Microstructure And Mechanical Property Evolution Mechanisms Of Cold-Rolled 316L Austenitic Stainless Steel Via Liquid Nitrogen Electropulsing Treatment

The microstructure and mechanical properties of metallic materials could be affected by the thermal and athermal effects of electropulsing treatment(EPT)through the interactions among the tremendous electrons and atoms or defects.By applying EPT in liquid nitrogen circumstances on the cold-rolled 316L stainless steel,the mechanical properties at room temperature and cryogenic temperature were systematically studied.In addition,the effect of recrystallization proportion on the strain rate sensitivity of samples tensile tested at cryogenic temperature were also discussed.The main contents are as follows:1.Electropulsing treatment in the liquid nitrogen(LN)circumstances,referred to as LN-EPT,was utilized to regulate the microstructure and tensile properties of a coldrolled 316L austenitic stainless steel.The electropulsing effects between the samples treated in the ambient environment at room temperature(RT)and in the LN temperature were compared.The effect of LN-EPT on 316L stainless steel and its mechanism were investigated with the Joule heat was significantly suppressed,and the deformation mechanisms of LN-EPT samples were also discussed.This attempt of LN-EPT may shed some new light on developing the cryogenic electropulsing assisted forming methods for metals in the future.2.LN-EPT was used to optimize the microstructure and mechanical properties of cold-rolled 316L austenitic stainless steel.The room temperature and cryogenic tensile properties and deformation mechanisms of the samples after EPT were compared.Tensile tests were conducted at 77 K and 293 K respectively,after characterizing the deformation morphologies by transmission electron microscopy,it was revealed that the main deformation mechanisms of the sample tensile at 293 K were dislocation slip and deformation twinning.However,a large amount of deformation-induced martensite transition was occurred during tension at 77 K.The martensite transitions and their subsequent deformation result in a significant increase in the strain hardening capability,thereby enhancing the strength-ductility synergy.Further analysis shows that the deformation mechanism transition was mainly attributed to the significant reduction of stacking fault energy at low temperatures.3.Three samples with different recrystallization proportion of 316L stainless steel were prepared by heat treatment,and the microstructure and tensile properties of those materials were compared with the EPT optimized samples.The cryogenic tensile tests were carried out with different strain rates.The strain rate sensitivity may be related to the dislocation density of the material.In the original cold-rolled sample,the microstructure was dominated by the nano-lamellar structure with high dislocation density,thus cryogenic strain rate sensitivity was higher than the samples with more recrystallized grains.