PSI - Issue 19

Fumiyosi Yoshinaka et al. / Procedia Structural Integrity 19 (2019) 214–223 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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investigated in detail in the future to clarify the effect of the deformation-induced transformation on the crack growth behavior.

Fig. 8. Post- fatigued microstructure in the fractured specimen tested at (a) Δ ε t = 2%, (b) σ a = 325 MPa, and (c) σ a = 275 MPa.

As shown in Fig. 7, surface relief emerged on the specimen that did not fracture up to 10 7 cycles, that is, the specimen tested at the stress level lower than the fatigue limit. Fig. 9 shows the EBSD results of the longitudinal section obtained from the center of this specimen. It was found that ε -martensite was formed even at the stress level lower than the fatigue limit. In addition to ε -martensite, γ -mechanical twins were detected at the tips of ε -martensite as demonstrated in the Inverse Pole Figure (IPF) map of γ -austenite shown in Fig. 9(c). In some places, including the region shown in Fig. 9, there are some grains occurring the ε -martensitic transformation. However, the amount of the deformation-induced microstructure at the stress level lower than the fatigue limit was limited. Here, the Schmid factor (SF) for the ε -martensitic transformation of the γ -phase with ε -martensite in the center of Fig. 9(b) was high (0.5), implying that a very high SF is one of the important factors impacting the ε -martensitic transformation at the stress level lower than the fatigue limit. On the other hand, there were γ -phases with a high SF but without ε -martensite. Hence, there may be other crucial factors impacting the ε -martensitic transformation.

Fig. 9. Post-fatigued microstructure in the specimen tested at σ a = 250 MPa for 10 7 cycles without fatigue failure: (a) SEM image, (b) phase map superimposed on the IQ map, and (c) IPF map of γ -austenite in the dashed area shown in (a). In (c), γ -mechanical twins are represented by circles.