Browsing by Author "Sitepu, H."

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    In situ structural and texture analyses of monoclinic phase for polycrystalline Ni-rich T49.86Ni50.14 alloy from neutron diffraction data
    Sitepu, H. (Cambridge University Press, 2008-03)
    Phase transformation temperatures of a polycrystalline Ni-rich Ti49.86Ni50.14 shape memory alloy were investigated using a differential scanning calorimeter. In situ structural and texture analyses of the monoclinic Ti49.86Ni50.14 were investigated using neutron powder diffractometer technique. Differential scanning calorimeter results showed that this Ni-rich alloy has a one-step cubic to monoclinic martensitic phase transformation on cooling and a one-step monoclinic to cubic transformation on heating. In situ high-resolution neutron powder diffraction data of the monoclinic phase from low temperatures to room temperature on heating are consistent with the differential scanning calorimeter's heating results. In addition, the refined monoclinic crystal structure parameters for all neutron diffraction data sets agree satisfactorily with single-crystal X-ray diffraction results. The multiple-data-set capabilities of the GSAS Rietveld refinement program, with a generalized spherical-harmonics description was used successfully to extract the texture description directly from a simultaneous refinement using 52 time-of-flight monoclinic neutron diffraction patterns, taken from a polycrystalline sample held in 13 orientations inside the diffractometer. (c) 2008 International Centre for Diffraction Data.
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    Structural refinement of neutron powder diffraction data of two-stage martensitic phase transformations in Ti50.75Ni47.75Fe1.50 shape memory alloy
    Sitepu, H. (Cambridge University Press, 2007-09)
    Transformation behaviors of the technologically important polycrystalline Ti50.75Ni47.75Fe1.50 shape memory alloy were investigated using differential scanning calorimeter (DSC) and powder diffraction techniques. DSC revealed that there are two-stage (i.e., cubic -> trigonal -> monoclinic) martensitic phase transformations on cooling and a one-step transformation (monoclinic -> cubic) on heating. In situ structural refinement of cubic -> trigonal -> monoclinic on cooling was carried out using the D1A high-resolution neutron powder diffractometer at the Institut Laue-Langevin Neutrons for Science in Grenoble, France. Results showed that the phases involved during the phase transition are consistent with the differential scanning calorimeter cooling curve, and the refined crystal structure parameters obtained from Rietveld refinements with the generalized spherical harmonic description agreed reasonably well with X-ray single-crystal data. Subsequently, a combined neutron and synchrotron structural refinement for each phase was conducted because the trial refinements initially using only the synchrotron data of trigonal phase yielded a false minimum with a somewhat high goodness-of-fit chi(2). Results obtained from the combined neutron and synchrotron data of the cubic, trigonal, and monoclinic phases show that the same minimum goodness-of-fit indices were always obtained. (C) 2007 International Centre for Diffraction Data.
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    Texture and structural refinement using neutron diffraction data from molybdite (MoO3) and calcite (CaCO3) powders and a Ni-rich Ni50.7Ti49.30 alloy
    Sitepu, H. (Cambridge University Press, 2009-12)
    Preferred orientation or texture is a common feature of experimental powder patterns. The mathematics of two commonly used models for preferred orientation-the March-Dollase and the generalized spherical-harmonic models-is reviewed. Both models were applied individually to neutron powder data from uniaxially pressed molybdite (MoO3) and calcite (CaCO3) powders in Rietveld analyses, as well as the as-received powders. The structural refinement results are compared to single-crystal structures. The results indicate that reasonable refinement of crystal structures can be obtained using either the March model or generalized spherical-harmonic description. However, the generalized spherical-harmonic description provided better Rietveld fits than the March model for the molybdite and calcite. Therefore, the generalized spherical-harmonic description is recommended for correction of preferred orientation in neutron diffraction analysis for both crystal structure refinement and phase composition analysis. Subsequently, the generalized spherical-harmonic description is extended to crystal structure refinement of annealed and the aged polycrystalline Ni-rich Ni50.7Ti49.30 shape memory alloys. (C) 2009 International Centre for Diffraction Data. [DOI: 10.1154/1.3257906]