Papers by Keyword: Nanocrystalline Material

Abstract: . In the present study, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, transmission electron microscope (TEM) observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure.

Abstract: Applying a bicolor coherent treatment of a Nd-YAG laser with wavelength 1.32 μm and its second harmonic generation, we have investigated photoinduced SHG in Au nanoparticledeposited ZnO nanocrystallinne (NC) films. We have established that coexistence of the such crystallites gives substantially better results compared to the pure ZnO NC deposited onto the glass substrate. It was shown that the value of the second order susceptibility obtained for the large incident angles of the fundamental and writing beams is equal to about 23 pm/V. The better results are obtained during the phototreatment at temperatures near the 60 oC for the Au doped samples and is almost non-sensitive to temperature for the pure samples. The further increase of the temperature leads to decay of the optical second harmonic generation

Abstract: In this study we show that in-situ tensile tests performed in a transmission electron microscope (TEM) in combination with high-resolution TEM are feasible, and, that this method is appropriate to elucidate the deformation processes in nanocrystalline materials directly. First results on nanocrystalline Pd produced by repeated cold-rolling with intermediate folding of metal sheets are presented revealing that the material ruptured during the in-situ tensile tests along grain boundaries. Deformation twinning was observed in grains next to the crack indicating that the deformation processes must have originated from the grain boundaries.

Abstract: The microstructure and mechanical properties of nano-crystalline 2219 Al alloy (Al-6.4Cu-0.29Mn, all in wt %) was studied. Nanocrystalline powders were produced from gas atomized 2219 Al alloy powders by high energy ball milling at room temperature. Powders were collected at different milling times and X-ray diffraction (XRD) analysis was used to evaluate grain size. High Vickers hardness (250HV), high compressive strength (920 MPa) and low ductility (2%) were observed in unimodal bulk nanostructured 2219 Al alloys consolidated to 99% density by hot pressing (HP). In addition, these nanocrystalline powders were blended with 15, 30 and 50% of (gas atomized) coarse-grained powders to obtain balanced mechanical properties of enhanced yield and ultimate strength and reasonable ductility and toughness as compared to either conventional or nanocrystalline 2219 alloys.

Abstract: The effect of capping with Cu, Au and Pt of an array of Co nanoparticles is revised. The magnetic surface anisotropy KS was found to be the dominant contribution to the effective anisotropy Keff of the particles. Recent X-ray Magnetic Circular Dichroism (XMCD) measurements show that there is hybridization between the 3d Co electrons and the d and 4p electrons of the capping metal. By comparison to the mechanisms which give rise to the surface anisotropy in thin films, it is argued that this hybridization governs the modification of KS, and hence, of Keff.

Abstract: Nanocrystalline Ni1-xZnxFe2O4 ferrites with 0 ≤ x ≤ 1 were prepared by sprayingcoprecipitation method. The microstructure was investigated by TG-DSC, XRD, SEM, TEM and BET. Magnetic properties were measured with vibrating sample magnetometer at room temperature. The results showed that uniform and fine nanocrystalline Ni1-xZnxFe2O4 ferrites are obtained. The grain size of sample calcined at 600°C for 1.5h is about 30nm. There are a few agglomerates with average sizes below 100nm. The specific saturation magnetization, Ms, of the sample increases with increasing Zn2+ concent x at room temperature, and the maximum Ms is 66.8 A·m2·kg-1 as the Zn2+ content x is around 0.5mol. As calcining temperature increased from 400°C to 1050°C, the Ms of Ni0.5Zn0.5Fe2O4 ferrite increases from 40.2 A·m2·kg-1 to 75.6 A·m2·kg-1. The coercivity maximum is about 5.97 kA·m-1 as its critical grain size is about 62.0nm. The relation between coercivity and grain size for nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite may be explained based on random anisotropy theory.

Abstract: Structural transformations taking place during annealing of rapidly-quenched soft magnetic ribbons from the Finemet type and Hitperm type alloys were investigated. The X-ray quantitative phase analysis and X-ray diffusion scattering methods were used. It was found that the structural changes in the alloys studied, including crystallisation process and short-range ordering in the amorphous phase, are reflected by the changes in their magnetic properties. These properties, and particularly the magnitude of saturation induction Bs , depend on the content of cobalt and silicon in these alloys and on short-range order in the amorphous phase.

Abstract: Wet gel obtained by sol-gel technique was dried in supercritical CO2 to prepare hydrated form of magnesium oxide. Calcination at 723 K under vacuum yielded nanocrystalline MgO aerogel. Structure studies were performed by X-ray diffraction, scanning and transmission electron microcopies. Electron microscopy images reveal rough, unfolded and ramified structure of solid skeleton. Specific surface area SBET was equal to 238 m2/g. X-ray pattern reveals the broadened diffraction lines of periclase, the only crystalline form of magnesium oxide. The gamma crystallite size distribution was determined using FW 5 4 / 5 1 M method proposed by R. Pielaszek. The obtained values of and σ (measure of polydispersity) of particle size parameters are equal to 6.5 nm and 1.8 nm, respectively, whereas the average crystallite size estimated by Williamson-Hall procedure was equal to 6.0 nm. The obtained at Rietveld refinement Rwp, and S fitting parameters equal to 6.62% and 1.77, respectively, seem to be satisfactory due to the nanosize of MgO crystallites and because of the presence of amorphous phase.

Abstract: High-strength nanocrystalline Ni-W alloys containing 16.9 at. % W with average grain size of about 6 nm in diameter has been obtained by electrodeposition. At room temperature, the nominal tensile strength of the alloy was attained to about 1600 MPa, while the plastic strain before fracture was a very low value of 0.05 %. In this case, highly localized shear bands were observed near the fractured surface of the tensile test specimen. When the samples were annealed at 300 °C under a static tensile stress of 327 MPa, the plastic strain was largely increased at the initial period of annealing and then tended to saturate, i.e., 0.54 % for 2 h, respectively. Grain size of the Ni-W alloys was almost saturated to 10 ~ 15 nm after annealing at 300°C for 2 h. It may be expected that the high tensile stress during grain growth might be effective to obtain the large uniform plastic deformation of nanocrystalline Ni-W alloys.

Abstract: The microstructure characteristic and wear behavior of the severe deformation surface constitution of Ti-4Al-2V alloy were studied in this paper. The results show that the wear resistance of the deformed surface is much better than that of the coarse grain surface. And the wear mechanism of the nanocrystalline surface is different from that of the coarse grain surface. The former is particle wear mechanism of harder material and the latter is adherence wear mechanism. The influence factors on the wear resistance of the nanocrystalline surface have been discussed.