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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A Vickers hardness, H V , of the aged alloy was H V = 259 which was nearly equivalent to maximum hardness by the aging. Large amounts of a Ni and Si content of about 7.5 wt% enhanced the driving force on precipitation because of the large difference between the supersaturated alloy matrix composition and the equilibrium solution limit (Kim et al. 2013), reducing the aging times (0.5 h) for the maximum hardness.

Fig. 1. Microstructure: (a) before aging ( H V = 171); (b) after aging ( H V = 259); (c) HR TEM images of the matrix; (d) SEM image of DPs.

Fig. 2 shows the S-N plots of the data collected during the stress controlled fatigue test. Fig. 3 shows the change in surface states around a fatal crack, which led to the fracture of the specimen, during cyclic stressing at σ a = 210 MPa. To clarify the positional relationship between the crack initiation sites and the microstructure, the specimen surface was etched prior to the fatigue tests to reveal the microstructure. After the optical micrographs of the etched surface were taken, a layer of a few micrometres was polished off the surface by buffing to remove the etching-induced damaged areas that would affect crack initiation during stressing; this was followed by fatigue tests. After initiation of fatigue cracks with grain-size length, the fatigue test was interrupted for etching to reveal the microstructure, and the test was resumed with the etched specimen. A crack was formed on the buffed surface at 2.0 × 10 5 cycles ( N / N f = 0.26). On the surface treated by etching, GBs and DP structures were recognized along the cracks. A comparison between pre-fatigued and fatigued surfaces suggested that the starting point of cracks was GBs, as shown by black arrows. Goto et al. (2016) indicated that GBs were the crack initiation sites of Cu-Ni-Si alloy with high solute concentrations fatigued at high stress amplitudes ( σ a ≥ 400 MPa) corresponding to a low-cycle fatigue regime. After the initiation, the cracks extended with shear mode along the slip planes of grains contiguous to the GBs (Fig. 4). The crack growth along GBs just after the initiation occurred occasionally in the cases in which neighbouring grains had no favourable crystallographic slip planes. The monitoring of subsequent crack propagation behaviour showed that large cracks greater than a 1-mm-length (stage II crack) continued to grow

Fig. 2. S-N curve under the stress controlled fatigue test of round bar specimens.