In this paper, a nanocrystalline surface layer without impurities was fabricated on Ti-6Al-4V alloy by means of surface mechanical attrition treatment (SMAT). The grain size in the nanocrystalline layer is about 10 nm and grain morphology displays a random crystallographic orientation distribution. Subsequently, the low-temperature boriding behaviors (at 600 °C) of the SMAT sample, including the phase composition, microstructure, micro-hardness, and brittleness, were investigated in comparison with those of coarse-grained sample borided at 1100 °C. The results showed that the boriding kinetics could be significantly enhanced by SMAT, resulting in the formation of a nano-structured boride layers on Ti-6Al-4V alloy at lower temperature. Compared to the coarse-grained boriding sample, the SMAT boriding sample exhibits a similar hardness value, but improved surface toughness. The satisfactory surface toughness may be attributed to the boriding treatment that was carried out at lower temperature.

It was demonstrated that the mechanical shot peening (MSP) technique was a viable way to obtain a nanocrystalline layer on a large size pure titanium plate due to the MSP provided for severe plastic deformation (SPD) of surface high velocity balls impacting. The MSP effects of various durations in producing the surface nanocrystalline layer was characterized by optical microscope (OM), X-ray diffraction (XRD), transmission electron microscope (TEM), and Vickers micro-hardness tester. The results showed that the thickness of the SPD layer gradually increased with the MSP processing time increase, but saturated at 230 μm after 30 min. The average grain size was refined to about 18.48 nm in the nanocrystalline layer. There was equiaxed grain morphology with random crystallographic orientation in the topmost surface. By comparing with the nanocrystalline layer, acquired by surface mechanical attrition treatment (SMAT), the microstructure and properties of the nanocrystalline layer acquired by MSP was evidently superior to that of the SMAT, but the production time was cut to about a quarter of the time used for the SMAT method.

Boronizing treatment on the surface of wear-resistant steel can both improve the hardness and abrasion resistance of the surface layer and reduce the difference between the hardness of the boride layer and substrate, which can also greatly improve the crush performance and effeciently prevent the occurrence of brittle and fragile boronizing layer in the service process. In this paper, a boride layer without impurities was fabricated on the surface of high strength wear resistant steel (HSWRS) by solid powder boronizing treatment. The boriding behaviors of the HSWRS sample, including phase composition, microstructure, micro-hardness, wear resistance and crushing performance were investigated in comparison with those of 45 carbon steel borided at 1050 °C for 5 h. The results showed that the treated HSWRS was covered by a homogeneous boride layer. The boride layer was mainly composeded of Fe2B, V2B3, CrB2, MC, M2C. The highest micro-hardness value of the boride layer was about 1525 HV and the hardness took on gradient distribution from the surface to the center, and finally reduced to the hardness of the substrate. Compared to the borided 45 carbon steel, the boriding samples exhibit a similar surface toughness, but improved crushing performance.

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.

Micro-arc oxidation (MAO) coatings with 2 μm, 5 μm and 10 μm in thickness were produced on Ti-6Al-4V titanium alloy by MAO treatment, and the effect of coating thickness on the high cycle fatigue property was investigated. The microscopies and phases of coatings were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The results indicated that the coatings were mainly composed of anatase TiO2. The MAO process generated micro pits at the substrate surface. The micro pits played a role of the notches, which induced the crack initiation during fatigue loading. The fatigue life of samples decreased with the increase of coating thickness. The micro pits were the main factor for the decrease of the fatigue life.