PSI - Issue 7

M. Goto et al. / Procedia Structural Integrity 7 (2017) 248–253 M. Goto et Al./ Structural Integrity Procedia 00 (2017) 000–000

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TEM image of GB areas (Fig. 6a), a band-like bright zone along the GBs indicated the precipitate-free zone (PFZ) that was formed as the result of solute atoms’ absorption near the heterogeneous precipitates. Accordingly, the soft PFZs and heterogeneous precipitates along GBs should give rise to localized high-stress/strain at GBs, which contribute to crack initiation. After the initiation, fatigue cracks grew along slip planes in grains sharing GBs because of a weakened slip resistance in PFZs, followed by continued propagation inside the grains as the result of high stress concentration at the crack tips. Fig. 6c shows the microstructure around matrix/DP phase boundaries, exhibiting no both heterogeneous precipitates and PFZs along the phase boundaries. The DP phases were composed of the fully coherent, stable interface between the precipitate and the matrix, resulting in the low values of the lattice mismatch δ (e.g., 0.0048 for the minimum δ, with 0.207 and 0.208 nm of the (021) plane of Ni 2 Si and the (111) Cu plane spacing, respectively (Han et al. 2016)). Accordingly, the crack generation is likely to be difficult at the interface between the precipitate and the matrix. In addition, DP phase formed inside a grain had the same crystallographic orientation as the grain (Han et al. 2016). This suggests that the matrix/DP phase boundaries are stable unlike original GBs, producing a lower strain concentration at the boundaries. Consequently, the fatal fatigue cracks initiated at GBs, not at DP phases.

Fig. 6. Microstructure: (a) TEM image of GB areas (a white arrow indicates heterogeneous precipitates); (b) HR-TEM image of heterogeneous precipitates at GBs; (c) TEM image of matrix/DP-phase boundaries.

Fig. 7 shows a crack path, exhibiting a zigzag manner. However, the path inside DP phases was nearly perpendicular to the loading direction. Fig. 8 shows the fracture surface. The fracture surface corresponding to the matrix showed a rough surface composed of both intergranular and transgranular facets resulted from the crack growth along a favorable slip plane of grains and GBs, whereas the fracture surface corresponding to DP phase were nearly perpendicular to the loading direction, showing a comparatively flat surface. In general, the crack paths along

Fig. 7. Crack growth path ( σ a = 240 MPa).

Fig. 8. Fracture surface at 1 mm beneath the surface ( σ a = 210 MPa).