Study On Deformation Mechanisms Of N-on-uniform Grain Structures Of FeCrNi Austenitic Stainless Steels

Because of its excellent mechanical properties and corrosion resistance,ascast austenitic stainless steel is widely used in industrial fields.However,the ductile processing of as-cast austenitic steel is greatly challenged due to its uneven microstructure(such as cracks and mixed crystals).Moreover,in the service process of large forgings,if the non-uniformity defects in these raw materials are not eliminated with appropriate technology,their mechanical properties and service life will be seriously damaged.Understanding the plastic deformation mechanism of non-uniform structure of austenitic steel can provide theoretical basis for optimizing the forging deformation process of austenitic steel and improving the mechanical properties of the material.Therefore,in this paper,by molecular dynamics simulation,Fe Cr Ni austenitic steel samples were prepared according to the proportion system of the main alloying elements of 18-8austenitic stainless steel,and the preset crack closure behavior and deformation law of the samples under different loading conditions were studied.Preset interface deformation behavior and plastic deformation mechanism of Fe Cr Ni austenitic steel samples under different loading conditions;Finally,the influence of different loading conditions on the plastic deformation of the samples with mixed crystal structure and the plastic deformation law of the central coarse crystal and the surrounding fine crystal are studied.In the study of the closure behavior of the preset crack,the specimens with the preset crack and the preset interface are respectively subjected to compression and shear loading.The stress-strain curve shows that the specimens with the interface show higher Young’s modulus and yield strength than the specimens with the crack no matter what the loading conditions are.These results show that the specimens under compression load have higher plastic resistance,while shear stress promotes plastic flow.Under compressive stress,dislocation dominates plastic deformation leading to crack closure,while under shear load,crack length shrinks due to high stress,and crack tips propagate along grain boundaries and interfaces.Dislocation activities,including dislocation emission,slip,and interactions with cracks,grain boundaries,and dislocations,contribute to the plasticity of the sample under compression loads.In addition to dislocation activity,grain boundary slip,grain rotation and twins are also potential plastic deformation mechanisms under shear loading.As the current experimental conditions cannot meet the requirements of nanoscale research,the hot compression experiments were carried out on the austenitic single-phase steel samples with prefabricated cracks at macroscopic scale.The experimental results are similar to the simulation results of molecular dynamics crack closure,indicating that the compression loading mode plays an important role in crack closure.In the study of the influence of different loading conditions on the plastic deformation of samples with mixed crystal structure,necessary improvements were made to the model.The computer C++ programming technology and LAMMPS software were used to construct Fe Cr Ni austenitic steel mixed crystal model with a central coarse crystal,and the compression,shear and tensile loading were carried out respectively.The simulation results show that under the same loading mode,the degree of plastic deformation in the fine crystal region is larger and the strain is more concentrated in the fine crystal region,while the degree of plastic deformation in the coarse crystal region is lower.Because of the higher grain boundary density,the fine grain zone is the high stress zone,while the coarse grain zone is the low stress zone.The coarse-grain refinement dominated by dislocation motion occurs under compression and shear loading.The formation of deformation twins can coordinate the plastic deformation of mixed crystals and promote the coarse-grain refinement under compression loading.However,under large plastic deformation conditions,dislocation activities of coarse crystals are activated under shear force,and then dislocation entanglement is formed,and dislocation walls are eventually formed,thus forming subcrystals and leading to coarse crystal refinement.Tensile loading has the strongest excitability for deformation twins of mixed crystals.However,tensile deformation twins and Stair-rod dislocation hinder the movement of dislocation,resulting in more resistance to plastic deformation,which is not conducive to the plastic deformation of mixed crystal samples.Therefore,compared with compression and shear loading,both coarse-grained grains and fine-grained grains have the lowest grain deformation rate under the same strain value.