摘要:

A total energy study has been performed on the NbCr2Laves phase using first-principles electronic structure calculations based on the full-potential linear muffin-tin orbital method. For the two Laves phase structures, C15 and C14, cohesive energies and heats of formation were obtained. A method was developed to calculate the stacking fault energy, γ, in C15 Laves phases, using only two quantities: the cohesive energy difference between C15 and C14 Laves phases, and the C15 lattice constant. For C15 NbCr2, the calculated stacking fault energy is 90mJ m−2. The calculated result is in good agreement with an experimental result deduced from an extended dislocation node.

ISSN:0950-0839    

DOI:10.1080/09500839508242445

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We have studied the structural changes in MgAl2O4and MgO·3A12O3spinel single crystals following 400 keV Xe2+irradiation at 100 K to a dose of 1 × 1016ions cm−2. The radiation-damaged layer was observed using cross-sectional transmission electron microscopy. The first-order Bragg reflections decreased in intensity with increasing damage dose and eventually disappeared, indicating a transformation to a metastable crystalline state. Further irradiation resulted in amorphization of the damaged layer. Considering the well known radiation resistance of spinel, these results suggest that the transformation to the metastable state could be an important step in the amorphization process. We have examined various models for the structure of the metastable phase by calculating the diffraction patterns and comparing them to the electron diffraction observations. Our results indicate that a fcc oxygen lattice with cation interstitials provides the best explanation for the observed structural changes.

ISSN:0950-0839    

DOI:10.1080/09500839508242446

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Thin films of cubic boron nitride were grown on Si(100) surfaces using an ion-beam-assisted physical vapour deposition process. We studied the film microstructure with transmission electron microscopy and selected-area electron diffraction. The films are polycrystalline with a grain size as large as 0.4 μm. Sometimes individual grains are found to extend through the film. Electron diffraction and large-angle tilting were used to identify clearly the structure as cubic boron nitride. Multiple twins on {111} planes are abundant in individual grains, as revealed by electron diffraction.

ISSN:0950-0839    

DOI:10.1080/09500839508242447

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We report the formation of a liquid-like amorphous metallic phase in the Al‒Ge alloy system at high pressures. We used Merrill-Bassett diamond anvil cells to apply pressures from 0 to 200kbar (from 0 to 20GPa) to nanosize two-phase mixtures of aluminium and germanium. At a pressure of approximately 25 kbar, we observed the formation of a metastable high-pressure hexagonal γ phase. On releasing the pressure, the γ phase transforms to a tetrahedrally bonded continuous random network structure, amorphous AlGe-I, similar to that found under high-pressure indents in crystalline pure germanium. On further increasing the pressure from 25 kbar, the γ phase remains stable up to 80 kbar. At 80 kbar, the γ phase transforms to a dense liquid-like phase, amorphous AlGe-II. This transformation is accompanied by an increase in density of about 15%. Both amorphous phases are metastable at ambient conditions of pressure and temperature and this facilitates the observations of the phases by transmission electron microscopy.

ISSN:0950-0839    

DOI:10.1080/09500839508242448

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The atomic structure of the interphase boundary enclosing an α (hcp) precipitate formed from the β (bcc) matrix in a Ti-Cr alloy was examined. All the interfaces of an α lath exhibit continuity of atomic planes between the matrix and the precipitate and thus are considered to be coherent or semicoherent. Such observations imply that there is an atomic site correspondence during transformation. The shear component of the transformation strains arising from the passage of (a/12) [111]βtransformation dislocations (structural ledges) on (112)β∥[1100)αplanes is accommodated by a set of glissile dislocation loops existing on every sixth (1100)αplane at the side facet and the edge of an α lath. The volumetric component of the transformation strain is accommodated by a set of sessile misfit dislocations (b= (a/2)[110]β= (c/2) [0001]αJ on the risers of the growth ledges at the side facet. The presence of sessile dislocations implies that the migration of α-β interfaces is a non-conservative process and should accompany the diffusion of substitutional atoms.

ISSN:0950-0839    

DOI:10.1080/09500839508242449

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

C70films deposited on highly oriented pyrolytic graphite (HOPG), Ag(110), Ag(111) and Pt(110) substrates have been investigated by scanning tunnelling microscopy. Interesting observations on novel molecular arrangements, as well as orientational disorder, are presented. Solid solutions of C60and C70show interesting packing of these molecules when deposited on HOPG.

ISSN:0950-0839    

DOI:10.1080/09500839508242450

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor