2.1 Cyclic hardening/softening characteristics
The stress amplitude evolution with cycles under strain amplitudes of 0.7%, 0.9%, and 1.1% are obtained, as shown in Fig.2-1. As demonstrated in Fig.2-1, the test steel shows the characteristics of cyclic hardening and then cyclic softening at strain amplitudes of 0.7% and 0.9%. The cyclic softening rate gradually slows down with the increase of cycles at the strain amplitude of 0.7% until the cyclic stability is achieved. Under the strain amplitude of 0.9%, the cyclic softening behavior develops until the material is damaged. When the strain amplitude is 1.1%, the test steel shows the characteristics of cyclic hardening at the initial stage of cycles, then cyclic softening, and finally cyclic hardening (secondary cyclic hardening).
2.2 The law of cyclic martensitic transformation
The variation curve of martensitic transformation of the test steel with cycles under strain amplitude of 1.1% is shown in Fig.2-2. As can be seen from the diagram, after the first few cycles, the martensite content increases with cycles and gradually reaches a relative saturation state at the late period of cycles.
2.3 Evolution of microstructure
The EBSD image of the test steel is shown in Fig.2-3, the ferrite (BCC) is red, and the austenite (FCC) is green.
The morphology and dislocations of the test steel under the original state and strain-controlled cyclic loading with a strain amplitude of 1.1% were observed by EBSD and transmission electron microscope. Fig.2-4 shows the dislocation structure of the original ferrite phase. In the original state, the ferrite phase is dominated by line defects, which are dislocation lines, and there is dislocation accumulation near the grain boundary. A small amount of dislocation pinning and dislocation cutting exist in the grains. The dislocation structure of the original austenite phase is shown in Fig.2-5. There are mainly stacking faults in the austenite phase, only a few dislocation lines, and there is superposition between stacking faults. Fig.2-6 shows the TEM picture of the ferrite phase at the 5-cycles. It can be seen that compared with the original state, the dislocation density in the ferrite phase increases significantly, there is much more dislocation entanglement, and the number of dislocation deposits at the grain boundary also increases significantly. The TEM picture of the austenite after 5 cycles is shown in Fig.2-7. There are mainly plane slip, dislocation parallel arrangements, and strain-induced ε martensite begins to appear in the austenite phase.
Fig.2-8 shows the dislocation structure of the test steel after 15 cycles. It can be seen that the dislocation structure of the ferrite phase is mainly dislocation wall and dislocation cluster. Compared with the 5 cycles, the dislocation structure is rearranged, from dislocation entanglement to dislocation cluster and dislocation wall. Previous studies have shown that ferrite with a body-centered cubic structure is prone to cross-slip and can activate multiple slip systems, thus making dislocation movement easier. It leads to the destruction and rearrangement of dislocations, which makes the dislocations evolve from entangled states to low-energy dislocation walls and dislocation clusters. For the austenitic phase, compared with the microstructure of the original condition, it can be seen that the number of stacking faults becomes less, and the length of stacking faults becomes shorter.
Fig.2-9 shows the microstructure of the test steel after 50 cycles. As can be seen from this figure, the dislocation clusters in the ferrite phase transform into dislocation walls and immature dislocation cells. It means that the multiplication and annihilation of dislocations gradually develop to equilibrium during this period; for the austenitic phase, the number of shear bands increases, and many shear band intersections are formed.
The microstructures of the test steel at fatigue (150 cycles) are shown in Fig.2-10. It can be seen that the dislocation structure in the ferrite phase has been transformed into a mature dislocation cell (subgrain). And the cell wall thickness of the dislocation cell is reduced compared with the dislocation wall under 50 cycles. There are still a lot of shear bands and their intersections in the austenite phase, which shows that the microstructure of the austenite phase does not change obviously from the 50th cycle to fatigue.
Fig.2-11a) and b) show the grain boundary of EBSD analysis under different cyclic periods. It can be seen that the low-angle grain boundary (subgrain boundary) increases significantly with cycles; that is to say, this low-energy dislocation structure is gradually growing. Thus the effect of dislocation structure on cyclic hardening/softening is slowly weakened.
Fig.2-12 shows the bright field of the microstructure of the austenite phase at fatigue, according to the diffraction spot, Fig.2-12(a) is ε martensite, and Fig.2-12(b) is α 'martensite.