The "devil's staircase"-type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q(c), defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q(c) is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.
The phase study of a Cd-Yb 1/1 approximant crystal over a wide pressure and temperature range is crucial for the comparison study between periodic and quasiperiodic crystals. The Cd(4) tetrahedra, the most inner part of the atomic clusters, exhibited various structural ordering in the orientation sensitive to pressure and temperature. Five ordered phases appeared in a P-T span up to 5.2 GPa and down to 10 K. The propagation direction of ordering alternated from [110] to <111> to at about 1.0 GPa and again to [110] at 3.5-4.3 GPa. The primarily ordered phases that appeared by cooling to 210-250 K between 1.0-5.2 GPa further transformed to finely ordered ones at 120-155 K. Besides the original short-range type interaction, a long-range type interaction was likely developed under pressure to lead to the primary ordering of Cd(4) tetrahedra. Coexistence of these interactions is responsible for the complicated phase behavior.
Despite enormous interest in calcium silicate hydrate (C-S-H), its detailed atomic structure and intrinsic deformation under an external load are lacking. This study demonstrates the nanostructural deformation process of C-S-H in tricalcium silicate (C 3 S) paste as a function of applied stress by interpreting atomic pair distribution function (PDF) based on in situ X-ray scattering. Three different strains in C 3 S paste under compression were compared using a strain gauge, Bragg peak shift, and the real space PDF. PDF refinement revealed that the C-S-H phase mostly contributed to PDF from 0 to 20 A whereas crystalline phases dominated that beyond 20 A. The short-range atomic strains exhibited two regions for C-S-H: I) plastic deformation (0-10 MPa) and II) linear elastic deformation (>10 MPa), whereas the long-range deformation beyond 20 A was similar to that of Ca(OH) 2 . Below 10 MPa, the short-range strain was caused by the densification of C-S-H induced by the removal of interlayer or gel-pore water. The strain is likely to be recovered when the removed water returns to C-S-H.
K E Y W O R D Scalcium silicate hydrate, deformation, portland cement, X-ray methods
Nanometer size oxide particles in 9Cr-ODS steel are dispersed finely and densely in a matrix by the hot-solidification process. The size and density distribution of dispersed oxide particles is recognized as one of the main issues for ensuring good microstructural stability and high temperature strength in a high temperature (<700 C) and neutron irradiation (250 dpa) environment. However, the behavior of oxide particles in the hot-solidification process has not been determined yet. This study evaluated the correlation between nano-size oxide particles and the heat treatment temperature and time in order to characterize the mechanism of formation and the behavior during growth and coalescence of these particles in 9Cr-ODS steel raw powder. XRD and SAXS measurements were made using high-energy synchrotron radiation X-rays in SPring-8. This is the first report of the oxide complex particles (Y 2 Ti 2 O 7 and Y 2 TiO 5 ) being formed from 800 to 960 C, and they were observed to grow and coalesce on increasing both heat-treatment temperature and time.
Titanium aluminides find application in modern light-weight, high-temperature turbines, such as aircraft engines, but suffer from poor plasticity during manufacturing and processing. Huge forging presses enable materials processing in the 10-GPa range, and hence, it is necessary to investigate the phase diagrams of candidate materials under these extreme conditions. Here, we report on an in situ synchrotron X-ray diffraction study in a large-volume press of a modern (α 2 + γ) two-phase material, Ti-45Al-7.5Nb-0.25C, under pressures up to 9.6 GPa and temperatures up to 1686 K. At room temperature, the volume response to pressure is accommodated by the transformation γ → α 2 , rather than volumetric strain, expressed by the apparently high bulk moduli of both constituent phases. Crystallographic aspects, specifically lattice strain and atomic order, are discussed in detail. It is interesting to note that this transformation takes place despite an increase in atomic volume, which is due to the high ordering energy of γ. Upon heating under high pressure, both the eutectoid and γ-solvus transition temperatures are elevated, and a third, cubic β-phase is stabilized above 1350 K. Earlier research has shown that this β-phase is very ductile during plastic deformation, essential in near-conventional forging processes. Here, we were able to identify an ideal processing window for near-conventional forging, while the presence of the detrimental β-phase is not present under operating conditions. Novel processing routes can be defined from these findings.
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