(c) (d)
Fig. 4. Typical dislocation structures observed in specimens subjected
to TMF cycling with constant strain amplitude 6×10-3to fracture; (a) IP-TMF, (b) OP-TMF, (c) IPD-TMF, (d) OPD-TMF.
Here the microstructure in specimens subjected to different
thermomechanical cycling using TEM in scanning mode (STEM) is reported.
Fig. 4 documents bright field STEM micrograph of typical dislocation
structures in specimens cyclically strained with total strain amplitude
of 6x10-3 to fracture in IP-TMF and OP-TMF as well as
IPD-TMF and OPD-TMF loading procedures.
Cyclic loading at high temperature results in a pronounced increase of
the dislocation density regardless of the testing conditions comparing
to the initial state 10. Similar dislocation
arrangement was observed here in TMF without dwells. Fig. 4a and Fig. 4b
show very high dislocation density with dislocations arranged in bands
parallel to the trace of the slip plane of the primary slip system.
Dislocation structure in specimens subjected to TMF with dwells differs
significantly from those without dwells. Nearly random distribution of
dislocation segments is typical for both loading procedures (Fig. 4c and
Fig. 4d). The density of dislocations is lower in the case of TMF-OPD
cycling than in the case of IPD-TMF cycling. Dislocation segments are
often curved and are pinned by nanoparticles.
The extensive investigation of the dislocations structure of specimens
cycled at high temperature by Heczko et al. 11, 26 was
devoted to a detailed analysis of Cu-rich and NbC particles formation
and their influence on the mechanical response in constant strain rate
cycling. Since the interaction of the dislocations with nanoparticles
contributes significantly to the high resistance of the material at high
temperature 2, 10, 11, 26 we have studied the
occurrence of these particles in specimens subjected to different
thermomechanical histories.