13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- during phase transformation, this interface plane is parallel to one prismatic plane in the hcp crystal of the alpha phase, which is a possible slip system [7]. Secondly, cracks are found to initiate on slip bands inside of primary alpha grains. Once initiated, different growth mechanisms were found for the short crack propagation in stage I. Inside of primary alpha grains or single lamellae, the crack grows on slip bands. By means of EBSD-measurements the cracks are identified to be either on the basal plane or the prismatic plane. The active slip bands are favorably orientated slip systems characterized by a high Schmid factor S. An example of such a crack is shown in Fig. 2a; the crack starts to grow on the basal plane with a Schmid factor of S = 0.43. After crossing a grain boundary the crack is slightly deflected and propagates on a prismatic plane, once again characterized by a high Schmid factor S = 0.46. Thus, the crack propagation rate is controlled by crack tip slide displacement range ∆CTSD. The grain boundaries act as obstacles to crack propagation, as they prevent a transmission of slip into the neighboring grain. This yields a dislocation pile up in front of the barrier resulting in a decreased crack growth rate. If the boundary is overcome, the stress intensity will be relieved by slip in the next grain and the crack propagation rate will increase again, resulting in an oscillating crack growth rate. In addition to that, crack growth is also found on grain boundaries, as can be seen at the right crack tip in Fig. 2a. Here, the crack path is in a plane perpendicular to the applied load so that the normal stress, which causes a crack opening, seems to be responsible for crack propagation. The crack in Fig. 2b grows on the interface between two lamellae. As this plane is equal to a prismatic plane with a Schmid factor of S = 0.49 the crack growth mechanism can be explained by slip as well. However, cracks were also found between two lamellae oriented normal to the applied load, where the Schmid factor on the respective prismatic plane is small. Figure 2. (a) Stage I-crack in primary alpha on slip plane and (b) crack growth on lamellae interface . Since orientation data from the surface was used to calculate the possible slip plane, FIB sectioning was applied to determine the crack path direction into the interior (Fig. 3). A comparison between the calculated tilt angle of the slip plane with the surface and the crack path in the depth shows a good agreement. This strongly supports the assumption of crack growth occurring on a single slip system. However, as soon as a grain boundary is crossed (indicated by the white line) the crack is slightly deflected as can be observed on the surface as well. a b
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