Browsing by Author "Zhang, JF"

Now showing 1 - 6 of 6
  • No Thumbnail Available
    Item
    Ab initio phonon dispersion curves used to check experimentally determined elastic constants of the MAX phase Ti3SiC2
    (Trans. Tech Publication Inc., 2011-07-04) Kirstein, O; Zhang, JF; Kisi, EH; Riley, DP
    The ternary carbide Ti3SiC2 is the archetype of MAX phases. To date, MAX phases have proven difficult to synthesize as sufficiently large single crystals from which single crystal elastic constants might be obtained. Therefore, the elastic properties not only of Ti3SiC2 but other MAX phases are extensively studied by ab initio methods. Recently single crystal elastic constants were experimentally determined for the first time using neutron diffraction. The experiment revealed extreme shear stiffness which is not only quite rare in hexagonal materials but also strongly contradicts the predictions of all published MAX phase elastic constants from ab initio calculations. In the present paper we would like to show that such shear stiffness can possibly be supported by ab initio calculations and the calculated phonon dispersion along high symmetry directions.© 2011, Trans Tech Publications
  • No Thumbnail Available
    Item
    Force chains in monodisperse spherical particle assemblies: three-dimensional measurements using neutrons
    (American Physical Society, 2014-10-03) Wensrich, CM; Kisi, EH; Luzin, V; Garbe, U; Kirstein, O; Smith, AL; Zhang, JF
    The full triaxial stress state within individual particles in a monodisperse spherical granular assembly has been measured. This was made possible by neutron imaging and computed tomography combined with neutron diffraction strain measurement techniques and associated stress reconstruction. The assembly in question consists of 549 precision steel ball bearings under an applied axial load of 85 MPa in a cylindrical die. Clear evidence of force chains was observed in terms of both the shape of the probability distribution function for normal stresses and the network formed by highly loaded particles. An extensive analysis of the source and magnitude of uncertainty in these measurements is also presented.©2014 American Physical Society.
  • No Thumbnail Available
    Item
    Measurement and analysis of the stress distribution during die compaction using neutron diffraction
    (Springer, 2012-11-01) Wensrich, CM; Kisi, EH; Zhang, JF; Kirstein, O
    The full axisymmetric stress state of a granular material undergoing compaction in a cylindrical die has been measured using a technique based on neutron powder diffraction. This technique allowed the detailed distribution of stress to be measured in situ, deep within a copper powder inside a solid die. Four components of normal strain were measured over a radial cross section. These components consisted of the axial, radial, hoop and an off-axis strain in the axial-radial direction. This allowed for the reconstruction of the full axisymmetric stress tensor as a distribution over the radial cross section. Many interesting features were observed in this distribution, such as exponential decay of the axial stress (described by Janssen in Zeitschrift des Vereines duetscher Ingenieure 39:1045, 1895), and highly localised regions of high shear stress. The potential of this type of data in the validation of numerical models is discussed. © 2012, Springer.
  • No Thumbnail Available
    Item
    Quantitative neutron diffraction texture measurement applied to alpha-phase alumina and Ti(3)AlC(2)
    (Wiley-Blackwell, 2011-10-01) Zhang, JF; Kisi, EH; Kirstein, O
    Orientation distribution functions, essential for making a quantitative connection between single-crystal and polycrystal properties, have been determined for extruded [alpha]-phase alumina, hot-pressed Ti3AlC2 and cold isostatically pressed Ti3AlC2 using experimental pole figures recorded on the fixed-wavelength neutron diffractometer KOWARI. Some practical improvements to the calculation of the pole-figure density from the raw area-detector data, and for constructing pole figures on an n x ndegrees hemispherical grid, are presented. The textures give some insight into particle flow during manufacture. Directly measured material textures were compared with one-dimensional pole density functions, such as the March and Rietveld functions commonly used for the correction of preferred orientation in Rietveld refinements, as a means of assessing the utility of the latter for the computation of diffraction elastic constants and other polycrystal properties from a given set of single-crystal properties. © 2011, Wiley-Blackwell
  • No Thumbnail Available
    Item
    Shear stiffness in nanolaminar Ti3SiC2 challenges ab initio calculations
    (Institute of Physics, 2010-04-28) Kisi, EH; Zhang, JF; Kirstein, O; Riley, DP; Styles, MJ; Paradowska, AM
    Nanolaminates such as the Mn + 1AXn (MAX) phases are a material class with ab initio derived elasticity tensors published for over 250 compounds. We have for the first time experimentally determined the full elasticity tensor of the archetype MAX phase, Ti3SiC2, using polycrystalline samples and in situ neutron diffraction. The experimental elastic constants show extreme shear stiffness, with c44 more than five times greater than expected for an isotropic material. Such shear stiffness is quite rare in hexagonal materials and strongly contradicts the predictions of all published MAX phase elastic constants derived from ab initio calculations. It is concluded that second order properties such as elastic moduli derived from ab initio calculations require careful experimental verification. The diffraction technique used currently provides the only method of verification for the elasticity tensor for the majority of new materials where single crystals are not available. © 2010, Institute of Physics
  • No Thumbnail Available
    Item
    Single crystal elastic constants of the MAX phase Ti3AlC 2 determined by neutron diffraction
    (Trans Tech Publications, 2010-01-01) Kirstein, O; Zhang, JF; Kisi, EH; Riley, DP; Styles, MJ; Paradowska, AM
    The ternary ceramic Ti3AlC2 has an interesting combination of electrical, thermal and mechanical properties. Single crystal elastic constants under the Reuss approximation for the micromechanical state were obtained by analysing the shifts of neutron diffraction peaks while a polycrystalline sample was subjected to a compressive load varying from 5 to 300 MPa. The values of Young’s modulus and Poisson’s ratio computed from the single crystal compliances are in good agreement with those obtained directly from strain gauges and from the average changes in the a and c unit cell parameters.