Study On Mechanical Behavior Of Pure Iron Subjected To Surface Mechanical Attrition Treatment And Nitriding

Surface nanocrystallization has been developed for synthesizing a nanostructured surface layer on metallic materials by means of some methods based on mechanical, physical and chemistry technology. A surface layer with nanocrystalline grains can be produced on a bulk metal by using surface mechanical attrition treatment (SMAT) without changing their structural and chemical composition. Metallic materials with surface nanostructured layer not only display unique mechanical properties, but also has lower nitriding temperature and faster nitriding process due to the high density of grain boundaries. Most of the previous studies were performed on the lower temperature nitriding behavior and surface properties of materials with nanostructured surface layer. Experimental results on mechanical property of materials with nanocrystallization surface layer nitrided at low temperature are limited.In this wok, an iron plate was treated by surface mechanical attrition treatment (SMAT), and the untreated and treated samples were nitrided in flowing, high-purity ammonia gas (NH3) at500℃for5hours. Microtructure of the SMATed Fe was characterized by means of optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Additionally, tensile properties of the pure iron processed by the surface nanocrystallization and nitriding were studied to supplement the shortage of previous study.A plastic deformation layer with a thichness of about100μm thick was fabricated on the pure iron by means of surface mechanical attrition treatment. The average grain size on the top surface layer is about15nm with random crystallographic orientation. Gradient structure of grain size was formed at different depth from the surface. Surface hardness of the SMATed Fe is much higher than that of the untreated sample, and it decreases with depth. Tensile test results indicate that yield stress of the SMATed Fe increases significantly compared with that of the untreated Fe. Whereas its plasticity decreases. Fracture surface of the treated sample is a combination of ductile and cleavage fracture with nanocrystalline surface layer behaves in a brittle mode, whereas the substrate tends to fracture in a ductile mode.Thickness of the compound layer and surface hardness of the nitrided SMAT Fe are much larger than those of the nitrided coarse-grained sample. The nitrided layer in the SMATed sample is composed of Fe2-3N phase with grain size in the range10-40nm. Additionally, a large amount of particle-like submicronstructured Fe2-3N phase with grain size in the range from tens to hundreds nanometer can be observed beneath the compound layer. Precipitation of the needle-like nitride is supposed to be retrained. Tensile tests showed that higher strength and plastic can be obtained in the nitrided SMAT sample in comparison with the nitrided coarse-grained sample. Strengthening mechanisms of the smated and nitrided Fe are supposed to be grain refinement, solution strengthening, dispersion strengthening and work-hardening. Satisfactory plasticity of the nitrided SMAT Fe was attributed to inhibition of needle-like nitrided precipitate, grains refinement and dispersive distribution of fine nitride.