PSI - Issue 23

Roman Petráš et al. / Procedia Structural Integrity 23 (2019) 209–214 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the twins in some grains. Moreover, dense slip lines can be found in some favorable oriented grain indicating homogeneous cyclic plastic strain development rather than localization to the PSBs. Well-developed cracks were observed mostly at the grain boundaries perpendicular to the stress axis.

3.3 FIB cutting

The early crack nucleation was investigated on the weakly developed secondary cracks. Accordingly, in depth profiles of the secondary cracks were investigated by means of SEM and FIB technique. Fig. 3 documents heavily oxidized grain boundary found on the specimen surface subjected to the LCF in the air. The direction of the oxide intrusion developed in the matrix follows the grain boundary.

Fig. 3. FIB cut produced perpendicularly to the surface and to the loading axis; heavily oxidized grain boundary in specimen subjected to LCF in air.

The different type of early crack formation was observed in LCF under vacuum conditions. Fig. 4 reveals the crack initiation at the grain boundary. The FIB cut was produced perpendicularly to the grain boundary investigated at the location denoted as a FIB cut line, see Fig. 4a.

(a)

(b)

Fig. 4. FIB cut produced perpendicularly to the surface and to grain boundary; (a) cracked grain boundary, (b) FIB profile revealing the cavity formation along the grain boundary.