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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Fig. 6. Relationship between the stress amplitude σ a and the number of cycles to failure N f .

Fig. 7. Post-fatigued specimen surface tested at σ a = 250 MPa for 10

7 cycles without fatigue failure.

3.4. Post-fatigued microstructure

The deformation-induced microstructure developed during the fatigue tests was investigated using EBSD microscopy for the following specimens: the LCF specimen tested at Δ ε t = 2% and the HCF specimens tested at σ a = 350 MPa, 275 MPa, and 250 MPa (not fractured). The results obtained for the three fractured specimens and unfractured specimen are discussed below in turn. As mentioned in Section 2, secondary cracks were found on the longitudinal sections of the fractured specimens. Fig. 8 shows the phase map for (a) the LCF and HCF specimens tested at (b) 325 MPa, and (c) 275 MPa around the secondary crack. The phase maps in Fig. 8 are superimposed on the Image Quality (IQ) maps, and the dark lines in the center indicate the secondary crack. The microstructures after the fatigue tests were significantly different depending on the test conditions. For the specimen tested at Δ ε t = 2%, most γ -austenite transformed into ε martensite, while the amount of ε - martensite was limited in the HCF specimens; especially little ε -martensite was detected in the specimen tested at 275 MPa, which fractured at the minimum stress in this work. Conversely, ε martensite can be formed in this alloy even at a low stress level where the fatigue life becomes of order 10 6 . Regarding the region along the crack, different tendencies compared to the region far from the crack were found in the specimen tested at Δ ε t = 2% and σ a = 325 MPa as follows. In the LCF specimen, body-centered- cubic α′ - martensite was detected in some places, while they were not fou nd in the region far from the crack. Here, α′ - martensite is considered to be formed by the two- stage phase transformation γ→ε→α′ (Jang et al. (1995)). For the HCF specimen tested at σ a = 325 MPa, ε -martensite seemed to be formed so as to cover the crack surface. Therefore, the stress/strain field at the crack tip is considered to promote the deformation-induced phase transformation under a relatively high stress/strain condition. Note, there were no such tendency observed for the HCF specimen tested at 275 MPa, at least on the section shown in Fig. 8. The local microstructural situation at the crack tip should be