摘要:

A new metastable chromium carbide Cr3C2-x, with a filled Re3B type structure, is formed during the crystallization of C-rich amorphous Cr–C alloys. Its crystals show tenfold twinning with different growth morphologies: faceted, dendritic, dense with tip splittings. A structural model of the twin boundaries, which agrees well with electron diffraction patterns and high-resolution electron microscopy images, is described.

ISSN:0950-0839    

DOI:10.1080/09500839108214659

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The improved resolution of synchrotron edge-topography is enabling thinner, < 100 μm, silicon carbide crystals to be studied, and is providing a more detailed and wider database on polytype depth profiles. Fine long-period and one-dimensionally-disordered layers, 5 to 25 μm thick, can now be confidently resolved and are found to be very common features, often in association with high-defect-density bands. These features are illustrated in this paper using three examples.

ISSN:0950-0839    

DOI:10.1080/09500839108214660

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

It is proposed that very low levels of solid solubility may be due to dynamic diffusion properties rather than thermodynamic equilibrium. In such systems, retrograde solid solubility will be observed at certain temperatures because of the thermal activation of an efficient solute diffusion mechanism or to a peak in the concentration of an emission-inducing defect. For the particular case of bismuth in copper we show that a likely cause of the solubility peak is the activation of thermal glide of dislocations.

ISSN:0950-0839    

DOI:10.1080/09500839108214661

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The electron diffraction contrast of stacking faults in a decagonal Al70Co15Ni15alloy has been analysed. The direction of the displacement vectorsRof the stacking faults have been determined to be parallel to theandzone axes, respectively, for the first time in quasicrystalline structures. The corresponding fault planes are parallel to (1 0 0 0 0 0) and (0 1 0 0 0 0) lattice planes, respectively. The characteristics of the diffraction contrast fringes of the stacking faults in the decagonal Al-Co-Ni alloy are similar to those in conventional crystals.

ISSN:0950-0839    

DOI:10.1080/09500839108214662

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The diffuse electron scattering accompanying the tweed contrast in oxygen deficient and Fe, Co and Al doped YBa2Cu3O7-δhas been qualitatively corrected for double scattering effects. The kinematical diffuse scattering which results from this work suggests that shear or twinning displacement causes the tweed contrast, and may provide a useful basis for a better insight into the detail microstructure of these phases which are tetragonal on a macroscopic scale.

ISSN:0950-0839    

DOI:10.1080/09500839108214663

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

It is proposed that the ‘cross-over’ of enthalpy associated with the structural relaxation of a vitreous solid can be conveniently studied by differential scanning calorimetry using thermal scans. Observations on a network structure (0·4 Na2O, 0·1 CaO, 0·05 A12O3, 0·05 SiO2, 0·4 P2O5), a linear-chain polymer (poly methyl methacrylate) and a nonpolar rigid molecule (1,3,5-tri-α-naphthylbenzene) glass illustrate the usefulness of the method for routinely studying this aspect of structural relaxation.

ISSN:0950-0839    

DOI:10.1080/09500839108214664

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Calculations by Dupree, Lewis and Smith (1986) on the ratio of aluminium in tetrahedral to octahedral sites in non-stoichiometric MgAl2O4have been revised taking into account the cation disorder ever present in synthetic crystals. Comparison with the experimental results also obtained by these authors shows that not only octahedrally, but also tetrahedrally coordinated, cation vacancies may be introduced for charge compensation.

ISSN:0950-0839    

DOI:10.1080/09500839108214665

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We present the results of the first X-ray measurement of diffuse scattering of an Al-Cu-Fe single quasicrystal. The measurement was carried out at room temperature using a specimen quenched from 1098 K. This specimen contains neither the linear phasons nor the quenched phasons having relatively large amplitude such as those contained in an icosahedral glass model. Contour maps of the diffuse scattering intensity have almost the same rhombic shape for all the four diffraction spots measured in this work. This result may be interpreted, if the specimen contains only a very few amounts of both thermal and quenched random phasons. It is also found that the diffuse scattering intensity is proportional to thenth power ofq, whereqmeans the difference in wave number vector from the corresponding Bragg reflection point. The exponentnis estimated to be -2 for the T-direction and between -3 and -4 for L-direction. The latter relation seems not to be explained by either phonon and phason strains assumed theoretically until now.

ISSN:0950-0839    

DOI:10.1080/09500839108214666

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor