| Austenitic stainless steel (SS) is a widely used engineering material nowadays due to itsexcellent corrosion resistance and good formability. However, its application is hindered tosome extent by its low mechanical strength and poor anti-friction property. Therefore, it isnecessary to enhance the strength and hardness of austenitic SS so as to improve its productquality. Nanocrystallization is an effective means to strengthen metallic materials andequal-channel angular pressing (ECAP) is one of the most important methods to produce bulknanocrystalline materials. How the corrosion resistance and formability change of SS afternanocrytallization will directly influence its application prospect.304austenitic SS was therefore used as the investigating object for nanocrytallization inthis study. The bulk nanocrystalline SS with a grain size of80-120nm was obtained by ECAPof8passes. Microstructure homogeneity and evolution, strengthening mechanism and,corrosion behaviors of SS during ECAP after were investigated in a great extend.Firstly, we designed a set of ECAP dies and auxiliary heating equipment and successfullysynthesized the bulk nanocrystalline SS without deforming defects on the basis of feasibleECAP parameters.Microstructure homogeneity of the ECAPed samples was inspected via microhardnessdistribution profiles over the cross-sectional plane of the samples, and microstructureobservation was made by means of optical microscope (OM) and transmission electronmicroscope (TEM). The results showed that microstructure homogeneity was increased andgrains were refined with the increasing of pressing number. Four passes of ECAP wereenough for refining the coarse grained SS into nanometer scale and further pressing did notresult in more refining effect, but a more uniform structure. According to the experimentalresults obtained we proposed a mixed microstructure evolution model of SS during ECAP,including slipping (subgrain bands), dislocation cell and twinning deformation. As a result,two types of nanocrystalline structures were obtained, namely the equiaxed nanocrystallinegrains (80-120nm) and nanocrystalline twins (a thickness of12-20nm, a length of60nm).XRD analysis showed that strain-induced martensite was present after one pass of ECAP,with a volume fraction of about9.2%. More passes of ECAP had less effect on the fraction ofstrain-induced martensite.Tensile test results showed that the strength of SS after one pass ECAP was increasedgreatly, but with an obvious decrease of plasticity. Strength and ductility did not changeobviously in the after passes of ECAP. Two strengthening mechanisms including strain hardening and grain refining strengthening were seen in SS during ECAP. Strain hardeningmechanism predominated during ECAP of1-3passes, however, grain refining strengtheningmechanism played an important role during ECAP of5-8passes, and both mechanismsworked in ECAP of4passes. It was also found that the relationship between yield stress andgrain size of SS fitted Hall-Petch relation after ECAP of5-8passes.The heat treatment process was also explored to restore the plasticity of nanocrystallineSS. It was found that annealing at400℃-10min of nanocrystalline SS did not have mucheffect on plasticity, where only a partial recovery occurred in nanocrystalline SS. Anannealing treatment of the nanocrystalline SS at600℃-10min could restore its plasticity to acertain extent, for example,8.5%elongation to failure before annealing was increased to15%after annealing, without much change of strength and grain size, where a full recoveryoccurred.Meanwhile, we also examined the tensile fracture of0-pass sample with original coarsegrains,1-pass sample with banded coarse grains and8-pass sample with nanocrystallinegrains and the influence of heat treatment on the tensile fractographies of1-and8-passsamples. Macro morphology observation of the fractures showed that0-and1-pass sampleshad a ductile fracture feature, but8-pass sample a brittle one. However, the tensile fractures ofthe three samples appeared to be ductile with dimple feature under high magnification ofSEM. Furthermore, the dimples became more shallow and finer with the increasing of ECAPpasses, which was related to the microstructure change of stainless steel after ECAP. Theeffect of annealing on the dimple morphology of1-pass specimen was greater than that on8-pass specimen, which seemed to be connected with the deforming mechanism of grainboundary sliding in nanocrystallized8-pass specimen.Lastly, we had a full analysis and characterization on the corrosion resistance and passivefilm performance of the nanocrystalline SS. The results of potentiostatic polarization test andMott-Schottky measurement showed that the passive film of8-pass sample with nanostructurewas more compact (lower porosity) and chemically stable (lower carrier density) than that of0-pass sample with coarse grains. XPS measurements showed that the thickness and chemicalcomposition of the passive film on both0-and8-pass samples were very similar, suggestingthat Cr diffusion normally enhanced by nanocrystallization structure did not occur at roomtemperature in the present study. The Cr/Fe ratio in the passive film in both0-pass and8-passsamples formed in0.5M H2SO4solution was higher than that formed in air due to thedissolution of passive film in solution and the higher dissolving rate of Fe than Cr. The Cr/Fe ratio in the passive film of8-pass sample was lower than that of0-pass sample at open circuitpotential in0.5M H2SO4solution, due to the more stable passive film of8-pass sample andconsequently the lower dissolving rate of Fe in its passive film.
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