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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Fig. 1. Cyclic hardening/softening curves in constant total strain amplitude loading at 700 °C .

3.2 Surface relief formation

Specimen surface relief was investigated at the end of the fatigue life. The feebly developed secondary cracks allow us to identify the crack nucleation process at the very beginning; therefore, these cracks were the matter of the scrutiny. The surface relief of the specimen subjected to LCF conditions in the air as well as in vacuum is documented in Fig. 2a and Fig. 2b, respectively. Fig. 2a reveals the surface of the specimen covered by rather thin uniform oxide layer developed. The thicker oxide bands formed preferentially at the grain boundaries can be recognized. Moreover, the oxidized grain boundaries arranged perpendicularly to the loading axis were found to be cracked. (a) (b)

Fig. 2. Specimen surface relief developed at the end of the fatigue life strained with total strain amplitude 7x10 -3 (a) LCF in air; (b) LCF in vacuum.

The surface of the specimen strained in the vacuum chamber is depicted in Fig. 2b. Fig. 2b reveals oxide-free surface relief. The precipitates developed within the grains as well as at the grain boundaries are noticeable along with