Nitriding of nanocrystalline pure Fe induced by surface mechanical attrition treatment

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.