13th International Conference on Fracture June 16–21, 2013, Beijing, China 4 Figures 3 and 4, respectively. The fractographs show that the fracture surface changes from the intergranular-dominated fracture mode in Figure 4(a) to the transgranular fracture as shown in Figure 4(b). Figure 4(a) shows the straight edges of the grains and shiny surfaces, which indicates that the grain boundary strength is weak in some areas in the as-cast materials. The fracture surface of the post-HIPped sample is mainly transgranular, indicating improved ductility. 3.3. Fatigue crack growth The fatigue crack growth curves obtained for these two conditions are presented in Figure 5, which plots the fatigue crack growth rate, da/dN, against the applied stress intensity factor range. As shown in Figure 5, the rate of fatigue crack growth in the as-cast condition is faster than that after post-HIPping. The crack paths in the two tested samples are shown in Figure 6. Crack deflection and branching were also found in these samples illustrated in Figure 7. The crack deflection at colony boundaries shows a zigzag path. When the crack propagated through lamellae of unfavorable orientations, it consumes more energy than a straight path, leading to a higher fatigue crack propagation resistance Figure 4. SEM fractographs of Ti64 fracture toughness samples: (a) as-cast, (b) post-HIPped. (a) (b) 10 11 12 13 14 15 16 17 18 19 20 21 10-6 10-5 10-4 10-3 As-cast sample HIPped sample （da/dN）/(mm·cycle -1) △K/(Mpa·m1/2) Figure 5. Fatigue crack growth resistance curves of Ti64 in both as-cast and post-HIPped state. Fatigue crack growth rate at R=0.5
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