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.