In-situ Study Of Mechanical Behavior For Micro/nanocrystalline Bimodal Strucutured Stainless Steel

Compared with traditional coarse-grained materials,nanocrystalline materials usually had high strength and hardness but poor toughness and plasticity at room temperature,which had become a bottleneck problem and greatly limited its research and application.It was found that micro/nanocrystalline bimodal strucutured metal could obtain excellent comprehensive mechanical properties for nanocrystalline metals.Alumina Thermite Reaction was an advanced and novel preparation method to obtain composite nanocrystalline materials,in which material system was expansibility,preparation process and microstructure were stable.This method could enhance plasticity by changing structure of nanocrystalline metals rather than by alloying means.The composition of metal after toughening had not changed,which was in line with current research trend of making materials plain.The most widely used 304,316 L austenitic stainless steel and 2205 duplex stainless steel with stronger corrostion resistance had been taken as research cases.Bulk of micro/nanocrystalline bimodal strucutured stainless steel was prepared by Alumina Thermite Reaction.Specimens with better mechanical properties were selected by room-temperature uniaxial tensile test.Microscopic intrinsic deformation mechanism of bimodal strucutured stainless steel were studied,interactional condition between microstructures and mechanical properties were revealed,various deformation mechanisms of grains with different sizes were obtained by means of in-situ tensile test.In-situ tensile test played an active role in analyzing mechanical properities and behaviors of bimodal structured,and summarizing mechanical laws involved.Based on selected specimens with the best match of mechanical properties,the constitutive model was established to get constitutive equation.Numerical calculation of mechanical properties of stainless steel with micro/nanocrystalline bimodal strucuture was carried out,and reason for the errors were analyzed by compared with experimental results.In this dissertation,the main work and conclustions were as follows:1.Tensile mechanical properties of stainless steel specimens which were prepared by Alumina Thermite Reaction were analysed with mechanical test equipments.The study found that mixed grain structure composed of microcrystalline and nanocrystalline grains were the main reason for good strength and ductility.2.The specimens with the best mechanical properties were selected as research objects.Technology of in-situ tensile table loading SEM was used to observe deformation behavior,changes of microstructure at the crack tip and propagation path of the crack for 304,316 L austenitic stainless steel during the tensile process.Dynamic relationship between mechanical properties and microstructure was studied at the micro-level.Two different yielding stages observed in engineering stress-strain curves,and there were two lines with different slopes.The fracture morphology was observed after fracture,there were many dimple nucleus location,dimples were smaller and shallow,boundaries were clearer.There were two sizes of dimples in general.3.On the basis of in-situ study of 304 and 316 L austenitic stainless steel with micro/nanocrystalline bimodal strucuture,bimodal structured 2205 duplex stainless steel was carried out,the factors affecting ferrite phase needed to be considered more than austenitic stainless steel.By studying deformation mechanism of duplex stainless steel with bimodal structure during in-situ tension,the influence of bimodal strucuture on dual phase microstructures and mechanical properties were discussed.4.In the material which composite structure of micro-sized plastic phases were embedded into nanoscale matrix,a large number of grain boundaries were contained in nanocrystalline grains resulting in high strength.At the same time,large slip bands and dislocation motion would be produced by accumulation of dislocations in microcrystalline grains.5.Based on classical Kocks-Mecking constitutive model for nanocrystalline materials and Taylor dislocation strengthening theory,the relationship between stress and strain was defined.Constitutive models of bimodal strucutured nanocrystalline materials were established by studying microscopic deformation behavior and the relationship between macroscopic stress and strain.Program was solved by numerical calculation software Matlab,and then engineering stress-strain curves of three kinds of specimens were obtained.Finally,reasons for the errors were analyzed.Through in-situ study of mechanical behavior for micro/nanocrystalline bimodal strucutured stainless steel,constitutive model was established to study relationship between microstructure and mechanical properties,which based on macroscopic mechanical properties.The numberical calculation method was used to guide adjustment of preparing conditions.Finally,stainless steel achieved the preset performance which prepared by Alumina Thermite Reaction.