Papers by Keyword: Polycrystal

Abstract: We investigate the predictive performance of specific analytical and numerical methods to determine the effective magnetic properties of two-phase steels at the macroscale. We utilize various mixture rules reported in the literature for the former, some of which correspond to rigorous bounds, e.g., Voigt (arithmetic) and Reuss (harmonic) averages. For the latter, we employ asymptotic homogenization together with the finite element method (FEM) and periodic boundary conditions (PBC). The voxel-based discretization of the representative volume element is conducted with digital image processing on the existing micrographs of DP600-grade steel. We show that unlike the considered isotropic mixture rules, which use only the phase volume fraction as the statistical microstructural descriptor, finite element method-based first-order asymptotic homogenization allows prediction of both phase content and directional dependence in the magnetic permeability by permitting an accurate consideration of the underlying phase geometry.

Abstract: A major challenge in forming polycrystalline aluminum alloy sheets is their strong plastic anisotropy, namely the flow stresses and plastic strains depend on the loading direction. This plastic anisotropy is due to the anisotropy of the constituent crystals and the preferred orientations that they assume in the polycrystalline material i.e. crystallographic texture. Recently, in [1] we developed a single-crystal yield criterion that involves the correct number of anisotropy coefficients such as to satisfy the intrinsic symmetries of the constituent crystals and the condition of yielding insensitivity to hydrostatic pressure. This single-crystal criterion is defined for any stress state. It is shown that a polycrystalline model that uses this single-crystal criterion in conjunction with appropriate homogenization procedures leads to an improved prediction of the plastic anisotropy in macroscopic properties under uniaxial tension and shear loadings for polycrystalline aluminum alloy 6016-T4. Moreover, results of FE simulations of cup forming operations demonstrate the predictive capabilities of this polycrystalline model.

Abstract: In this paper, polycrystal Ga2O3 thin films were grown on crystal n+-Si substrates by solution process. The XRD profile revealed that monoclinic β-Ga2O3 and rhombohedral α-Ga2O3 were coexisting in the film. The solution-process Ga2O3 film exhibited an ultrahigh transmittance (>97%) to a wavelength range of 280 nm~800 nm. The optical bandgap of ~5.0 eV and breakdown field of 4.2 MV/cm of the Ga2O3 thin film was obtained. Dielectric parameters such as capacitance, dielectric permittivity and loss tangent were investigated. It was observed that these parameters have a strong dependence on frequency.

Abstract: 3C-SiC layers of different microstructures (monocrystalline (100) and (111) oriented and polycrystalline) were implanted with high energy (800 keV) ¹²⁹Xe⁺⁺ ions. Implantations were performed at room temperature (RT) and at 500 °C using two different fluences of Φ₁ = 1x10¹⁶ and Φ₂ = 1x10¹⁷ at/cm². Surface blistering was only observed for RT and Φ₂ implantations into poly-SiC material while mono-SiC kept rather smooth surface. This was due to more homogeneous Xe bubbles distribution (200 nm deep) in the mono-SiC than in the poly-SiC. Xe retention was found to be almost complete for all samples. Some Xe enhanced diffusion was detected in the poly-SiC material which was attributed to grain boundaries. Some irradiation-induced oxidation effect was evidenced, O element being located at the depth where Xe bubbles are accumulating. This was more pronounced for poly than for mono-SiC. These results demonstrate that SiC microstructure affects many aspects of its behavior upon Xe irradiation.

Abstract: A crystal plasticity analysis of polycrystalline pure magnesium is conducted to investigate deformation twinning behavior at the crystal grain scale. A dominant factor in the onset of deformation twinning is the resolved shear stress on a twinning system. More than one twin system may simultaneously be activated in a crystal grain, resulting from inhomogeneous stress distribution caused by constraints imposed by neighboring grains. In this study, a pure magnesium polycrystal is modeled using a fine finite element mesh and analyzed using the crystal plasticity model involving deformation twinning. The evolution of deformation twinning at the crystalline scale is numerically investigated, and the present approach demonstrates that two or more twinning systems are be activated in a single crystal grain because of the strong inhomogeneity in the grain.

Abstract: The use of finite element simulations has become one of the main tools of the mechanical engineer. The method is applied to the analysis and design of engineering structures, the study of manufacturing processes and even to perform virtual experiments. Traditionally, the constitutive laws chosen for finite element analysis have been as simple as possible, mainly due to the limitation imposed by the available computing power. However, the development of more powerful computers and more efficient methods is opening the possibility of using more elaborated (and, most often, more accurate) material models. In particular, polycrystal models capable of predicting not only the mechanical behaviour of the material, but also of the evolution of properties with increasing strain, are particularly well suited for the simulation of forming processes, for which a precise knowledge of the properties of the resulting product is of paramount importance.The present work studies how the Visco Plastic Self-Consistent model (VPSC) can be used in combination with the implicit finite element package Abaqus/Standard to simulate the behaviour of Ti-6Al-4V sheet, and compares it with the more common (and much simpler) Johnson-Cook model. More specifically, the goal of this study is to determine whether or not, with using similar experimental calibration data, the use of the much more complex polycrystal model, justifies the increased complexity and execution time. Using standard tensile experiments at different strain rates, the parameters of the VPSC and Johnson-Cook models are fitted using a minimization method. Then, both models are used in finite element simulations and the results given by both models are compared.

Abstract: 3C-SiC layers were grown on Si substrates using standard precursors (SiH₄ and C3H₈) and by adding methyl trichloro silane (MTS) to the gas phase, with growth temperatures between 1200 and 1300 °C. Characterization of the 3C-SiC layers shows that 3C-SiC grown with MTS has higher polycrystalline and amorphous content as well as lower residual stress.

Abstract: Textured polycrystalline Ni-Mn-Ga alloys were prepared by directional solidification. Alloys were chosen to have the 5M modulated martensitic structure after proper heat treatment. A two-side mechanical training decreased the twin boundary pinning stress. The stress-strain behaviour for the training process and the magnetically induced stress depends on the training direction. Magnetic field induced strain was demonstrated in samples with a plate-like geometry. The influence of the microstructure on the magneto-mechanical behaviour is discussed.

Abstract: A well known method was used to convert the solid silica spheres to porous silicon spheres with high surface area, with the starting shape retained. The presented work demonstrated that the porous silicon spheres are composed of polycrystal morphology. Due to the quantum confinement effect, the photoluminescence emission centered at 621 nm. The results of N2 adsorption and desorption analysis indicates the mesoporous silicon spheres possess a surface area around 88 m2 g-1 and single point micro-pore volume 0.173 cm3 g-1.

Abstract: The flow of material out from under regions in compression must occur by the operation of many slip systems, which together produce rotational flow. Such flow requires the accumulation of geometrically necessary dislocations, and leads to the indentation size effect: smaller indents produce higher hardness, a component of the hardness being inversely proportional to the square-root of the indenter size. A pattern of flow in polycrystals which satisfies both continuity of normal stress and continuity of matter at boundaries can be achieved by rotational flow, and it leads to a grain-size effect. Under most circumstances, the flow stress has a component which is inversely proportional to the square-root of the grain size, the Hall-Petch law. The flow is accompanied by the build-up of internal stress which can be relieved by intercrystalline cracking, thereby limiting the cohesive strength of polycrystals. The relationship between these ideas and traditional views is briefly explained, and an analysis is given of recent experimental results.