PSI - Issue 23

Jaroslav Polák et al. / Procedia Structural Integrity 23 (2019) 275–280 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3: Surface of the specimen cyclically strained with constant strain rate 2 × 10 -3 s -1 at 700 °C with constant strain amplitude; (a)  a = 3 × 10 -3 , cycling without dwells, (b)  a = 4 × 10 -3 , cycling with 10 min dwells in maximum tension.

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Fig. 4. Wedge crack developed after constant strain cycling (  a = 4 × 10 -3 ) with 10 min dwells in maximum tension to fracture; (a) grain boundary sliding leading to the wedge crack formation on the material surface, (b) FIB cut revealing its in-depth profile.

Fig. 4a shows the wedge crack and Fig. 4b FIB cut at one end of the this crack showing the sliding of the grain boundary and accompanying oxidation. Fully opened cracks are much more prone to further oxidation during tensile part of the cycle and preferentially during the dwell in maximum tension. Oxide cracking at the tip of oxide intrusion is thus the driving force for the crack growth. Another contribution to the damage in cycling with dwells in the maximum tension was formation of internal cracks. Fig. 5a shows the early stage of internal crack formation by precipitation of cavities along the grain boundary and Fig. 5b shows their linkage and formation of the internal crack. Numerous internal cracks were produced during cycling with dwells. The internal cracks can join the principal crack starting at the surface. The resulting crack path is mostly intergranular as found using EBSD contrary to transgranular crack path found when cycling without dwells.