Investigations On Mechanical Behavior Of18Cr-18Mn-0.63N Ni-free Austenitic Stainless Steel

As a kind of newly-developed stainless steel materials with wide applications and excellent performance, high nitrogen nickel free austenitic stainless steel would be inevitably subjected to various kinds of uniaxial or cyclic loads during service. Therefore, studies on its mechanical properties and fatigue behavior are of particular importance and practical significance. Since no nickel is contained in the steel, no additional allergic reaction comes into being when high nitrogen nickel free stainless steel is implanted into human body, and it is usually made into various sizes of devices to implant into the body. Therefore, studying the size effect on mechanical behavior of the material has a certain practical significance. However, until now, no systematic investigations in this area have yet been performed. In light of this, the18Cr-18Mn-0.63N austenitic stainless steel was selected in the present work as the target material to investigate its uniaxial deformation and damage behavior at different strain rates, and on this basis, deformation mechanisms under various types of loads and effects of the sample thickness (i.e., geometric dimension) on its tensile and fatigue behaviors will be focused on.Compared with the conventional nickel containing stainless steels,18Cr-18Mn-0.63N stainless steel shows high strength, but relatively low ductility. During tensile deformation, with the increase of strain rate, the yield strength of samples increases, while ultimate tensile strength and the total elongation decrease. This is closely related to the appearance of a large number of microstructures, like twins and twin-like bands. Compressive deformation behavior is not so sensitive to the strain rate adopted. Compared with tensile deformation, the surface fluctuation is more serious as the specimen is subjected to compressive deformation. Nearly no twins are formed inside the compressed samples, and planar slip dislocations along three slip directions are generally found in most of grains.Pull-Push fatigue experiments of18Cr-18Mn-0.63N stainless steel under different total strain amplitudes show that fatigue and fracture behavior of this material is closely related to strain amplitude. Stress saturation phenomenon is generally found after an initial significant softening stage. At low strain amplitudes, cracks initiate and extend along slip band, and the internal dislocation configuration is mainly occupied by the formation of planar slip dislocation structure. At high strain amplitudes, the intergranular cracks are generally found on the surface of deformed specimen. In this case, the corresponding dislocation structures are mainly composed of dislocation cells and stacking faults, and some deformation twins tend to form in local grains.The specimen thickness (around submillimeter-scale) has an effect, to some extent, on the tensile and fatigue properties, fracture behavior and microstructures of18Cr-18Mn-0.63N austenitic stainless steel. At the same strain rate, with the increase of sample size (thickness), the tensile yield strength, ultimate tensile strength and elongation increase, meanwhile, deformation and damage on the surface are found to be more serious, the necking phenomenon becomes more apparent, and the size and depth of dimples in the fibrous area on the fracture surface increase. TEM observations demonstrate that, with increasing thickness, the deformation microstructures transform from pure planar slip dislocation structure into deformation twins and twin-like bands in addition to normal dislocation structures. As the specimens are deformed under pull-pull fatigue at the same stress amplitude, with increasing specimen thickness, the fatigue life increases and the major damage features on the surface change from slip cracking into intergranular cracking, meanwhile, the micro-features in the fatigue crack source zone changes gradually from quasi-cleavage to cleavage, the plastic characteristics of fatigue striations become more evident, and the size and density of dimples in the finial rapid fracture areas increase slightly. With increasing thickness, the microstructures transform from planar slip bands along multiple slip directions and twins (or twin-like bands) into planar slip bands along single slip direction and stacking faults.Pull-pull fatigue experiments under constant stress amplitude (Ac/2) control show that the curve of fatigue life (Nf) versus stress amplitude of the18Cr-18Mn-0.63N stainless steel meets the bilinear Basquin relationship. With the increase of stress amplitude, the damage features on the surface of specimen change from the slip cracking into the intergranular cracking.18Cr-18Mn-0.63N steel shows a strong stress amplitude sensitive of its deformation microstructure. At low stress amplitudes, deformation microstructures consist mainly of planar slip dislocations and stacking faults, accompanied by the formation of a few twins. In contrast, at high stress amplitudes, deformation microstructures comprise mainly planar slip dislocations and twins, meanwhile, wavy slips and twin-like bands are found within local grains. The difference in deformation micromechanism under various stress amplitudes should be the major reason for the occurrence of an apparent turning point in the S-N curve (i.e., the curve of Nf-△σ/2meets the bilinear Basquin relationship).