共查询到20条相似文献,搜索用时 10 毫秒
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J. W. Elmer JOE Wong M. Fröba P. A. Waide E. M. Larson 《Metallurgical and Materials Transactions A》1996,27(3):775-783
Spatially resolved X-ray diffraction (SRXRD) consists of producing a submillimeter size X-ray beam from an intense synchrotron
radiation source to perform real-time diffraction measurements on solid materials. This technique was used in this study to
investigate the crystal phases surrounding a liquid weld pool in commercial purity titanium and to determine the location
of the phase boundary separating the high-temperature body-centered-cubic (bcc) β phase from the low-temperature hexagonalclose-packed
(hcp) α phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0.25 × 0.50
mm X-ray probe that could be positioned with 10-μm precision on the surface of a quasistationary gas tungsten arc weld (GTAW).
The SRXRD patterns were collected using a position-sensitive photodiode array in a φ-2φ geometry. For this probe size, integration times of 10 s/scan at each location on the specimen were found adequate to produce
high signal-to-noise (S/N) ratios and quality diffraction patterns for phase identification, thus allowing real-time diffraction
measurements to be made during welding. The SRXRD results showed characteristic hcp, bcc, and liquid diffraction patterns
at various points along the sample, starting from the base metal through the heat-affected zone (HAZ) and into the weld pool,
respectively. Analyses of the SRXRD data show the coexistence of bcc and hcp phases in the partially transformed (outer) region
of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examinations of
the HAZ, combined with a conduction-based thermal model of the weld, were correlated with the SRXRD results. Finally, analysis
of the diffraction intensities of the hcp and bcc phases was performed on the SRXRD data to provide additional information
about the microstructural conditions that may exist in the HAZ at temperature during welding. This work represents the first
directin situ mapping of phase boundaries in fusion welds. 相似文献
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T. Egami W. Dmowski Yi He R. B. Schwarz 《Metallurgical and Materials Transactions A》1998,29(7):1805-1809
The atomic structure of Pd-Ni-P bulk amorphous alloys was studied by the anomalous (resonance) X-ray scattering technique
using synchrotron radiation tuned near the Pd K-edge. Bulk samples of Pd40Ni40P20, Pd30Ni50P20, and Pd50Ni34P16 amorphous alloys were prepared by the flux method in the form of rods. The results show that the structure of these alloys
can be described basically by the dense random packed structure with small chemical short-range order. It is suggested that
the exceptional stability of these glasses originates mainly from the instability of the competing crystalline phases rather
than the atomic ordering in these glasses.
This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part
of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy
Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees,
and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory. 相似文献
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G. Bruno G. Schumacher H. Cavalcanti Pinto C. Schulze 《Metallurgical and Materials Transactions A》2003,34(2):193-197
High-energy, high-resolution synchrotron radiation diffraction was successfully used to measure the lattice misfit in the
single-crystal nickel-base superalloy SC16. A three-peak model, one belonging to the precipitate phase γ′ and two to the matrix γ, was used to fit the diffraction peaks. Room-temperature (RT) contour plots and a temperature scan up to 1170 K revealed
that the misfit evolves together with the measured thermal expansion difference between the γ and γ′ phases. This suggests that the origin of the misfit lies in the different thermal expansion coefficient of the two phases.
The misfit was found to be positive at RT and to evolve toward negative values as the temperature increases. 相似文献
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M Sunde LC Serpell M Bartlam PE Fraser MB Pepys CC Blake 《Canadian Metallurgical Quarterly》1997,273(3):729-739
Tissue deposition of normally soluble proteins as insoluble amyloid fibrils is associated with serious diseases including the systemic amyloidoses, maturity onset diabetes, Alzheimer's disease and transmissible spongiform encephalopathy. Although the precursor proteins in different diseases do not share sequence homology or related native structure, the morphology and properties of all amyloid fibrils are remarkably similar. Using intense synchrotron sources we observed that six different ex vivo amyloid fibrils and two synthetic fibril preparations all gave similar high-resolution X-ray fibre diffraction patterns, consistent with a helical array of beta-sheets parallel to the fibre long axis, with the strands perpendicular to this axis. This confirms that amyloid fibrils comprise a structural superfamily and share a common protofilament substructure, irrespective of the nature of their precursor proteins. 相似文献
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Two alternative, general methods are presented that lead to enhanced signal intensity of rapidly exchanging protons. Both methods work by avoiding saturation of the water resonance, and are convenient to implement since they do not use any selective pulses. One method carefully chooses proton pulse phases and gradient strength and position in such a way that the water is realigned along the +z axis at the beginning of the acquisition time. An alternative method is proposed for cases where the pulse sequence does not allow such phase cycling. The latter uses radiation damping to bring water back to the +z axis 20-30 ms after acquisition. The methods are applied to the triple-resonance experiments HNCA, HNCO and HN(CO)CA. Both methods require pulsed B0 field gradients and can result in higher signal intensity by a factor of two or more. 相似文献