Studies On The Effects Of Strain Rate And Cyclic Pre-Deformation On Uniaxial Tensile Behavior Of Cr17 High-Purity Ferritic Stainless Steel

In this paper,Cr17 high-purity ferritic stainless steel is chosen as the target material,and the effects of strain rate,respectively at low and high strain rate ranges,and cyclic predeformation on the uniaxial tensile behavior of Cr17 high-purity ferritic stainless steel are systematically investigated,meanwhile,the surface deformation and damage characteristics as well as the microstructures of the Cr17 high purity ferritic stainless steel are studied using scanning electron microscopy and transmission electron microscopy.The aim is to reveal the micro-mechanisms of the effects of the strain rate and cyclic pre-deformation on the mechanical behavior of Cr17 high-purity ferritic stainless steel,and further provide a reference data for its practical application.The obtained results in the paper are mainly as follows:(1)In the low strain rate range from 10-4 s-1 to 10-2 s-1,with the increase of strain rate,the density of slip bands increases,and the slip bands formed in different regions inside a coarse grain have different orientations,which weakens the extrusion and intrusion of grain boundaries,further leading to the changing of micro-void formation sites from nearby grain boundaries to inside grains and the decrease in the elongation at a strain rate of 10-2 s-1.The dislocation structures are transformed from loose dislocation cells into wall-cell structures,and the size of dislocation cells decreases,which promotes the increased yield strength and tensile strength as strain rate is raised.In this low strain rate range,the sensitivity index of strain rate m is 0.0235.(2)At the high strain rate range from 10-1 s-1 to 100 s-1,the micro-cavities are observed in grains and nearby grain boundaries as strain rate is lower,whereas are mainly formed nearby grain boundaries at higher strain rates.The TEM microstructures mainly consist of wall-cell structures,and with the increase of strain rate,the size of cell blocks in wall-cell structures decreases and the degree of wall bending increases,which leads to the synchronous increase of strength and uniform strain.In this high strain rate range,the sensitivity index of strain rate m is 0.032.(3)The density of slip bands on the surfaces first increases with the increase of D and then decreases at D?50%after Cr17 high-purity ferritic stainless steel is cyclically predeformed,and micro-cavities are observed on the surfaces of specimens at D=10%and 75%,especially at D=75%,which results in the obtaining of a lower uniform strain during subsequent tension.The cyclic pre-deformation results in the increase in dislocation density,and dislocation structures are evolved from dislocation lines at the un-fatigued state to the loose dislocation cells at the D range from 5%to 25%,but at D? 50%,the dislocation density decreases,and its structures are developed from larger loose dislocation cells at D=50%to dislocation lines,or even dislocation tangles at D=75%,which is mainly related to the dynamic recovery induced by cyclic pre-deformation of higher cycles.(4)The cyclic pre-deformation leads to a notable increase in yield strength and the decrease in uniform strain of Cr17 high purity ferritic stainless steel.With the increase of D,yield strength increases firstly and then decreases,and their values at different D are higher than that at the un-fatigued state.The uniform strain first decreases and then increases,and is reduced again at D=75%,whereas tensile strength does not exhibit the obvious change with D.The density of slip bands on the surfaces increases with increasing D,and the extrusion and intrusion near grain boundaries is weakened,which leads to the decreased probability of the formation of micro-cracks nearby grain boundaries.The dislocation structures are mainly typical wall-cell structures after tensile deformation.The almost unidirectional dislocation walls in the unfatigued sample are observed,and the spacing between walls is wider.For the samples cyclically pre-deformed at D=5%-25%,the walls with two orientations are formed and with the increase of D,the spacing between the walls and the cell blocks become smaller.However,as D is as high as 50%and 75%,the unidirectional walls are formed and the spacing between the walls is increased,but compared with the unfatigued sample,the dislocation density is higher and the spacing between the walls and the size of dislocation cells decrease.