Investigations On Fatigue Dislocation Structures And Their Thermal Stabilities Of AL6XN Super-austenitic Stainless Steel

As a kind of newly-developed stainless steel materials with excellent performance, the AL6XN super-austenitic stainless steel has been given much attention by numerous researchers, and a great deal of systematic research has been investigated in macro-mechanical properties. However, studies of the fatigue deformation micro-mechanisms of this material have not been involved; in particular, knowledge of the thermal stability of the fatigue dislocation structures of the AL6XN steel still remains much less. Therefore, in the present work, the dislocation structures of the AL6XN steel fatigued under tension and compression with constant plastic-strain-amplitude control and under torsion were observed systematically by transmission electron microscopy (TEM), and the changes of those fatigue dislocation structures occurring in annealing at different temperatures were detected by TEM observations. Some new research findings have been achieved on understanding of fatigue microstructures and their thermal stabilities of the AL6XN steel.Cyclic stress-strain behavior and dislocation structures of the AL6XN stainless steel push-pull fatigued under plastic strain amplitude (△εp1/2) control exhibit different characteristics at high and low plastic amplitudes, respectively. As△εp1/2<5×10-3, the material presents a sustained cyclic softening behavior, and the fatigue-ductility exponent measured from the Coffin-Manson curve is comparatively high. In this case, the corresponding dislocation structures are mainly composed of typical planar slip structures, e.g., planar slip bands, persistent Luders bands (PLBs), etc. However, as△εp1/2≥5×10-3, the material exhibits an initial hardening stage followed by a striking softening stage, and the fatigue-ductility exponent is lower, and under this circumstance, the dislocation structures consist jointly of typical planar slip and wavy slip structures. Quite different cyclic stress-strain features of the AL6XN steel occurring at high and low△εp1/2, respectively, result primarily from such a variation of microscopic slip deformation mode. The strain-life curve of AL6XN alloy under torsion fatigue exhibits three characteristic regions with a distinct plateau, which is closely related to the different dislocation structures induced at different shear strain amplitudes (Ay/2). At the low stain amplitude region, i.e.,△γ/2<1.04×10-2, the dislocation structures are mainly composed of planar slip structures. As△γ/2<1.04×10-2, the fatigue life changes within a certain range, which corresponds to a plateau region in the strain-life curve. In this case, both planar slip and wavy slip dislocation structures were simultaneously observed in the AL6XN steel. As the shear strain amplitude is higher than that of the plateau, i.e.,△γ/2<1.04×10-2, the wavy slip dislocation structures have become dominant ones. The occurrence of the distinct plateau is probably consequent upon the variation of relative quantities of the planar slip and wavy slip dislocation structures due to the inhomogeneity of torsion fatigue deformation.TEM observations were carried out on the dislocation structures of the AL6XN stainless steel fatigued under tension-compression and torsion, respectively, and then annealed at different temperatures. The experimental results show that, the dislocation structures have undergone a certain extent process of recovery at600℃, and the recovery degree of wavy slip dislocation structures is slightly much than that of planar slip ones. In contrast, when the annealing temperature is as high as800℃, a violent recovery of fatigue dislocation structures occurs, and the wavy slip dislocation structures (e.g., dislocation cells, dislocation walls, persistent slip bands, etc) disappear completely by recovery; however, the planar slip bands with a decreased dislocation density still exist. Clearly, as compared to the wavy slip dislocation structures, the planar slip ones possess a higher thermal stability. Interestingly, recrystallization phenomenon was observed to occur in the microstructures of the AL6XN steel fatigued under torsion at high shear strain amplitudes and then annealed at800℃for30min. The occurrence of recrystallization should be closely related to the quantity of accumulated plastic strain (namely deformation storage energy), which can be reached after fatigue deformation. Calculations show that the quantities of accumulated plastic strain for torsion fatigue are obviously higher than those for push-pull fatigue, and such a higher accumulated plastic strain could provide a higher driving force for the occurrence of recrystallization. In addition, a enough high applied strain amplitude is also a prerequisite for the occurrence of recrystallization, since it can causes plastic deformation concentrations during cycling of the AL6XN steel. These influencing factors, i.e., high accumulated plastic strain and high plastic strain amplitude, would make it possible that local recrystallization indeed takes place at plastic deformation concentration region, e.g., grain boundaries, in the AL6XN steel fatigued under torsion and then annealed at a high temperature of800℃.