Abstract
Properties of nanocrystalline (nc) materials are different from, and often superior to those of conventional coarse-grained counterparts. Unfortunately, it is still difficult to obtain ideal (e.g. full-density, residual stress-free, flaw-free, porosity-free and contamination-free) nc bulk sample by using the present preparation methods. Recently, a new technique named surface mechanical attrition treatment (SMAT) was developed. SMAT enables the fabrication of an nc surface layer on various bulk metals. The nc layer is free of contamination and porosity because the nanocrystallization process is induced by the severe plastic deformation at very high strain rates. In this work, a pure Fe plate was subjected to the SMAT and its microstructure were characterized. The effect of the surface nanocrystalline layer on the gas nitriding process at a lower temperature was also investigated by using structural analysis. The surface nanocrystallization evidently enhances nitriding kinetics and promotes the formation of an ultra-fine polycrystalline compound layer. The results of the investigation showed that this new gas nitriding technique can effectively increase the hardness of the resulting surface layer in comparison with conventional nitriding, demonstrating a significant advancement for materials processing.
Highlights
Nitriding is a widely used chemical treatment to form surface nitrides
It has been demonstrated experimentally that chemical reaction kinetics are greatly enhanced during mechanical attrition of solids in which the grain size is significantly reduced into the nm scale and structural defects are created due to the severe plastic deformation [3]
We studied enhanced chemical reaction kinetics at lower temperatures when the surface layer of a metal is transformed into nanocrystalline structure, using the nitriding of Fe as an example
Summary
Nitriding is a widely used chemical treatment to form surface nitrides. This technique is of great industrial interest as it forms a unique composite structure with a hard surface (nitride compounds layer) and tough interior so that the global mechanical performance and wear/corrosion resistance of alloys and steels could be greatly improved [1]. Experiments showed that apparent grain growth of the nanocrystalline -Fe phase in the surface layer may occur at 773 K or above, and the average grain size of the -Fe nanophase was measured to be about 40 nm at 723 K after 9 h, indicating a rather weak grain growth tendency at this temperature These results make it possible to investigate nitridation of nc iron and steels at lower temperature. The subsequent nitriding kinetics of the as-treated iron with the nanostructured surface layer is greatly enhanced so that the nitriding temperature can be reduce to 573 ∼ 673 K regions, and the ultrafine polycrystalline -Fe2∼3N and -Fe4N compounds layer were formed in nanocrystalline grains layer. The effective activation energy for diffusion of nitrogen in composite nanocrystalline -Fe2∼3N and -Fe4N nitrides layer can be calculated from the temperature dependence of compound layer thickness ( = 4h), as shown, in comparison with that for the coarse-grained Fe nitrides at high temperature (above 773 K). The obtained value of activation energy for diffusion being about 54.39 kJ/mol, is smaller than that for coarse-grained phase (80.7 kJ/mol) and phase (88.3 kJ/mol)
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