample conditions have similar fatigue fracture mechanisms, although, more secondary cracks were found in the as-cast Ti-6Al-4V. Figure 6. Crack profile of Ti64: (a) as-cast, (b) post-HIPped. (a) (b) Crack growth direction Figure 7. Crack path morphologies of Ti64: (a) as-cast, (b) post-HIPped. (a) (b) Crack branching Crack branching
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