(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.