The Relationship Between Fatigue Damage Behavior And Orientation Of AL6XN Super Austenitic Stainless Steel

Supercritical water cooled reactor (SCWR) used above the thermodynamic critical point of water (372 ?,22.1MPa), is the only reactor in the fourth generation of nuclear power system that employs a light water cooled. The reactor has many outstanding features, such as high thermal efficiency, simple system, sustainable and high fuel utilization rate. Hence, it is the main reactor type which is suitable for large-scale power generation. However, the supercritical water is extremely corrosive and the reactor operates in the high temperature and pressure conditions at about 500 ?. It puts forward high requirements on the high temperature corrosion resistance, creep and fatigue properties of the material. The selection of the materials on supercritical water reactor is still a major problem in the development of the supercritical water reactor so far. The current research on practical nuclear materials especially on fatigue is mainly limited to the specific properties of certain materials under specific conditions.The study of material deformation mechanism especially cross scale deformation mechanism is neither detailed nor scientific. Research on the relationship between anisotropy and fatigue behaviors is limit to certain single crystal materials, encountering great difficulties in promoting polycrystalline materials properties. AL6XN super austenitic stainless steel, one of the SCWR candidate materials, is selected as the object of the study in order to solve the problem and provide a basic experimental data for materials across scales of fatigue research. The relationship between fatigue and orientation is studied by transmission electron microscope, electron backscattered diffraction and in-situ neutron diffraction.In the conventional macro-scale fatigue test, fatigue life of the sample with total amplitude of ± 0.3% is about 38000 cycles, and the fatigue life of the sample with total amplitude of ± 0.8% is about 2100 cycles. Two groups of samples exhibited cyclic softening and initial hardening then softening phenomenon in the fatigue process. At the same time, changes of plastic amplitude are contrast to stress. In the low stress condition, it can be explained by the Cottrell theory and the back stress theory, while in the high stress state it is related to the transformation of dislocation structures in the material.Dislocation structures and deformation mechanisms are found different according to different stress states. In the low stress state, the density of dislocation is low and the major dislocation structures are the primary array of dislocations and dislocation walls in the form of simple planar slip typical. In the high stress state, the density of dislocation is very high and the main dislocation structures are the dislocation network, PSB, veins and the dislocation cells and other complex dislocation structure, which have wavy slip characteristics, showing a transform from planar slip to wavy slip. The deformation and damage of the material show certain orientation dependence according to the dislocation structures distribution. It may have some differences in the type of expression forms in different stress states.In-situ neutron diffraction, it has similar performance on the conventional macro fatigue test. Strong orientation dependence is observed in different planes (normal) elastic micro strain statistics. In fatigue process, the grains near [111] and [101] mainly provide plastic deformation damage on tensile and compression direction. [101] has stronger ability to respond to the elastic micro strain, hence, [111] is much easier damaged. There is slight elastic micro strain and no damage on [001].The results of EBSD analysis shows much more details on micro texture and crystallographic orientation. It is found that the grain boundary becomes higher when the damage accumulates. The micro texture changes consistent with the cumulative damage orientation. The higher precision EBSD analysis shows that small angle grain boundary representing the damage and dislocation stack mainly exists in the interface of grains at the low stress condition and widely distributed in the intragranular under high stress state. It proves that the anisotropy affects between different grains in the low stress state and affects between the different structures in the high stress state.