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).