Papers by Keyword: Surface Nanocrystallization (SNC)

Abstract: The effect of the strain rate on the surface nanocrystallization of titanium is investigated both theoretically and experimentally in this paper. The strain rate variation and stress distribution from surface to the interior of titanium during shot peening are estimated firstly using finite element method. Then shot peening experiment is carried out on a commercially pure titanium (CP-Ti) plate, and the obtained surface microstructures is characterized by transmission electron microscopy (TEM). Combining theoretical simulations and experimental observations, the effect of strain rate on the strain accommodation mechanism and plastic deformation mode are discussed. It is concluded that the strain rate and stress achieve the highest at the top surface layer of CP-Ti, and the strain rate decrease dramatically from the surface to the interior. The strain rate at the top surface layer is up to 104 s-1, which leads to superplastic deformation of Ti. There is no mechanical twin in the surface layer, instead, deformation lamella and adiabatic shear bands are the dominating microstructures. By means of rotation recrystallization, those deformation bands evolve to nanocrystallines.

Abstract: 35 steel was processed in order to investigate its structure and performance after strengthened shot peening and gas tufftriding process. The composition,structure and phase of the samples were analyzed by means of scanning electron micrograph(SEM),metallographic microscope and X-ray diffraction(XRD)respectively. The results show that the hardness of samples after strengthened shot peening increases by about 100 HV0.025,and the friction coefficient and corrosion rate decrease.However,the samples treated with gas tufftriding after strengthened shot peening exhibit the performance of approximate 3 times hardness of the original samples, lower friction coefficient,more excellent tribological characteristics,stronger corrosion-resistance with a stable and broad passivation region.

Abstract: Nanocrystalline surface layer was fabricated on a quenched and tempered Cr-Si alloy steel by using Surface mechanical treatment. The microstructure features of various sections in the surface layer were characterized by using transmission electron microscopy (TEM). By analyzing the microstructural characteristic at different depths in the treated surface, the effect of the initial microstructures on grain refinement process of quenched and tempered steel was investigated. Experimental evidence showed the initial subgrains with small angle boundary and lower dislocation density were firstly developed into Lamellar-type dislocation cells (DCs) with dense dislocation walls (DDWs). Some initial subboundaries were moved to DDWs by dislocation activities. The width of lamellar-type DCs was 2-3 times of that of initial lathy subgrains. The size of the DCs and subgrains formed in the interim of refinement process was not uniformity. On the top surface the cementite granules were decomposed or fragmented to hyperfine particles, and the size of the grains tended to uniformity. Experimental analysis indicated the configuration of microstructure was affect by the initial microstructure in the initial stage and the interim of the grain refinement process. Surface nanocrystallization of Cr-Si steel can be attributed to dislocation activities.

Abstract: Commercially pure Titanium (CP-Ti) TIG weld joint was treated by means of high energy shot peening (HESP) using a shot peening equipment commonly used in industry. The nanostructured surface layer was characterized by XRD, TEM, SEM and Microhardometer. The results showed that surface nanocrystallization of CP-Ti TIG weld joint were realized by high energy shot peening treatment. The finest grain size in the top surface layer is about 40nm. The hardness of the surface layer is enhanced significantly after shot peening compared with that of the as-welded joint, which resulted in a remarkable surface hardening effect. Surface welded defects such as air pores are eliminated successfully so that relative uniform surface layer was obtained.

Abstract: This paper discusses the influence of ultrasonic surface peening on the fatigue property of 7B04 high strength and toughness aluminum alloy. The results showed that a nano-layer of 10-20 μm was formed after ultrasonic surface peening. This nano-layer had a gradient structure with grain size of 10 to 50nm. Further, the ultrasonic surface peening process created residual compressive stress of 200MPa in this nano-layer. This stress caused micro surface cracks to be compressed or closed . Consequently, after ultrasonic surface peening the fatigue life of 7B04 alloy was extended to 5 times.

Abstract: A nanostructure surface layer was produced on low carbon steel and commercially pure titanium using high-speed rotating wire-wheel deformation (HRWD). The microstructural features of the surface layer were systematically characterized by cross-sectional optical microcopy observations, transmission electron microscopy, and microhadness measurement was conducted along the depth from top surface layer to matrix of the samples. The results show that nearly equiaxed nanocrystalline layer is formed on the surface of the low carbon steel and pure titanium, in which the average grain size is about 8 nm and 15 nm respectively. The microhardness of the top surface is enhanced obviously compared with that of the coarse-grained matrix.

Abstract: Based on strain-induced grain refinement, a novel surface mechanical attrition treatment (SMAT) technique has been developed to synthesize a nanostructured surface layer on metallic materials in order to upgrade their overall properties and performance without changing their chemical compositions. In recent several years, the microstructures and properties of surface layer were systematically investigated in various SMAT metals and alloys, including b.c.c., f.c.c. and h.c.p. crystal structures. Different grain refinement approaches and nanocrystalline formation mechanisms were identified in these deformed materials, involving dislocation activities, mechanical twinning and interaction of dislocations with mechanical twins. The properties of the surface layer were measured by means of hardness, tensile, fatigue and wear tests. The enhanced properties of the surface layer are mainly attributed to the strain-induced grain refinement. In this work, we reviewed the microstructures and properties of surface layer in the SMAT materials.

Abstract: The nanostructured surface layers were fabricated on a hardened and tempered chrome-silicon alloy steel and a normalized medium carbon steel by mean of Supersonic Fine Particles Bombarding (SFPB). The microstructure features in the treated surface layer were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. Experimental results show the nanostructured surface layer is fabricated on both samples after SFPB treatment. The microstructure of the top surface is characterized by uniformly distributed nano-scale grains with equiaxed shape and random crystallographic orientations. The mean size of equiaxed nanocrystallites on the top surface layer is approximately 15-20nm for the SFPB treated medium carbon steel and Chrome-silicon alloy steel. During severe deformation the grain refinement in ferrite and cementite phases is observed, the cementite phases are exposed to breaking and dissolution due to mechanical alloying resulting in the formation of a supersaturated solid solution of carbon in α-Fe matrix. In the ferrite phase, the grains are refined by the process of dislocation actives and forming cell structures separated by dense dislocation walls (DDWs), as well as evolution of dislocation to subboundaries and grain boundaries.

Abstract: Supersonic fine particles bombarding (SFPB) is an important way to perform surface nanocrystallization. Harder the material is, higher the bombarding particles speed is needed. The gun with excellent features is premise to obtain the nano-structured layer.This paper analysed the advantages of rectangle Laval nozzle compared to annular-shape one and calculated critical structured parameters of the nozzle.Numerical simulation analysis of flow field of the nozzle at inlet temperature 300K and inlet pressure 0.25 MP, 1.0 MP, 1.7 MP and 2.5MP and velocity field of the gun with divergent-angle extended-barrel conduced by a commercial finite volume code FLUENT software of CFD; Consequently, the overall structure of the gun was optimized and determined finally. The sample of 38CrSi steel was treated by this gun. And the nano-structure layer on the surface was observed by TEM.

Abstract: Nanostructured surface layer was produced on the Ti-6Al-4V alloy by surface mechanical attrition treatment (SMAT) technique, and then the ceramic composite coatings of surface nanocrystallization/ micro-arc oxidation (SNC/MAO) were prepared on the nanocrystallized surface of Ti-6Al-4V alloy by pulsed single-polar micro-arc oxidation in NaAlO2 solution. The phase composition, morphology of surface and section and element content of the nanostructured surface layer and the ceramic coatings were investigated by XRD, TEM, SEM and EDS, respectively. The results showed that after the SMAT technique treatment for a short period of time, the surface layer was refined into ultrafine grains. The thickness of the coatings on the substrate treated by SMAT technique is about 10μm by micro-arc oxidation, which is thicker than the ones on the substrate untreated by SMAT technique, and the ceramic coatings are mainly composed of Al2TiO5. Besides, a bright interface layer comes out between the substrate and the coating when the substrate treated by SMAT technique. The influences of the SMAT technique on the surface morphology of the ceramic coatings also changed. The coatings using the SMAT technique were more compact and less porous than the ones on the substrate untreated by SMAT technique. The content of elements in the nanostructured surface layer on Ti-6Al-4V alloy had changed after the SMAT technique: the content of Ti increased, the content of V decreased, and the content of Al changed a little. The contents of Al and P in the coating increased while the content of Ti decreased.