PSI - Issue 7

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

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M. Goto et Al./ Structural Integrity Procedia 00 (2017) 000–000

with the shear-mode. The shear-mode growth of stage II crack along crystallographic slip planes was commonly observed in other classes of precipitate strengthened Cu alloys (Goto et al. 2012).

Fig. 3. The change in surface states of Cu-6Ni-1.5Si alloy at σ a = 210 MPa, ( N f = 7.46×10 5 ): To prevent the crack initiation from GBs damaged by etching, etched specimens were fatigued after polishing off a layer of a few micrometers from the surface. After a grain-sized crack was initiated, the surface was etched to reveal the microstructure.

Fig. 4. SEM image of crack paths just after the crack initiation ( σ a = 240 MPa, N = 1.6×10 5 ). Fig.5 shows the behaviour of a crack initiated at DP phases. A 10 µ m-length crack was initiated inside the DP phases at N = 3.2 × 10 5 ( N / N f = 0.62). Generally, the crack initiation from DP phases occurred at a later fatigue stage ( N / N f > 0.5), whereas fatal cracks initiated from GBs at an extremely early fatigue stage ( N / N f < 0.2), suggesting the harmlessness of sporadic DP phases on fatigue strength.

Fig. 5. Crack initiation inside DP phases at σ a = 240 MPa, ( N f = 5.187×10 5 ).

The bright-field TEM, and HR-TEM micrographs in Fig. 6a–c present the microstructure around GB areas; large particles with a few tens of nanometers heterogeneously precipitated along the GBs can be observed (highlighted by a white arrow in Fig. 6a). The heterogeneous precipitates were δ -Ni 2 Si intermetallic compounds (Fig. 6b). The fast diffusion along the GBs is likely to lead the formation of precipitates at GBs and subsequent rapid extension. In the