PSI - Issue 43

Jiří Man et al. / Procedia Structural Integrity 43 (2023) 203 – 208 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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The results of ferritescope measurements performed after finishing of fatigue tests at different locations of fatigued specimens are for both SLMed steels summarized in Table 2. In agreement with our previous study on the stability of wrought 316L steel produced conventionally and fatigued at room temperature (see Man et al. (2011)) the maximum volume fraction of DIM was detected on the fracture surfaces in all cases. The enhanced population of DIM in the close vicinity of a long fatigue crack shown in Fig. 3 supports this finding. The DIM particles within individual grains are mainly present at the intersection of slip events coming from two slip systems. As indicated recent TEM study of foils prepared from the thin slices taken close to the fracture surface of fatigued SLMed 316L steel DIM particles may form at the intersection of thin deformation micro-twins which was however overlooked by Cui et al. (2021). Note that the stress-strain state in the vicinity of both the crack tip and rugged crack path may deviate from that present in the crack-free volume of material. In addition to fracture surface a limited amount of DIM has been detected also within the gauge parts of fatigued SLMed 316L steels with a tendency for higher DIM content at the specimen surface (see Table 2). The morphology and distribution of DIM in the bulk of fatigued 316L-TUK steel was clearly dependent on the applied stress Fig. 3. DIM particles formed in the vicinity of a long fatigue crack developed in SLMed 316L-IPM steel cycled with  a = 0.8% until failure as revealed by SEM on the longitudinally sectioned gauge part of specimen after color etching (modified Beraha II).

Fig. 4. Two different characteristic morphologies of DIM developed in the volume of SLMed 316L-TUK steel cycled with (a)  a = 360 MPa and (b)  a = 440 MPa until failure as revealed by SEM on the longitudinally sectioned gauge part of specimen after color etching.

(a)

(c)

(b)

Fig. 5. Morphology of DIM as revealed by TEM in thin foils prepared using FIB from the longitudinal cross-section of SLMed 316L-TUK steel fatigued with  a = 360 MPa until failure. (a) Pt layer deposited across the bands containing DIM islands showing the place of nascent TEM lamella (SEM); STEM micrographs showing in dark field (b) overview and (c) detail of DIM islands together with selective area electron diffraction (SAED) patterns corresponding to places A and B indicated.