3.1 Stress-strain curve
The monotonic tensile stress-strain curve and cyclic stress-strain curve of GS-20Mn5 used in this test are shown in Fig. 5.
The results show that GS-20Mn5 has an obvious yield platform under monotonic tensile process. The strain range of the platform section is about 0.00156 ~ 0.0099, the yield limit is 310MPa, the tensile strength is 542MPa and the elastic modulus is 212GPa.
The cyclic stress-strain curve in Fig. 5 is formed by connecting the stress amplitudes of cyclic stability under different strain amplitudes, which reflects the mechanical properties of the material under cyclic loading. Due to cyclic softening/hardening, it can be seen from the figure that the cyclic stress-strain curve of the sample steel does not coincide with the uniaxial tensile stress-strain curve, according to the curve, the strain at the intersection is about 0.3%. That is to say, when the strain is less than 0.3%, the test steel shows cyclic softening characteristics; When the strain is higher than 0.3%, it shows cyclic hardening.
3.2 Cyclic soft/ hardening characteristics
Hysteresis loops with strain amplitudes of 0.16%, 0.20%, 0.30%, 0.40% and 0.50% are obtained according to the experiment. The hysteresis loops of typical cycle amplitude with strain amplitudes of 0.16%, 0.20%, 0.30%, 0.40% and 0.50% are presented here, as shown in Fig.6. The hysteretic loops of the stable cycle under various strain amplitudes are shown in Fig. 7.
It can be seen from Fig. 6 that the cyclic stress-strain hysteresis loops of GS-20Mn5 materials are different under different strain amplitudes. As the increase of cycles, the shape and the peak-valley value of the hysteresis loop will change. Under the strain amplitudes of 0.16%,0.2% and 0.3%, the hysteresis loop first becomes sharp, and the peak and valley of stress increase gradually, which shows the initial cyclic hardening. Then the hysteresis loop became flat, the peak and valley of stress decrease gradually, which shows the cyclic softening. Then it changes from flat to sharp, the peak and valley of stress increase gradually, it occurs cyclic hardening again (secondary cyclic hardening). Under the strain amplitudes of 0.4% and 0.5%, the cyclic hysteresis loop gradually becomes sharp, the peak and valley of stress gradually increase, which always shows cyclic hardening. It can be seen from Fig. 7, under low strain amplitude, the loop is slender and sharp. With the increase of strain amplitude, the loop becomes wider, while the peak and valley of stress increase and gradually flatten out.
The variation curve of stress amplitude of test steel under different strain amplitudes with cycles is shown in Fig. 8. It can be seen from the figure that the cyclic softening/hardening characteristics of the test steel are sensitive to amplitude variation and cycles. When the strain amplitudes are 0.16%, 0.2% and 0.3%, the stress amplitude increases gradually at the initial cyclic stage, showing slow cyclic hardening. Then the stress amplitude gradually decreased with the cycles, showing cyclic softening; The subsequent stress amplitude gradually increases, which showing cyclic hardening, i.e., the secondary cyclic hardening. The larger the strain amplitude is, the shorter the cyclic softening stage is and the higher the softening rate is. When the strain amplitude is 0.4% and 0.5%, the stress amplitude increases rapidly at the beginning of the cycles (2-10 cycles), showing rapid hardening followed by slow hardening(1-2cycles) and there is no cyclic softening phenomenon in the whole cyclic loading.
3.3 Evolution of microstructures
The steel samples with strain amplitude of 0.2% showed obvious initial cyclic hardening, the subsequent cyclic softening, and occur secondary cyclic hardening at last, the loop softening/hardening characteristics change significantly, therefore, choose the strain amplitude cyclic loading tests of different cycles. The steel cyclic softening/hardening characteristic of microscopic mechanisms is analyzed through microstructure observation and analysis. In addition, microscopic observations are also made for samples with strain amplitudes of 0.16%, 0.3% lower cycle for 3000 cycles, and 0.4% lower cycle for 2450 cycles, respectively.
Fig. 9 shows the dislocation structure of the original sample. Fig. 9 (a) shows the dislocation structure of ferrite. From which it can be concluded that some short-range dislocation lines are mainly scattered and generated in the heat treatment process of the test steel. Fig. 9 (b) shows the dislocation structure in pearlite, which the fine-rod-shaped pearlite is lamellar. And some spherical pearlites can be observed between the lamellar pearlites, some short-range dislocation structures can be also observed in the cracks between pearlites.
Figure 10 shows dislocation structure at the strain amplitude of 0.2% under 10 cycles corresponding to the stage of cyclic hardening. Dislocations occur at the grain boundaries. The initiation of dislocations and the interaction between dislocations make the pearlite/ferrite interface has a very high dislocation density, and entangle each other. Fig. 10 (a) shows the dislocations at the boundary of ferritic grains. Due to the discordant intergranular deformation, grain boundary will act as a source of dislocation then continuously insert dislocations in the grain, which makes the material develop towards hardening. Fig. 10 (b) shows the situation of cementite pinning dislocation inside ferrite grains. The pinning effect of cementite on dislocation hinds dislocation movement, which increases the deformation resistance of the material and makes the material develop towards hardening. Figure 10 (c) shows the dislocation stacking at the ferrite grain boundaries. Since the ferrite grain boundary obstructs the dislocation movement, the dislocation stacking occurs here and there is a reverse force on the dislocation source. Consequently, it needs a larger shear force to make the dislocation move which causes the hardening of the material. Fig. 10 (d) shows the high-density dislocation entanglements in ferrite grains. These dislocation entanglements hinder dislocation movement and cause the material to develop towards hardening. At this stage, the dislocation accumulation at the grain boundary of ferrite and the cementite pin in the grain lead to the hardening of the material. Since the dislocation characteristics of pearlite at this stage are not significantly different from that at the initial state, it has little hardening effect on the material.
After cyclic deformation under 60 cycles at a strain range of 0.2%, The steel samples with strain amplitude of 0.2% showed obviously cyclic softening. The dislocation structure is shown in Fig. 11. Dislocation annihilation occurs in ferrite grains. Fig. 11 (a) shows dislocation walls and dislocation spots begin to form. However, at the transformation stage, the dislocation arrangement is still disorder. On the one hand, dislocation structure form in ferrite that make high stacking fault energy, so under the action of dislocation interaction, the dislocation of multiple sliding systems start repeatedly positive and reverse slip, some different direction dislocations offset each other during movement process lead to the dislocation annihilation. Eventually, the movement leads to the decrease of the dislocation density then dislocation slip resistance decreases and the materials softening occur; On the other hand, because of the dislocation mechanism of pinning, the dislocations pinned by carbides in the dislocation spots are dislocated during cyclic deformation, which resulting in the decrease of friction stress. Consequently, dislocation entanglement and grain boundaries are also destroyed under cyclic loading, which makes dislocation disorder and distributed in the shape of network. It enhances the mobility of dislocations, which lead to the softening of the material. As shown in figure 11 (b) and (c), the pearlite lamellar have the same orientation at the initial stage. And the dislocation sources at the pearlite interface continuously emit dislocations outwards after deformation. The dislocations begin to accumulate, which expands into ferrite in the form of dislocation rings and gradually forms dislocation walls. But it began to form dislocation lines across ferrite and pearlite when the dislocation loop front-end and adjacent pearlite lamellar intersect. In the dislocation wall as shown in Fig.11 (c), lamellar pearlite ruptures and dislocation annihilation at the fracture, which leads to softening [21]. At this stage, the offset of hetero-sign dislocations in ferrite grains, the mechanism of dislocation pinning and the annihilation of lamellar dislocation in pearlite grains all lead to the cyclic softening of the materials.
Fig. 12 shows the dislocation structure with 3000 cycles at strain amplitude of 0.2%, which corresponds to the secondary cyclic hardening stage. Compared with the dislocation structure of the 60 cycles, the dislocation structure in ferrite grains during this period is mainly mature dislocation walls, as shown in Fig. 12 (a). This dislocation structure is in a stable state and has little influence on the softening/hardening properties of the material. It can be seen from Fig. 12 (b) that there are many dislocation lines between the lamellar pearlite. These entanglements in the pearlite hinder the movement of the dislocation. The deformation resistance increase which leads to obvious cyclic hardening of the material. So the secondary hardening is directly related to the dislocation in the pearlescent body.
In summary, at the early cyclic stage, the dislocation accumulation at the ferrite grain boundary and the dislocation tangle in the grain dominate the hardening characteristics of the material; in the subsequent cycles, the dislocation in the ferrite annihilates and gradually forms the dislocation walls, which dominates the softening characteristics of the material; at the late cyclic stage, the dislocation of the pearlite begins to entangle in the grain, which prompts the material to present the hardening characteristics again.
The dislocation structures in ferrite with strain amplitudes of 0.16%, 0.3%, and 0.4% for about 3000 cycles are shown in Figure 13. When the strain amplitude is 0.16%, the dislocation structure is mainly the dislocation "spot" with higher dislocation density, and there is a channel with lower dislocation density between the dislocation "spot"; when the strain amplitude is 0.2%, the dislocation structure in the ferrite is mainly the parallel wall (shown in Figure 12 a); when the strain amplitude is 0.3%, there is also the dislocation wall structure, but the dislocation wall is mainly wavy, there are a large number of dislocations between the walls which cut the dislocation walls, and a small number of dislocation walls have been surrounded by cellar structures; when the strain amplitude is 0.4%, a large number of dislocation cells (substructures) are generated. It has been shown that during the deformation of the material [22-24], the change process of dislocation structure inside the grain is: dislocation tangle - dislocation spot - dislocation wall - immature dislocation cell - mature dislocation cell. It can be seen that with the increase of strain amplitude, the test steel is more likely to form dislocation walls and more stable dislocation structures. Different dislocation structures dominate the cyclic softening/hardening characteristics of each stage of the material at different cycles under different strain amplitudes.