Fatigue Property And Mechanism Of Hierarchical Nanostructured 304 Austenitic Stainless Steel

As one of the most widely used stainless steels in industry,304 stainless steel were widely used in construction and Shipping car accessories materials due to their excellent corrosion resistance and good formability.However,the strength and fatigue limit of 304 stainless steel are very low,which limit their application in complicated environment.Traditional strengthening methods such as fine grain strengthening and second phase strengthening can improve the strength and high cycle fatigue(HCF)properties of austenitic stainless steels.But the poor plasticity of fine grained metal lead to the decrease of low cycle fatigue(LCF)resistance,and the deformation incoordination between the strengthening phase and matrix often leads to serious deformation localization during fatigue.Recent studies show that the strength of austenitic stainless steels can be improved effectively without serious loss of plasticity by introducing nanotwins(NT)and gradient nanostructures(GNS).In this work,two kinds of multistage nano-structured 304 austenitic stainless steels were prepared by dynamic plastic deformation(DPD)and cyclic torsion(CT)methods.The fatigue properties and mechanism of these two kinds of samples were studied systematically.The main results are as follows:1.Two types of 304 austenitic stainless steel with nanotwin/dislocation structure and nanotwin/recrystallization structure was prepared by dynamic plastic deformation technique and annealing process.The stress-controlled tension-compression high cycle fatigue properties and mechanism of these two types of samples were investigatedThe volume fraction of NT grains is 30%for both nanotwin/dislocation and nanotwin/recrystallization 304 SS.The yield strength and tensile strength of the nanotwin/dislocation 304 SS are 928 MPa and 1312 MPa respectively,and the uniform elongation is about 1.5%.After annealing treatment,the yield strength and tensile strength of the nanotwin/recrystallization 304 SS are decreased to 522 MPa and 936 MPa respectively,but its uniform elongation is increased to 34%significantly.Stress-controlled HCF test suggest although the strength of nanotwin/recrystallization 304 SS lower than that of nanotwin/dislocation 304 SS,its fatigue limit and fatigue ratio are both increasd,which enhanced from 300 MPa,0.23 to 330 MPa,0.36,respectively.The fatigue mechanism of nanotwinned 304 SS does not follow the trend predicted by Basquin equation that the HCF properties of metals are generally dominated by their ultimate tensile strength.The statistical results show that the fatigue fluctuation morphology of nanotwin/dislocation 304 SS is observed only in part of the dislocation structure(about 2.5%in volume fraction)at lower stress amplitude.But in nanotwin/recrystallization 304 SS the fatigue morphology is observed in both nanotwin and static recrystallization(SRX)grains,of which the fatigue morphology are formed in numerous SRX grains located in the vicinity of NT grains(about 7%in volume fraction)but only a small number of SRX grains(3.5%volume fraction)far away from NT grains.The microstructure observation shows that the excellent fatigue resistance of nanotwin/recrystallization 304 SS is derived from a variety of cyclic deformation mechanism.The nanotwin grains with lots of correlated necklace dislocations(CNDs)and partial detwinning can effectively coordinate the plastic deformation,moreover,NT grains can coordinate the deformation by promoting the dislocation activity and martensitic transformation in numerous surrounding SRX grains,which can reduce the deformation localization and improve the fatigue resistance of the material effectively.2.The plastic strain-controlled tension-compression low cycle fatigue properties and mechanism of nanotwin/recrystallization 304 SS were investigatedPlastic strain-controlled LCF test suggest nanotwin/recrystallization 304 SS exhibit cyclic softening behavior as CG 304 SS.However,the softening ratio of nanotwin/recrystallization 304 SS is 0.11 at high plastic strain amplitude,which is significantly lower than the softening ratio of low plastic strain amplitude and CG 304 SS(about 0.2),due to the low degree of cyclic softening of NT grains at large plastic strain amplitude.With the increase of plastic strain amplitude,the deformation behavior of NT grains changes from dislocation activity and detwinning to dislocation activity and shear bands,which makes the NT grains keep cyclic stability without obvious softening.Thus the stress response of the nanotwin/recrystallization 304 SS is much higher than that of the CG 304 SS.Moreover,there is little fatigue cracks form at the interface of NT and SRX grains at higher plastic strain amplitude due to its excellent anti-deformation localization ability,which makes the nanotwin/recrystallization 304 SS maintain a good fatigue life.3.304 SS with gradient dislocation structures(GDS)was prepared by cyclic tortion technique.The stress-controlled tension-compression high cycle fatigue properties and mechanism of GDS 304 SS were investigatedGDS 304 SS with different dislocation densities were prepared by changing the torsion angle at small deformation rate.When the torsion angle was 16°,the surface hardness of the sample is 3.7 GPa due to the nanotwin structure produced on the surface.The yield strength and tensile strength of GDS 304 SS are 464 MPa and 664 MPa respectively,the uniform elongation is 55%,which has good synthesis between strength and ductility.Stress-controlled HCF test suggest the fatigue limit and fatigue ratio of GDS 304 SS is 320 MPa,0.48 respectively,both comparable to the gradient nanosructured(GNS)austenitic stainless steels with higher surface hardness(about 5 GPa).At the low stress amplitude,the nanotwins in surface layer is slightly detwinned and a large number of stacking faults are generated in the dislocation cells to coordinate the deformation.The CG in the core with low strength assume the main plastic deformation through dislocation activity and martensitic transformation.The multiple cyclic deformation mechanisms and the anti-deformation localization ability brought by the gradient structure cause the superior high-cycle fatigue property of GDS 304 SS.The cyclic deformation mechanism of GDS 304 SS at higher stress amplitude is different from the core-CG coordinated plastic deformation mechanism at lower stress amplitude.The nanotwins in surface layer have significant detwinning at higher stress amplitude,which leads to a significant decrease in surface hardness.The surface layer coordinates the plastic deformation through detwinning,dislocation activity and stacking faults,the CG in the core coordinates the plastic deformation through dislocation activity and stacking faults,which increases the hardness of the core and forms a nearly uniform hardness distribution from the surface to the core.