摘要:

A transformation-induced elastic strain effect on the decomposition kinetics of a disordered phase into a mixture of ordered and disordered phases was investigated for a prototype binary alloy in two dimensions (2-D) by using a computer simulation technique. The simulation technique not only described simultaneously different processes, such as atomic ordering, clustering, disordering and coarsening, but also produced automatically ordered structures and alloy morphologies. It was found that irrespective of the degree of the crystal lattice mismatch between precipitates and the matrix, the alloy morphologies, developed during the congruent ordering stage which preceded the decomposition, showed no alignment in any of the crystallographic directions. Alignment developed during subbequent decomposition of the congruently ordered single phase, which resulted in the appearance of an equilibrium disordered phase preferentially along the antiphase domain boundaries. It became increasingly pronounced as the time of alloy aging increased. The aligned morphology finally formed the modulated structure found in many alloys. This mechanism of modulated structure formation is new since almost all previous experimentally observed modulated structures were interpreted as a result of a homogeneous spinodal decomposition. The predicted kinetics was found to be in excellent agreement with recent experimental results in a Fe-Si system and should be expected in most two-phase alloys with an ordered intermetallic precipitate whose stress-free lattice constant is significantly different from that of the disordered matrix.

ISSN:0950-0839    

DOI:10.1080/09500839108214618

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Low energy (50 eV) implantation of boron and silicon into crystalline Si through the {110} and {100} faces is studied by a molecular dynamics simulation and the results compared with a binary-collision crystalline computer model and SIMS data. It is found that long-range channelling of the B particles takes place and that their ranges far exceed those predicted by transport theory in random media or Monte-Carlo computer models. It is found that channelling of B occurs only in the ⟨110⟩ direction. The Si atoms which are displaced by more than the nearest-neighbour spacing always originate from near to the surface as a result of direct knock-ons from ions which do not channel. These displacements show a distinct angular dependence because of the crystalline nature of the solid. For the Si implantation, the attractive forces between the incoming ion and the crystal atoms play an important role in limiting the ion range.

ISSN:0950-0839    

DOI:10.1080/09500839108214619

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The technique of reflection electron microscopy (REM) has been combined with wedge transmission electron microscopy (WTEM) in a single instrument using a single wedge-shaped specimen. This combination needs only a commercial transmission electron microscope (CTEM) with a double tilt (or less preferably a single tilt) specimen stage which allows a minimum 25° rotation about one principal axis. Additional specimen preparation for WTEM and instrumental modifications to a CTEM are not required. We have demonstrated that, using a single wedge-shaped GaAs/AlxGa1-x, As specimen, much useful information can be obtained by combining the complementary information provided by REM and WTEM.

ISSN:0950-0839    

DOI:10.1080/09500839108214620

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We demonstrate the application of quantitative chemical lattice imaging techniques to a metallic system. The first results establish that the chemical interface between γ and γ′ phases of a nickel-based superalloy can be quantitatively identified.

ISSN:0950-0839    

DOI:10.1080/09500839108214621

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

An incoherent {112} twin boundary in aluminium has been observed by high resolution electron microscopy. No rigid body translation parallel to the boundary plane has been found. The experimental structure is in agreement with one of the calculated models. The junction of the {112} segment with a coherent {111} boundary is also studied.

ISSN:0950-0839    

DOI:10.1080/09500839108214622

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The atomic mean square displacement of aluminium and noble metals are calculated as a function of temperature. The anharmonic contributions are taken into account by relating the atomic displacements to the lineshifts and linewidths of phonon modes. A force constant model potential is used to parametrize the harmonic and anharmonic interatomic forces. It is shown that the inclusion of the anharmonic effects in the phonon spectral function allows results to be obtained that are in good agreement with the experimental data.

ISSN:0950-0839    

DOI:10.1080/09500839108214623

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The lifetime-spectra of photoexcited carriers in a-Si: H were calculated by Monte-Carlo simulations of the relaxation and recombination process. Good agreement with experimental findings from Frequency-Resolved Spectroscopy (FRS) was achieved by a modified interpretation of FRS spectra.

ISSN:0950-0839    

DOI:10.1080/09500839108214624

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Five elastic constants define the stiffness of coupled phonons and phasons in icosahedral quasicrystals. Requirements of elastic stability impose constraints (inequalities) on combinations of these elastic constants. This paper derives two sets of inequalities, one against the onset of spatially uniform strains, and a weaker set against the onset of spatially varying strains. When the elastic constants approach a violation of this weaker set of inequalities, Bragg peak intensities vanish continuously, while diffuse scattering intensities grow. Depending on which combination of elastic constants violates its stability condition, the diffuse scattering patterns peak in directions corresponding to icosahedron vertices, faces, or edges. Qualitative examination of diffuse scattering patterns therefore may yield an insight into the mechanism driving quasicrystal-to-crystal phase transitions, while quantitative studies may yield numerical values of the elastic constants.

ISSN:0950-0839    

DOI:10.1080/09500839108214625

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Melt-spun Al65Cu15Co20ribbons annealed at 550°C for 62 days have been studied in high-resolution electron microscopy (HREM). No structural transformation of the decagonal phase has been observed. Using the centres of tenfold ring cluster, both HREM pictures for the long-term annealed and unannealed samples can be covered completely with Penrose tiles from set P1 and set P3, with the tile length 12·4 Å, which is a τ−1scaling of our previous tile length (20 Å). Phason fluctuations calculated for both samples are found to be similar. The observed structures can be described by a random tiling model based on Penrose tiles from set P1 and P3, which is different from conventional random binary tiling. The important roles of composition and frozen-in phason disorder in the stability of the decagonal phase are discussed.

ISSN:0950-0839    

DOI:10.1080/09500839108214626

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Local density functional theory is used to evaluate the structure, vibrational frequencies and electronic properties of substitutional phosphorus in diamond. The method uses a 71-atom H-terminated cluster with Gaussian basis functions. We find the defect to remain on-site with the four P—C lengths at 1·69 Å and a triplet resonance at ≈ 380 cm−1. There are also carbon-related modes around the Raman frequency. The latter are due to slightly contracted C—C bonds neighbouring P. The defect also possesses a half-filled level high up in the band gap.

ISSN:0950-0839    

DOI:10.1080/09500839108214627

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor