2. Experimental
2.1. Material
Superaustenitic heat resistant stainless steel Sanicro 25 grade UNS S31035 supplied by Sandvik, (Sweden) in the form of a cylindrical rod of 150 mm in diameter was investigated. The chemical composition of the material in wt. % is 0.1 C, 22.5 Cr, 25.0 Ni, 3.6 W, 1.5 Co, 3.0 Cu, 0.5 Mn, 0.5 Nb, 0.23 N, 0.2 Si and the rest Fe. Semi-products of specimens having the diameter of 7 mm on the gauge length of 16 mm were manufactured parallel to the rod axis. Specimens were heat-treated by solution annealing at the temperature of 1200 °C for one hour and cooled in air. Later they were finished to the final dimensions. To facilitate the surface damage assessment, the gauge length of the specimens was mechanically and electrolytically polished.
2.2. Mechanical testing
Specimens were subjected to thermomechanical fatigue (TMF) loading in out-of-phase cycling with 10 min dwell in maximum compression (OPD-TMF). The dwell was thus applied at maximum temperature in a cycle. TMF experiments were carried out by means of computer-controlled servo-hydraulic mechanical testing MTS 880 system with hydraulic grips. More details on the high temperature thermomechanical testing can be found elsewhere 25. In all cases mechanical strainε , mechanical strain amplitude εa or mechanical plastic strain amplitude εap are reported. Hysteresis loops were recorded during cycling and plastic strain amplitude has been evaluated offline from the half of the width of the hysteresis loop.
2.3. TEM internal structure investigation
TEM was adopted for the study of the specimen internal structure. Thin plates were cut from the specimen gauge length parallel to the loading axis. The samples were mechanically grinded and once the thickness of 0.08 mm of the thin plate was reached, discs having a diameter of 3 mm were produced. Afterwards, discs were electrolytically polished utilizing a double jet device TenuPol5. The TEM foils were studied in high resolution TEM in scanning mode (STEM) along with EDS STEM (Oxford X-Max80) analysis of precipitates on Jeol JEM-2100F STEM operating at 200 kV.
2.4. SEM, FIB and EBSD study of fatigue crack nucleation and growth
The surface relief of cyclically strained specimens was studied by means of Tescan Lyra3 XMU FESEM equipped with a focused ion beam (FIB). The depths of secondary cracks were revealed by producing FIB trenches at the location chosen to find the in-depth profile of the crack. In addition to the damage evolution from the surface of the specimens, the internal damage was studied as well. In order to reveal the crack paths and chemical composition of the oxide layer the longitudinal sections parallel to the loading axis of specimens were produced. The longitudinal cuts were polished and the crack paths and their relation to the grain orientation were studied using electron back scatter diffraction (EBSD - Oxford Instruments). The chemical analysis of the oxide layer was documented utilizing energy dispersive spectroscopy (EDS - Oxford Instruments).