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Weisheng Liu - One of the best experts on this subject based on the ideXlab platform.

Xiuping Gao - One of the best experts on this subject based on the ideXlab platform.

Xiaoliang Tang - One of the best experts on this subject based on the ideXlab platform.

Randi Holmestad - One of the best experts on this subject based on the ideXlab platform.

  • 3d modelling of β in al mg si towards an atomistic level ab initio based examination of a full precipitate enclosed in a Host Lattice
    Computational Materials Science, 2014
    Co-Authors: Flemming J H Ehlers, Stephane Dumoulin, Randi Holmestad
    Abstract:

    Abstract We extend a first principles based hierarchical multi-scale model scheme for describing a fully coherent precipitate in a Host Lattice to 3D simulations. As our test system, the needle-shaped main hardening Al–Mg–Si alloy precipitate β′′ is chosen. We show that computational costs do not impose practical limits on the modelling: the scheme can probe the full interface energy for physically sized and well isolated precipitates. Examining a series of energetically competitive bulk β′′ configurations, we highlight a series of results: (i) the scatter in the structural parameters for different β′′ configurations clearly exceeds experimental uncertainties also when interaction with the Host Lattice is taken into account. (ii) Structural and compositional β′′/Al interfaces generally coincide. This implies that precipitate stoichiometry is retained only for the two β′′ configurations with the lowest formation energy (compositions Mg5Al2Si4, Mg4Al3Si4). (iii) β′′–Mg4Al3Si4 emerges as a minimum energy configuration for large precipitates. Finally, (iv) more complete modelling, with precipitates surrounded by Al in all three dimensions, is expected to highlight a non-negligible influence of the precipitate misfit along the main growth (needle) direction.

  • phase stabilization principle and precipitate Host Lattice influences for al mg si cu alloy precipitates
    Journal of Materials Science, 2014
    Co-Authors: Flemming J H Ehlers, Sigurd Wenner, Sigmund Jarle Andersen, Calin Daniel Marioara, W Lefebvre, C B Boothroyd, Randi Holmestad
    Abstract:

    In this work, we seek to elucidate a common stabilization principle for the metastable and equilibrium phases of the Al–Mg–Si–Cu alloy system, through combined experimental and theoretical studies. We examine the structurally known well-ordered Al–Mg–Si–Cu alloy metastable precipitates along with experimentally observed disordered phases, using high angle annular dark field scanning transmission electron microscopy. A small set of local geometries is found to fully explain all structures. Density functional theory based calculations have been carried out on a larger set of structures, all fully constructed by the same local geometries. The results reveal that experimentally reported and hypothetical Cu-free phases from the set are practically indistinguishable with regard to formation enthalpy and composition. This strongly supports a connection of the geometries with a bulk phase stabilization principle. We relate our findings to the Si network substructure commonly observed in all Mg–Al–Si(–Cu) metastable precipitates, showing how this structure can be regarded as a direct consequence of the local geometries. Further, our proposed phase stabilization principle clearly rests on the importance of metal-Si interactions. Close links to the Al–Mg–Si precipitation sequence are proposed.

  • applying precipitate Host Lattice coherency for compositional determination of precipitates in al mg si cu alloys
    Philosophical Magazine, 2012
    Co-Authors: Malin Torsaeter, Flemming J H Ehlers, Sigmund Jarle Andersen, Calin Daniel Marioara, Randi Holmestad
    Abstract:

    The present paper reports experimental and theoretical studies on the structure of the metastable C precipitate forming during precipitation hardening in Al–Mg–Si–Cu (6xxx) alloys. We describe the procedure of deriving an initial unit cell model based on experimental data and how this is further refined by quantitative use of nanobeam electron diffraction patterns. A reliable 3D refinement was prevented by the small precipitate thickness and its disorder/intergrowth with other phases, necessitating the development of a more theoretically based methodology for precipitate composition determination. We find that for experimental results to be acceptably reproduced in density functional theory-based calculations on bulk candidate structures, these would have to not only minimise precipitate formation enthalpy, but also reproduce the experimentally reported negligible Lattice mismatch with the Al matrix along the precipitate main growth direction. We argue, through comparison of the isostructural Q′ and Q pre...

Wolfgang Schnick - One of the best experts on this subject based on the ideXlab platform.

  • ba3ga3n5 a novel Host Lattice for eu2 doped luminescent materials with unexpected nitridogallate substructure
    ChemInform, 2012
    Co-Authors: Frauke Hintze, Franziska Hummel, Peter J Schmidt, Detlef U Wiechert, Wolfgang Schnick
    Abstract:

    Single crystals of Ba3Ga3N5 are prepared from a mixture of Ba, Sr, Ga, Mg, and NaN3 in a sodium flux (Ta ampule, 760 °C, 48 h).

  • ba3ga3n5 a novel Host Lattice for eu2 doped luminescent materials with unexpected nitridogallate substructure
    Chemistry of Materials, 2012
    Co-Authors: Frauke Hintze, Franziska Hummel, Peter J Schmidt, Detlef U Wiechert, Wolfgang Schnick
    Abstract:

    The alkaline earth nitridogallate Ba3Ga3N5 was synthesized from the elements in a sodium flux at 760 °C utilizing weld shut tantalum ampules. The crystal structure was solved and refined on the basis of single-crystal X-ray diffraction data. Ba3Ga3N5 (space group C2/c (No. 15), a = 16.801(3), b = 8.3301(2), c = 11.623(2) A, β = 109.92(3)°, Z = 8) contains a hitherto unknown structural motif in nitridogallates, namely, infinite strands made up of GaN4 tetrahedra, each sharing two edges and at least one corner with neighboring GaN4 units. There are three Ba2+ sites with coordination numbers six or eight, respectively, and one Ba2+ position exhibiting a low coordination number 4 corresponding to a distorted tetrahedron. Eu2+-doped samples show red luminescence when excited by UV irradiation at room temperature. Luminescence investigations revealed a maximum emission intensity at 638 nm (FWHM =2123 cm–1). Ba3Ga3N5 is the first nitridogallate for which parity allowed broadband emission due to Eu2+-doping has b...

  • structure elucidation of basi2o2n2 a Host Lattice for rare earth doped luminescent materials in phosphor converted pc leds
    Solid State Sciences, 2009
    Co-Authors: Juliane A Kechele, Oliver Oeckler, Florian Stadler, Wolfgang Schnick
    Abstract:

    Abstract BaSi2O2N2 is a promising Host Lattice for rare-earth doped luminescent materials in phosphor-converted (pc)-LEDs. Applying a combined approach, its orthorhombic average structure (space group Cmcm (no. 63), a = 14.3902(3) A, b = 5.3433(1) A, c = 4.83256(7) A and V = 371.58(2) A3, Z = 4) has been elucidated by electron diffraction and structure solution from X-ray and neutron powder diffraction data with subsequent Rietveld refinement (wRp = 0.0491 for X-ray data). The structure contains layers of highly condensed SiON3 tetrahedra with O terminally bound to Si. The Ba2+ ions are situated between the layers and are surrounded by a cuboid of O atoms capped by two N atoms. In the structure, there is only one Ba site and one Si site, respectively, which is in accordance with a single sharp 29Si NMR signal observed at −52.8 ppm typical for SiON3 tetrahedra in MSi2O2N2 type oxonitridosilicates. Lattice energy calculations support the results of the structure determination.