PSI - Issue 43

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A. Shanyavskiy et al. / Procedia Structural Integrity 43 (2023) 215–220 Author ame / Structural Integrity Pro edi 00 (2022) 000 – 000

Fig. 3. Transgranular relief of fracture surface with formed fatigue striations at (a) 23 ° C and (b) 150 ° C.

Fig. 4. (a) Mixed trans- and intergranular relief on fracture surface at 650 ° C and (b) intergranular relief on fracture surface at 750 ° C.

Under creep-fatigue interaction test conditions at T = 450 ° C, the change of dominant fracture mechanism occurs with an increase in crack length. The initial crack growth is realized by the dominant transgranular fracture mechanism. Fatigue striations with spacing of 125 nm start being revealed after reaching the crack length of 18.9 mm which corresponds to K 1 = 32.5 MPa·m 0.5 . Formation of fracture surface relief is characterized by the existence of intensive oxidation products. At T = 550 ° C, more intensive oxidation of fracture surface appears and process of intergranular fracture is accompanied transgranular fracture. The portion of fracture surface area with transgranular pattern amounts of approximately 20%. At T > 650 ° C, the intergranular fracture with the formation of small secondary crystals on the grain boundary of primary grain (Fig. 5) becomes dominant. The nature of secondary crystal formation on the grain boundary is related with the realization of local melting process caused by combined effect of dwell time, temperature and local heat in the vicinity of crack tip due to plastic zone. Fatigue striations is revealed on the final stage of fatigue fracture only, with their spacing of 3.7 μ m.