Investigation Of Surface Nanocrystallinzation And Alloying Of Commercial Iron And Diffusion Behavior

The metallic materials are often treated at high temperature for a long time todiffuse the alloying elements into them during the existing surface alloying process ofmetallic materials. Which not only has an adverse effect on the microstructure andperformance of the metallic materials, but also causes resource and energy wasting. Tosolve this problem, the surface self-nanocrystallization （SSNC） technology and surfacealloying were combined to realize the surface modification of commercial iron in thispaper. There were many grain boundaries and defects in the self-nanocrystalline iron,which will provide a large number of diffusion channels. Therefore the atoms maydiffuse more rapidly in the self-nanocrystalline iron than in the coarse iron, which leadthe surface alloying of commercial iron at lower temperature in a short time. Thecombination of the surface self-nanocrystallization （SSNC） technology and surfacealloying was called surface nanocrystallinzation and alloying （SNA） in this paper.Firstly self-nanocrystallinzed layers were prepared on the commercial pure ironsamples by means of high energy shot peening （HESP） method, and the microstructureof the self-nanocrystallinzed layers was characterized through optical microscopy （OM）,scanning electron microscopy （SEM）, X-ray diffraction （XRD） and electronbackscattered diffraction （EBSD）. Then different elements were diffused into theSSNCed commercial iron and different SNAed layers were obtained. The diffusion ofatoms in nanocrystallined iron, phase transformation during SNA and the performanceof the SNAed layers were investigated detailedly.Results showed that under0.6MPa press,50mm distance from shotting gun to thesurface of samples, a nanocrystalline surface layer without oxidation, porosity andcontamination was obtained by means of HESP to a commercial iron cylinder with a1mm diameter cast iron ball after6min shot. The SSNCed commercial iron containedan average grain size of43.9nm and a microstrain of0.0652%, and the grain begun togrow when annealing at600℃.The mechanism of SSNC process of commercial iron involved three aspects. Themotion of the dislocations was the main reason, which lead the formation of bigorientation subgrain boundaries in coarse iron grain. After continued annihilation andrearrangement of the dislocations, the subgrain boundaries evolved into grainboundaries and the coarse grains were refined. Secondly big orientation lead the nonsynchronous deformation appear among different grains and the different parts ofthe coarse grains, which would separate grains and the different parts of coarse grains.Thirdly recrystallization produced fine grains on the deformed grain boundaries duringHESP.The diffusion test showed that the diffusion coefficient of nickel in SSNCedcommercial iron improved by one order of magnitude than that in coarse iron under a10MPa constant pressure on the diffusion couple. The diffusion coefficient of nickel inSSNCed commercial iron would be further improved if imposing a8-16MPa pulsepressure on the diffusion couple. When using Arrhenius formula to calculate thediffusion coefficient of atoms in nanocrystallines, the effect of temperature on the grainsof nanocrystallines must be considered.During SNA of commercial iron, intermetallic compounds such as FeNi₃, Fe₂Ni_0.25,NiCr and NiCr₂formed in the SNAed layers. But after annealing at900℃, theintermetallic compounds reduced and transformed into solid solution, and a certainamount of austenite appeared in the SNAed layers.The hardness of the SSNCed commercial iron enhanced because of grainrefinement, stress concentration and lattice distortion, while solid solution strengtheningeffect was the reason of the enhancement of hardness of the SNAed layers. Thecorrosion resistance of the commercial iron decreased after SSNC treating, but SNAtreatment can enhance the corrosion resistance of the commercial iron. Chemicalcorrosion and electrochemical corrosion were the corrosion principles of the SSNCedcommercial iron and the SNAed layers respectively. The wear resistance of commercialiron got a different level of improvement after SSNC and SNA treatment. The motion ofthe dislocations emitted from the fatigue source would be subjected to block of innercoarse and outer fine grain boundaries in the gradient structure of the SSNCedcommercial iron, which lead the enhancement of the wear resistance of SSNCed layers.As for the SNAed layers, the formation of hard and brittle intermetallic compounds wasthe main reason why the wear resistance improved.