Microstructure Evolution Of Adiabatic Shear Bands In AISI201Austenitic Stainless Steel

Abstract:Grain refinement can greatly improve the comprehensive properties of low nickel AISI201austenitic stainless steel. Revealing the characteristic of fine-grained stainless steel under high strain rates would have great significance for developing new generation of stainless steel.Multi-axial compression(MAC) was first carried out to refine the annealed AISI201stainless steel, and then hat-shaped specimens were dynamically compressed by means of the Split-Hopkinson pressure bar (SHPB). The microstructure and microtexture evolution during the first cycle of MAC and dynamic loading were studied by optical micrograph(OM), transmission electron microscopy(TEM) and electron backscatter diffraction(EBSD). The results show that:The accumulation of stress and strain curve during the first cycle of MAC exhibited dynamic recovery feature, the grains were remarkable refined from about35μm to5μm in diameter. When the deformation direction change, microbands formed after each pass would mutually cross each other, and divided the original grains into several parts of subgrains, the fraction of High grain boundaries(HABs) monotonically increased during the process. The progressive increasing in misorientations between deformation subgrains during plastic deformation was a typical feature of continuous dynamic recrystallization (CDRX), a model was proposed to explain the microstructure evolution in the first cycle of MAC.EBSD results of annealed hat-shape specimen indicated that:the grain boundaries in ASB were GNBs with high-angles, ODFs and the orientation line analysis show that recrystallization microtextures were formed within ASB. The temperature in ASB was about943K which was high enough to meet the needs of recrystallization. The kinetics of formation of the fine grains was successfully calculated by RDR mechanism. Therefore, these results confirmed that the grain refinement in ASB of annealed AISI201austenitic stainless steel was result of dynamic recrystallization. The fine-grained AISI201austenitic stainless steel had a higher adiabatic shear sensitivity comparing to the annealed sample. Moreover, the dynamic behavior was also different, the first peak stress increased to1135MPa. However, new recrystallization microtexture have been formed in both fine-grain and annealed samples. It consists of a few ultrafine equiaxed grains of diameters varying from30nm to80nm with low dislocation density and some serious deformed grains with blurred grain boundaries. The kinetic calculations revealed that the recrystallized nanosized grains can be formed in the shear band when the subgrain’s size was lower than80nm or the temperature in the shear band was higher than0.5Tm (834K) during the deformation process and they did not undergo significant growth by grain boundary migration at the cooling stage. Due to the inhomogeneous temperature distribution in the shear band, some ultrafine grains with recovery characteristics were also formed. Therefore, dynamic recovery and dynamic recrystallization took effect on the formation of microstructure in the ASB, and the microstructure did not undergone significant growth by grain boundary migration in the cooling stage.