Papers by Keyword: Nanocrystalline Material

Abstract: In hardening stage, a model was used to study the plastic deformation behaviors of nanocrystalline materials. The material was considered as a composite of grain interior phase and grain boundary (GB) phase. The constitutive equations of the two phases were determined in term of their main deformation mechanisms. In softening stage, a shear band deformation mechanism and the corresponding constitutive relation were presented. Calculation results have shown that the predications fit well with experimental data. The investigation using the finite-element method (FEM) provided a direct insight into quantifying shear localization effect in nanocrystalline materials.

Abstract: We postulated a softening model involving grain rotation that results in diffusion-accommodated grain-boundary sliding. This numerical model was used to compute the proportion evolution of grains within shear bands and was also employed to predict the softening of nanocrystalline materials considering non-homogeneous plastic deformation due to shear bands. The effect of softening mechanism for total stress-strain relation and the grain size and mean maximum Schmid factor effect was also considered in our model.

Abstract: Nanocrystalline CoFe₂O₄ ferrite have been synthesized by a spraying-coprecipitation method. X-ray diffraction (XRD) and Transmission electron microscope (TEM) confirmed the formation of single-phase CoFe₂O₄ ferrite nanoparticles in the range 10 nm ~ 100 nm depending on the calcining temperature. The magnetic measurements show that specific saturation magnetization of nanocrystalline CoFe₂O₄ ferrite increases from 36.1 A•m2•kg-1 to 88.6 A•m2•kg-1 as grain size increases from 12 nm to 98 nm. The coercivity shows a peak with grain size, peaking at around 43 nm. The magnetic behaviour of nanocrystalline CoFe₂O₄ ferrite might be attributed to the effects of surface and random anisotropy.

Abstract: β-Ni(OH)₂ flower-like spheres and nanoflakes were synthesized with strong ammonia water as precipitator and nickel nitrate as nickel source by the hydrothermal method. The phase structure and morphologies were analyzed using XRD and SEM. The results showed that under the same conditions of the pH value and the reaction time, the low temperature was propitious to synthesize β-Ni(OH)₂ flower-like spheres. High temperature was in favor of the synthesis of β-Ni(OH)₂ nanoflakes. Namely, β-Ni(OH)₂ flower-like spheres were obtained at 180 °C for 48.0 h with pH=9.0. β-Ni(OH)₂ nanoflakes were prepared at 240 °C for 48.0 h with pH=9.0. The formation mechanism was explored through observing influence of the conditions of the hydrothermal method on the morphologies of the samples.

Abstract: Application of nanocrystalline magnetic materials in electromechanical devices is increasingly being adopted, helping to solve energy-saving problems and global warming. Compared with conventional silicon steel materials, nanocrystalline materials show faster flux reversal, lower magnetic loss and more versatile property modification, which result in the successful application in modern electronic devices. Nanocrystalline magnetic materials will be increasingly popularized and used in power electronics, telecommunication equipment and electronic article surveillance systems due to the demands for smaller and efficient devices in the future.

Abstract: A nanocrystalline surface layer is produced in Co plate by surface mechanical attrition treatment (SMAT). The characterization of microstructure and composition indicates that elements of Fe, Cr diffuse from hardened steel balls into the surface layer during SMAT. The diffusion phenomenon results in the composition deviation in the surface layer, leading to higher value of saturation magnetization (M_s) for nanocrystalline Co surface layer in comparison with its coarse-grained counterpart.

Abstract: Here, we report the synthesis of nanoporous gold (NPG) with a uniform structure by chemical dealloying of an AlAu intermetallic compound under free corrosion conditions. The length scales of ligaments/channels in NPG can be tuned by changing the dealloying solution. We verify that Cl^- can accelerate the surface diffusion of Au atoms and induce the coarsening of ligaments/channels in NPG. In addition, a metastable intermediate Al₂Au₅ phase appears during the dealloying of AlAu.

Abstract: A theoretical model based on self-consistent approximation is proposed to explore the effect of grain size distribution on the local mechanical response of nanocrystalline (nc) materials. The representative volume element (RVE) is composed of grains randomly distributed with a grain size distribution following a log-normal statistical function. The grain interior and grain boundary are taken as an integral object to sustain deformation mechanisms of grain-boundary sliding, grain-boundary diffusion and grain-interior plasticity. Local plastic strains and internal stresses, developing within the RVE, have been recorded and discussed.

Abstract: A constitutive model was presented for nanocrystalline metallic materials that can experience large plastic deformation with shear band. The model was composed of two parts for different deformation stage: hardening stage and softening stage. In the hardening stage, the phase mixture model was used, and in the softening stage, a shear band deformation mechanism was proposed. Based on the model presented, numerical simulations were carried out to prove that the predications kept in good agreement with experimental data.

Abstract: Suggested methods describe the process of self-diffusion along grain boundaries and triple junctions in polycrystals without using geometric models of the grain boundaries structure. The calculation method introduced diffusion characteristics along grain boundaries derived from the results of molecular dynamic simulations of nanocrystalline materials. The diffusion experiments were imposed to establish relationship between introduced diffusion characteristics and the diffusion parameters along grain boundaries and triple junctions of the Fisher’s grain boundary diffusion model. By the example of copper for the first time the characteristics of self-diffusion along grain boundaries of nanocrystalline materials and coarse grained analog defined in the same temperature range was compared for the first time. It was found that values of the self-diffusion characteristics along grain boundaries in high purity nanocrystalline and polycrystalline copper are equal at the same temperatures.