The Atoms Diffusion Behabiror And Oxidation Resistance Of Fe-based Metal With Surface Nanocrystallization

High temperature oxidation can lead to the decline of service performance and shortened of service life in the use of metal parts or devices.Generally,the corrosion of metallic materials begins at their surface,and the overall properties are extremely sensitive to the structure and properties of the material's surface.Therefore,optimizing the surface structure and properties of the metal material will greatly improve the overall performance.Using the surface nanocrystallization treatment,the grain in the surface is refined to nano-scale,and the volume fraction of grain boundary is increased,which can provide more access for the rapid diffusion of elements in the metal surface.In this paper,the metal surface nanocrystallization and alloying were combined to improve the high temperature oxidation resistance of materials.The diffusion behavior of the atomics and the evolution of the organizational structure were studied from the microscopic point of view,the transmission behavior of solute atoms in the nano-structured metal in-depth understanding were researched in-deep,and the effect of surface nanocrystallization on high temperature oxidation behavior was analyzed,in order to technical support to improve the high temperature oxidation resistance of metalmaterial.The surface nanocrystallization of industrial pure iron was carried out by Surface Mechanical Attrition Treatment(SMAT).At the same time,the relationship between the surface nanocrystalline process parameters and the surface structure and properties was studied.The surface self-nanocrystallization of the pure iron was realized by SMAT,and the equiaxed nanocrystalline structures were formed in the outer surface with the grain size of 12 nm.The microstructure of nano-sized pure iron presented a gradient change along the depth,which could be divided into nanostructures(0 ~ 15 ?m),submicron structure area(15 ~ 40 ?m),microstructure area(40 ~ 60 ?m)and matrix.The mechanism for grain refinement was summarized as the parting of dislocation.After SMATed,the microhardness of the surface layer increased significantly,and decreased gradually to a steady value along the thickness direction.The increase of the hardness was the result of the combination of grain refinement and work hardening.In addition,the corrosion resistance of the nanostructured surface layer was decreased,contrary to the improve of the tensile strength.The effect of metal surface nanocrystallization on the atoms diffusion behavior was studied.After SMATed,the iron-nickel mixed layer with a thickness of 110 ?m was formed on the surface of pure iron when nickel powder was added.Due to the refinement of grains,the formof large number of defects including dislocations,the impact energy formed by the impact of the projectile,besides of the annealing heat treatment,the diffusion of Ni was strengthened,leading to the produce of Fe Ni solid solution phase.Combined the treatment of SMAT and ion injection,the concentration of Ti atoms on the surface of the surface was greatly increased.The main reasons were explained as follows: the surface by SMAT contained more defects(supersaturated vacancy,high density dislocation,non-equilibrium crystal,and so on),and the solute atoms interacted with these defects,leading to an additional increase in solid solubility.Using the molten salt method,the pure iron was siliconizinged after SMATed.It was found that the time of silicon infiltration was shorten and he depth was increased for silicon infiltration by SMATed.The diffusion rate and concentration of surface atoms in the metal surface can be improved by metal surface nanocrystallization,therefore,it is a feasible way to improve the surface properties of metal materials by the combination of surface nanocrystallization and chemical treatments.The effects of element diffusion,oxide film structures,oxidation product types and corrosion damage mechanisms on the oxidation process of 410 S,304and 430 stainless steels were investigated by means of SEM,EDS and XRD.The results showed that the selective oxidation of Cr in the oxidation process occured first with the form of Cr2O3 film,and thenthe film grown outwards as the result of the outward expanded of the alloying elements.The key factors affecting the oxidation rate were the temperature and oxidation atmosphere.At the meantime,high temperature water vapor accelerated the oxidation rate,as the role of self-catalysis.The process of high temperature oxidation can be summarized as follows.Firstly,the dense film appeared and covered over the metal surface,then the film thickened in length and became loose.With the increase in oxidation time,the cracks occurred and divided the film into two layers.To continue,voids appeared between the inside and outside of the oxide film,as well as the oxide film and matrix.Finally,the oxide film peeled off from matrix partly or whole.By SMATed,nanocrystalline structure was obtained in the surface of stainless steel,and the volume fraction of surface grain boundary was increased,so the probability of nuclear nucleation was improved.A large number of crystal defects in the deformation layer provide more channels for the diffusion of elements,and promoted the selective oxidation of Cr,Ni,Fe and other elements.At the beginning of oxidation,the oxidation curves showed a linear law and the weight increased rapidly.However,the time to achieve passivation was shortened,and the oxide film became much denser,making a greatly improved in antioxidant properties.The continuous oxide film was formed much easier in nano-surface layer than the coarse crystal,which had a small inner stress,excellent toughness,and strongerbonding force with the matrix.Because of these,the oxidation film could exist for a long time at high temperatures,which was one of the reasons for better oxidation resistance of SMATed samples.