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Browsing by Author "He, Baoping"

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    A simplified procedure for obtaining relative x‐ray intensities when a texture and atomic displacements are present
    He, Baoping; Rao, Satish I.; Houska, Charles R. (American Institute of Physics, 1994-05-01)
    The orientation problem in polycrystalline cubic materials has been simplified, using fundamental relationships, so that the determination of a quantitative interrelationship between the various Bragg peak intensities is no longer a formidable task. This is demonstrated with a cubic Cu-Be-Co alloy having a fiber texture, and a conventional focusing diffractometer. Because data are required which extend over a larger range in d spacings, extinction, thermal, and static atomic displacements must be included into the analysis of intensities. The displacement terms and the extinction parameters may be of primary interest or used as a correction. Seventeen diffraction peaks are used in the example. These must be internally consistent with a crystallite orientation function, the cubic symmetry of the sample, extinction effects influencing the two strongest peaks, and attenuation due to atomic displacements. Tabulated coefficients are presented which greatly reduce the task of calculating the orientation function. A correction is given for instrumental smearing which should be considered for stronger textures than the intermediate case examined or for intermediate textures and nonfocusing instrumental conditions.
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    X-ray diffraction from point-like imperfection
    He, Baoping (Virginia Tech, 1992-01-15)
    Displacement fields from point-like defects are investigated by x-ray diffraction. The atomic volume changes in the interstitial compounds using crystallographic information has been found to correlate with the size of filled octahedral sites. Systematic correlations enable estimates to be made of the components of the dipole tensor for interstitials in octahedral sites for binary systems containing N, C, and 0 in V, Nb, Ta, Cr, Mo, W, and Fe lattices. X-ray diffraction analysis of the concentration and residual stress gradients in N implanted Mo crystals and Nb films show that the dominant source of internal strain arises from N located in octahedral sites. For Nb implanted at LNT, these distortion centers are aligned equally along three mutually perpendicular directions to maintain cubic symmetry. Large biaxial residual strains are developed after a 5at% implantation of N into Nb and Mo. Radiation damage is present as small vacancy and interstitial loops. A method was developed to obtain the orientation function for samples containing a fiber texture. A slit correction is included and the final results from this simplified approach are compared with the pole figure measured by direct x-scanning. Knowing the orientation function allows one to correct the integrated intensities to that for an ideal powder, thereby allowing thermal and static displacements to be obtained from textured samples. Anisotropic displacements about coherent Be rich GP zones were investigated in a Cull-at%Be alloy. The results indicate that the attenuation factor 2M which determines the relative integrated intensities of Bragg, quasiline, and static diffuse scattering can be expressed in a simplified form. Experimental data of 2M for aged Cu-Be samples show an anisotropy. The anisotropy increases with increasing aging time when the equiaxed-GP zones formed in the early stage collapse into platelike GP zones.
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    X‐ray diffraction analysis of concentration and residual stress gradients in nitrogen‐implanted niobium and molybdenum
    Rao, Satish I.; He, Baoping; Houska, Charles R.; Grabowski, Kenneth (American Institute of Physics, 1991-06-15)
    Large biaxial residual strains are developed after a 5-at.% implantation of N into Nb and Mo. The results indicate that the dominant source of internal strain arises from N located in interstitial sites. For Nb implanted at liquid-nitrogen temperature, the N atoms are located in octahedral sites. However, the data allow for some clustering as di- or tri-interstitials at the highest concentration (approximately 5 at. % N). Radiation damage is present as small vacancy and interstitial loops. Since vacancies and self-interstitials are present in nearly equal concentrations, the overall bulk dilatation cancels. However, because of their small size, a lesser core expansion has been included as a correction to the overall residual strain. Although one can obtain an estimate of the N distribution from TRIM, a more accurate description must include the distribution of knock-on energy. The latter has an important influence on the redistribution of N relative to that predicted by TRIM. Both host lattices (Nb and Mo) behave like "rigid containers" in directions parallel to the free surface and give a magnified elastic response normal to the free surface.