PSI - Issue 13

Taketo Kaida et al. / Procedia Structural Integrity 13 (2018) 1076–1081 Taketo Kaida et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 3. GROD maps at (a) ∆ K = 8.1, (b) ∆ K = 22.9, and (c) ∆ K = 43.0 MPa √m . Figure 4 shows an ECC image at a location ~20 μm from the fracture surface at 22.9 MPa √m . A band-like dislocation structure, which is known as vein type (Rieux et al., (1978)), was aligned with a certain direction. Figure 5(a) shows an overview of the ECC image at 43.0 MPa √m . Figures 5(b) and (c) exhibit magnified images at locations 20 and 35 μm from the fracture surface, respectively. Dislocation walls arranged in two directions, known as labyrinth type, (Rieux et al., (1978)) were observed at 20 μm fro the fracture surface. According to the EBSD analysis, the vein type microstructure was formed parallel to the (111) plane, and the dislocation walls aligned with the two directions (labyrinth type microstructure) were formed parallel to (0-11) and (100) planes. Moreover, Figure 5(a) shows the alignment of the dislocation walls, which form vein structure gradually tilted, as the observation region approaches the fracture surface. This exhibits the transition towards the formation of the labyrinth structure. According to the ECC image, dislocations were active on one slip plane at 22.9 MPa √m , and on multiple slip planes at 43.0 MPa √m . Therefore, at the 20 μm from the fracture surface, the band-like structures were aligned to one direction to form vein structure at 22.9 MPa √m , or aligned to two directions to form labyrinth. It is noteworthy that the dislocation walls forming vein and labyrinth structures form on the planes vertical to the resultant Bugers vectors. (Rieux et al., (1978)) To estimate the active slip systems from the tilt status of the dislocation walls, slip factors on {110} and {112} planes, the typical slip system of the bcc crystals, were calculated. A crack front is in multi-axial stress state, due to stress singularity. Therefore, a slip factor ξ , which takes the stress specificity of the crack tip into account was introduced, when discussing the slip systems of the crack tip. The slip factor is defined as (Kimura et al., 2003)   2 2 3 3 cos sin 2sin cos sin cos cos si c { n } os 2 2 2 2              (3) where θ is the angle between the crack propagation direction and the slip line vector on the specimen surface, and φ is the angle between Burgers vector and the slip line vector. The slip factors were calculated, using crystallographic orientations obtained through EBSD analyses. The slip systems having large slip factors are listed in Table 1. From the crystal orientation information obtained through EBSD analyses, it was found that the arrangement plane of the dislocation wall forming vein structure is parallel to the plane perpendicular to the [11-1] direction. Alternatively, the planes of the dislocation walls forming labyrinth are parallel to the planes perpendicular to the [100] and [0-11] that are the resultant vector of [11-1] and [1-11]. By comparison between the alignment of the dislocation walls and the slip factors, it was found that the vein structure was formed by dislocation motion in the (2-11)[11-1] slip system, and labyrinth was formed by the (2-11)[11-1] and (-112)[1-11] slip systems. At locations closer to the fracture surface, the stress promoting the slip motion increases. As the stress increases, the secondary slip systems starts to activate. Hence, as the observation area approached the fracture surface, the contribution of the secondary slip system increased, and the dislocation walls were tilted in the transient process from vein into labyrinth-type microstructures. from the fracture surface.