Fig. 9. STEM image of the grain boundary. Marked area was subjected to EDS analysis. M23C6 Cr-rich carbides nucleated at the grain boundary along with Cu- and Nb-rich particles are documented. Location of M23C6 Cr-rich carbides is depleted in Fe and Ni content.
3.3. Damage mechanisms in-phase thermomechanical fatigue with dwells
Fatigue damage in thermomechanical loading started by oxidation of the surface. Thin surface oxide layer has been formed with enhanced oxidation at the grain boundaries. Preferentially the boundaries which were on the surface perpendicular to the loading axis were heavily oxidized and oxide extrusions prolapse on the surface. Preferential oxidation continued along the grain boundary (in IP-TMF cycling) and fatigue crack has been formed similarly to the case of high temperature fatigue loading 8. In the case of thermomechanical fatigue loading without dwells 20, cracks arise due to cracking of the thick oxide layer in the tensile part of the cycle. In order to find the effect of the dwells the surface relief of the specimens subjected to the IPD-TMF and OPD-TMF at the end of the fatigue life was studied and compared with that during TMF cycling without dwells.
3.3.1. IPD-TMF cycling
Fig. 10a represents the surface of the specimen subjected to IPD-TMF cycling with total strain amplitude 6×10−3. Oxide layer formed on the specimen surface can be easily recognized along with the bands of the thicker oxide developed preferentially at the grain boundaries. The early fatigue cracks initiated exclusively due to cracking of the highly oxidized grain boundaries, preferentially those which were on the surface oriented perpendicularly to the stress axis.
The in-depth profile of the secondary crack is imaged in Fig. 10b. The oxidation and cracking of the oxidized grain boundary are essential in terms of crack formation. The crack here follows the grain boundary perpendicularly to the loading axis. Since oxygen diffusion and also subsequent oxidation of the crack tip are temperature and time dependent the 10 minute tensile dwell during high temperature exposure of the opened crack to the air results in the increase of oxidation rate. Subsequently, the oxidation and cracking follow the grain boundary as documents the longitudinal cut of a specimen subjected to IPD-TMF cycling in Fig. 11. SEM image in Fig. 11a shows the crack developed from the specimen surface propagating through the material volume. The respective EBSD image in Fig. 11b reveals intergranular crack growth.