Study On In-Situ Surface Alloying Of Pure Iron

Nano-diffusion layer with a certain thickness can be fabricated in the surface of material by surface mechanical attrition treatment (SMAT). During the process of SMAT, steel balls with a certain frequency impact the surface of material repeatedly, so that the plastic deformation is produced in the surface and a large stress strain energy, non-equilibrium grain boundary, crystal defects (vacancies, dislocations, etc.)are formed, which can contribute to the diffusion of elements. Mechanical alloying (MA) technology is defined as a method making two or more kinds of powders to achieve elements alloying after high-energy ball milling methods and the principle of MA is that powders occur diffusion reaction to form alloy through the role of constantly rubbing, crushing, welding. Through combining the features of SMAT and MA technologies in the thesis, adding different powders during the process of SMAT to get nanocrystallization and the effect of elements diffusion to reach alloying at the same time by the projectiles hitting the surface continually, then in-situ formation of alloying in the surface of materials.In this paper, Ni powder, Cr powder, Zn powder and Al powder were added during SMAT in the iron substrate surface, by materials analysis and detection methods such as optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy spectrum analysis(EDS), glow discharge tester (GDS), electrochemical workstation and so on, the surface morphology, the cross-sectional structure, phase composition of the surface, surface analysis of corrosion resistance of pure iron were discussed and analysised with different powders and SMAT times. And also the factors influencing the diffusion of alloy layer between adding elements and the substrate were studied. Finally, the diffusion coefficient of SAMT added elements in the surface of the substrate were calculated simply from the thermodynamic point.The conclusion is as follows:(1) After pure Fe/SMAT/Nil20min and200min,～25nm and～17nm of the surface grain size were get respectively,～30μm and50～100μm diffusion layers were formed respectively. The composition of diffusion layer was FeNi after SMAT120min. The diffusion layer components from the surface to the core was Ni, FeNi3, FeNi, pure Fe after SMAT200min. On the corrosion resistance research showed that electrode potential will enhance after SMAT. The corrosi--on resistance of SMAT200min was better than SMAT120min. The diffusion coefficient increased after SMAT.(2) After pure Fe/SMAT/Crl20min, a diffusion layer whose surface grain size was～19.4nm, thickness was～50μm and composition was Fe(Cr) solid solution was get. With the processing time extending to240min, the highest thickness of the diffusion layer was up to～110μm, the surface of grain refined as～12nm. The surface roughness increased greatly with extending SMAT time, no-covered by Cr powder areas existed on the surface. The corrosion resistance research indicated that composed by SMAT120min and SMAT200min, without Cr layer on the surface can increase electrode potential. The corrosion resistance of the samples decreased after SMAT and the diffusion coefficient increased.(3) The diffusion mechanism of pure Fe/SMAT samples with adding Ni and Cr powders:There were something in common.When the powders were embedded in the matrix surface, because of the effect of projectile stressing, powder will be welded in the substrate surface, high volume of grain boundary and surface defects after SMAT offered elements short-circuit diffusion channel; Projectile high-speed repeating impact specimen surface maked new interfaces and exposed surface of the powder appeared constantly, which can increase the number of solder point between powder and Fe matrix; Matrix constantly absorbed the energy of the projectile impacts caused the matrix of high strain and high strain energy in the process of SMAT, which can reduce the active atomic migration; Finally,the local heating during treated process can facilitate diffusion. It was the common of pure iron added Ni and Cr powder diffusion mechanism. The difference was that the pure Fe/SMAT/Ni atom diffusion formed in the substrate was Fe-Ni compound, and the pure Fe/SMAT/Cr atom diffusion formed in the substrate was Fe(Cr) solid solution, and the thickness of the diffusion layer was also different more or less.(4) There were neither corresponding alloy diffusion layer nor solid solution diffusion layer formed in both of pure Fe/SMAT/Zn samples with different treated times and pure Fe/SMAT/A1120min sample. The cause of the failure can result in the soft texture of zinc and aluminum.Because the powers adhered to the substrate surface presenting as lamellar in the process SMAT, thus making it difficult to get active atoms and then subsequent diffusion reaction. The influencing factors of pure Fe/SMAT diffusion layer formed can be attributed to:surface smoothness of the balls, SMAT times, hardness of adding powders and the substrate, crystal structure and stacking fault energy, etc.