Investigation Of Surface Nanocrystallization And Relative Problems In Pure Iron And 35CrMo Steel

In recent years, because surface mechanical attrition treatment (SMAT)enables the fabrication of a porosity-free and contamination-freenanocrystalline surface layer which is induced by the severe plasticdeformation at very high rates, the method of SMAT, which is used tofabricate bulk nanostructured materials (NSM), has attracted the growinginterest of specialists in materials science.In this paper, the nanostructured surface layer is obtained on a pure ironplate and a 35CrMo steel plate by using SMAT. The average grain size of thenanocrystallites approximates to 10nm and 15nm in the surface layers of pureiron and 35CrMo steel, respectively. In order to investigate the effect ofnanocrystallite on atomic diffusion kinetics, the gas nitriding and plasmanitriding for both the original coarse-grained samples and the SMAT samplesare carded out. The iron samples surface is cleaned and electroplated with alayer of pure nickel (Ni, about 5μm in thickness). Titanium (Ti) ions are implanted into the surface layers of the iron samples. Microstructure featuresof various sections in the surface layer and properties of the SMAT samplesare systematically characterized by using optical microscope, X-raydiffraction (XRD), transmission electron microscope (TEM), microhardnesstesting machine. The nitrided samples are also characterized and theconcentration of nitrogen (N) is measured along the depth after gas nitridingand plasma nitriding by means of Glow discharge spectroscopy (GDS).Scanning electron microscope (SEM) is used as analyzing diffusion behaviorof Ni atoms in the pure iron. Ti distribution is measured by making use ofAuger electron spectroscopy (AES). It shows that:1. Equiaxed nanocrystallites with random crystallographic orientationsare formed in the top surface layer of the iron and 35CrMo plates. DuringSMAT, severe plastic deformation occurs randomly in the surface layer, anddecreases gradually along the depth of the treated samples, which results inthe gradient variation of the microstructure.2. The microhardness of the nanostructured surface layer is found to beobviously increased, and is twice more than that of the original sample. Themicrohardness decreases gradually to that of the matrix along the depth of theSMAT sample. The polarization curves show that the nanostructured surfacelayer can improve the corrosion resistance behavior of the samples.3. For both samples nitrided, it can be seen that microhardness of the SMAT sample nitrided is a little higher than that of the original coarse-grainedsample nitrided. And the diffusion depth of N atoms in the SMAT sample isenhanced with respect to that in the coarse-grained sample. This can beattributed to the much enhanced heterogeneous nucleation rate at numerousgrain boundaries in the nanocrystallization iron.4. The diffusion phenomenon of Ni atoms in the SMAT iron canobviously appears at 300~C after diffusion annealing, otherwise there ishardly diffusion of Ni atoms at 500~C in the coarse-grained sample. So thediffusion of Ni can be facilitated due to the non-equilibrium defects and grainboundaries of triple junctions.5. The distributions of Ti concentration after ion implantation in thecoarse-grained iron and SMAT iron agree with the Gaussian distribution.Compared with that in the coarse-grained sample, Ti concentration in theSMAT sample is enhanced, but the implantation depths of both are nodifferent.