Showing papers in "Philosophical Magazine Letters in 1990"
TL;DR: Finnis-Sinclair potentials were developed for computer simulations in which van der Waals type interactions between well separated atomic clusters are as important as the description of metallic bonding at short range as mentioned in this paper.
Abstract: Finnis–Sinclair potentials are developed for computer simulations in which van der Waals type interactions between well separated atomic clusters are as important as the description of metallic bonding at short range. The potentials always favour f.c.c. and h.c.p. structures over the b.c.c. structure. They display convenient scaling properties for both length and energy, and a number of properties of the perfect crystal may be derived analytically.
1,098 citations
TL;DR: In this article, the elastic constants and shear fault energies of TiAl were analyzed and the anomalous temperature dependence of the flow stress was explained by the cross-slip-pinning and fault-dragging mechanisms.
Abstract: First-principles total-energy calculations of the elastic constants and shear fault energies of TiAl are presented. We find a large elastic-shear anisotropy along the [011] direction, and high antiphase-boundary energies due to the strong cohesion between Ti and A1 layers as well as the formation of directional d bonds in the Ti layer. Shear faults of intrinsic type, extrinsic type and twin boundary type are predicted to be prevalent owing to their low energies. The anomalous temperature dependence of the flow stress is explained by the cross-slip-pinning and fault-dragging mechanisms. The intrinsic brittleness of TiAl is discussed in terms of these results.
184 citations
TL;DR: In this article, the influence of small amounts of copper on the neutron-irradiation-induced embrittlement of reactor pressure vessel steels is investigated using fluorescence extended X-ray absorption spectroscopy.
Abstract: The influence of small amounts of copper on the neutron-irradiation-induced embrittlement of reactor pressure vessel steels is of considerable current interest. Previous work has shown that the embrittlement is associated with the formation of copper-rich precipitates, but uncertainties remain regarding their composition and form. This Letter reports the preliminary results from a structural investigation of such precipitates in solution-treated and thermally aged Fe–-Cu and Fe–-Cu–-Ni model alloys using fluorescence extended X-ray absorption spectroscopy. The most significant result is the first direct observation that small clusters in the peak hardness condition have a b.c.c. structure.
122 citations
TL;DR: In this article, an icosahedral quasicrystalline phase was found to be formed in rapidly solidified Al-Pd-Mn alloys, and it was confirmed to have a superlattice structure.
Abstract: An icosahedral quasicrystalline phase was found to be formed in rapidly solidified Al–Pd–Mn alloys. The quasicrystalline structure was confirmed to have a superlattice structure of the simple icosahedral quasicrystal, similar to the face-centred icosahedral structure for Al–Cu–Fe quasicrystals. The intensity of the superlattice reflections increases successively with increasing Pd content, and hence Pd atoms seem to occupy predominantly the superlattice site rather than the fundamental lattice in the icosahedral structure.
96 citations
TL;DR: In this article, the effects of magnetic annealing on recrystallization and grain-boundary character distribution (GBCD), the type and frequency of grain boundaries, have been studied in iron-9 mol% cobalt alloy polycrystals in the range of magnetic field strength up to 4 MA m−1 (5kOe).
Abstract: The effects of magnetic annealing on recrystallization and grain-boundary character distribution (GBCD), the type and frequency of grain boundaries, have been studied in iron-9 mol% cobalt alloy polycrystals in the range of magnetic field strength up to 04 MA m−1 (5kOe). It was found that magnetic annealing retards recrystallization and affects the GBCD, particularly the frequency of low-angle boundaries in recrystallized polycrystals of the alloy. The frequency of low-angle boundaries increases with increasing strength of the magnetic field, exceeding the value predicted for a random polycrystal.
90 citations
TL;DR: In this paper, a single-crystal X-ray structure analysis of decagonal Al65 Cu20Co15 has been performed using the n-dimensional approach using the Patterson analysis and subsequent least squares refinement, both in the five-dimensional description.
Abstract: A single-crystal X-ray structure analysis of decagonal Al65 Cu20Co15 has been performed using the n-dimensional approach. The structure has been solved by Patterson analysis and subsequent least-squares refinement, both in the five-dimensional description. A final value of wR=0.097 for 259 independent reflections was achieved with I≥2σ(1) and 12 variables. There are three atoms in the asymmetric unit. The calculated density of the refined, structure model is ρc = 4.8 M gm −3. The three-dimensional decagonal structure consists of two planar quasiperiodic layers, related by the tenfold screw axis, stacked periodically one upon the other. Characteristic large elements of the crystalline Al13 Fe4 structure type can be found in new combinations in the quasiperiodic layers. It is remarkable that pentagonal channels appear which are empty or occupied statistically.
66 citations
TL;DR: A decaprismatic growth morphology has been found for the decagonal quasicrystal in the Al65Cu20Co15 alloy slowly cooled from the melt as discussed by the authors, which was proved later by electron and X-ray single crystal diffraction analyses.
Abstract: A decaprismatic growth morphology has been found for the first time for the decagonal quasicrystal in the Al65Cu20Co15 alloy slowly cooled from the melt. Substituting a little si for Al in this alloy, for example, Al62Si3Cu20Co15, results in quasicrystal decaprisms of 0·5 mm in diameter and several millimetres in length. These decaprisms showed possibly a 10/mmm morphology which was proved later by electron and X-ray single crystal diffraction analyses. Moreover, the sharp selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) patterns show clearly a high degree of perfection and the X-ray precession photographs prove the single crystalline nature across a few millimetres of this decagonal quasicrystal.
63 citations
TL;DR: In this paper, two states of Fe atoms with different parameters of the electric and magnetic hyperfine structure have been found to exist in submicrometre-grained iron, one corresponding to a grain boundary phase the parameters of which are related to the physical width of the intercrystalline boundaries.
Abstract: Two states of Fe atoms with different parameters of the electric and magnetic hyperfine structure have been found to exist in submicrometre-grained iron. One of these states coincides with that of Fe atoms in conventional iron. The second state is regarded as a state corresponding to a grain boundary phase the parameters of which are related to the physical width of the intercrystalline boundaries.
58 citations
TL;DR: In this paper, the role of kinks in moving locked screw superdislocations in L12 alloys is studied and a geometrical glissile model for the kink is proposed.
Abstract: The role of kinks in moving locked screw superdislocations in L12 alloys is studied. In Ni3Al single crystals deformed at room temperature, two types of kinks are identified: single kinks that sit on one superpartial and paired ones. It is shown that both types glide on the primary octahedral plane and interconnect screw superdislocations which are themselves dissociated in the cube cross-slip plane. A geometrical glissile model for the kink is proposed.
58 citations
TL;DR: The icosahedral structure of A170 Pd20 Mn10 and A165Cu20Fe15 quasicrystals in rapidly solidified and annealed states has been examined by electron and X-ray diffraction techniques as discussed by the authors.
Abstract: The icosahedral structure of A170 Pd20 Mn10 and A165Cu20Fe15 quasicrystals in rapidly solidified and annealed states has been examined by electron and X-ray diffraction techniques. Broadening and distortion of the diffraction peaks due to atomic disordering were observed in rapidly solidified A165Cu20Fe15, but were not observed in rapidly solidified A170Pd20Mn10. The rapidly solidified A170Pd20Mn10 quasicrystal has a highly ordered atomic arrangement which is nearly the same as that for an Al65Cu20Fe15 quasicrystal annealed at a temperature just below the melting point. On the basis of the thermodynamical data, it is presumed that a strong chemical order(bonding) between Pd and Al or Mn prevents the A170Pd20Mn10 quasicrystal from generating atomic disordering and phason strains during rapid solidification.
53 citations
TL;DR: In this article, a model for the hole formation process is proposed and the holes develop from the electron exit face of the crystal, have a profile which is clearly related to that of the electron probe, and can be generated with incident electron energies as low as 40keV Unlike other inorganic materials, there appears to be no lower current density threshold for hole formation, suggesting that the MgO is desorbed as molecules or clusters.
Abstract: Square cross-sectional holes with widths of about 1 nm can be formed in MgO crystals using circular cross-section high-current-density electron probes which are focused and oriented close to High-resolution scanning transmission electron microscopy has been used to reveal, on a unit cell scale, the three-dimensional geometry of the hole structures as they grow The holes develop from the electron exit face of the crystal, have a profile which is clearly related to that of the electron probe, and can be generated with incident electron energies as low as 40keV Unlike other inorganic materials, there appears to be no lower current density threshold for hole formation and there is no excess Mg or 0 present in the MgO crystal after hole formation, suggesting that the MgO is desorbed as molecules or clusters A model for the hole formation process is proposed
TL;DR: In this paper, the yield stress shows an anomalous temperature dependence with a peak at around 1120 K. The slip planes are determined to be {1121} by two-surface trace analysis and dislocations observed have Burgers vector ⅓,〈1 126〉.
Abstract: Ti3Al single crystals have been compressed along the c axis at temperatures from 300 to 1273 K. The yield stress shows an anomalous temperature dependence with a peak at around 1120 K. Slip planes are determined to be {1121} by two-surface trace analysis and dislocations observed have Burgers vector ⅓,〈1 126〉 and are mostly of edge character. A possible mechanism for the anomaly is briefly discussed.
TL;DR: In this paper, a field-emission-gun scanning electron microscope (FEG SEM) was fitted with a back-scattered electron detector in order to image dislocations by the electron channelling contrast mechanism.
Abstract: A field-emission-gun scanning electron microscope (FEG SEM) has been fitted with a back-scattered electron detector in order to image dislocations by the electron channelling contrast mechanism. By using an efficient detector, optimizing its orientation with respect to the specimen and the incident beam, and applying an image-enhancement technique, it has been possible to image and characterize dislocations near the surface of bulk samples of a number of materials without the need of an electron-energy filter in front of the detector. Examples are presented of preliminary studies of dislocations in Si and Ni, Ga; the image widths are typically 20 nm, and the normal g·b = 0 invisibility criterion for screw dislocations applies. By dispensing with the electron-energy filter used by Morin et al., the design and operation of the system become very simple, and the technique should become available in commercial FEG SEMs. This non-destructive technique is likely to have wide applications to situations ...
TL;DR: In this article, high-resolution electron microscopy combined with computer-imaging techniques is used to delineate the distorted atomic configurations that underlie the tweed structure in Y-Ba-Cu-O oxides.
Abstract: A new approach high-resolution electron microscopy combined with computer-imaging techniques is used to delineate the distorted atomic configurations that underlie the tweed structure in Y-Ba-Cu-O oxides. It appears that the quasi-periodic shear displacements along [110] and directions are directly responsible for both the tweed contrast observed under the two-beam condition and the streaking in diffraction. The modulation is associated with the presence of off-stoichiometric oxygen occurring either when the trivalent elements substitute for Cu in the chain site or when oxygen is removed from YBa2 Cu3 O7.
TL;DR: In this paper, the electron diffraction contrast of dislocations in icosahedral Al65Cu20Fe15 quasicrystals has been analyzed and the displacements in the translational degree of freedom were characterized by a Burgers vector parallel to a twofold quasICYstal axis.
Abstract: The electron diffraction contrast of dislocations in icosahedral Al65Cu20Fe15 quasicrystals has been analysed. The displacements in the translational degree of freedom were characterized by a Burgers vector parallel to a twofold quasicrystal axis. Evidence of displacements in the phason degree of freedom was obtained by high-resolution electron microscopy.
TL;DR: In this article, the as-cast state is not icosahedral but trigonal and exhibits a microstructure such that the overall symmetry is pesudo-icosahedral.
Abstract: Thin fragments of dodecahedral particles, extracted from an as-cast Al65Fe15Cu20 alloy ingot, have been studied by transmission electron microscopy. The as-cast state is not icosahedral but trigonal and exhibits a microstructure such that the overall symmetry is pesudo-icosahedral; annealed within the microscope, this microcrystalline state transforms homogeneously into the icosahedral phase, stable until its melting point. During this second transformation (surface melting or sublimation), faceted holes of a dodecahedra] shape were observed to develop within the quasicrystalline phase.
TL;DR: In this paper, an atomic-resolution image of a symmetrical Σ99 {557} 〈110〉 tilt boundary in aluminium is compared with images simulated from models based on atomistic calculations using pair potentials and the embedded-atom method.
Abstract: An atomic-resolution image of a symmetrical Σ99 {557} 〈110〉 tilt boundary in aluminium is compared with images simulated from models based on atomistic calculations using pair potentials and the embedded-atom method. The two methods for atomistic modelling result in very similar structures, and image simulations based on these structures closely match the experimental results. This study shows that high-resolution electron microscopy can now be used to assess quantitatively the degree of coincidence between experimental and theoretical atomic structures of high-Σ grain boundaries.
TL;DR: In this article, the microstructure of a 60% reduced sample showed the formation of amorphous bands within the finely twinned crystalline matrix, and the fracture surface extended straight and inclined 30° to 50° to the compression direction.
Abstract: Alloy sheets of Ni–49·2at% Ti were cold rolled at room temperature. The microstructure of a 60% reduced sample showed the formation of amorphous bands within the finely twinned crystalline matrix. Sample fracture was observed for reductions of more than 65%. The fracture surface extended straight and inclined 30° to 50° to the compression direction. Similarities in morphology to the shear bands observed in heavily cold-rolled metals suggest that an amorphous phase forms in the shear band, caused by the mechanical instability against shear stress.
TL;DR: In this article, the structural properties of aluminium-based alloys are investigated using high-resolution electron microscopy and isothermal calorimetry measurements. But the results show that the structure of these alloys is truly amorphous, rather than micro-crystalline.
Abstract: The study of crystallization processes of rapidly solidified alloys by means of high-resolution electron microscopy and isothermal calorimetry measurements can help to resolve the long-standing controversies between the amorphous and microcrystalline models in metallic glasses. Close examination of a melt-spun Al90Fe5Gd5 alloy based on these techniques shows that the structural nature of this new family of aluminium-based metallic glasses is truly amorphous, rather than microcrystalline. Based on our calorimetric measurements, the unusual formability of aluminium-rich metallic glasses is also discussed.
TL;DR: In this paper, the authors investigated hole transport in films of solid solutions of N,N′-diphenyl-N,N€-bis(3-methylphenyl)-[1,1′-biphenYL]-4,4′-Diamine (TPD) in bisphenol A polycarbonate and polystyrene.
Abstract: Hole transport has been investigated in films of solid solutions of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) in bisphenol A polycarbonate and polystyrene. Rather than being ‘inert’ as previously assumed, the binder plays a major role in influencing the rate of charge exchange between molecules. It is found that the absolute values of the drift mobilities, their electric-field dependence and the activation energies are strong functions of the binder polymer employed to cast the films. At equivalent low molecular concentrations, the hole mobility in films of molecular dispersions in polystyrene is two orders of magnitude higher than that observed in dispersions in polycarbonate, on which most investigations reported in the literature have been carried out. The increased mobility of dispersions in polystyrene is directly related to the reduction in the activation energies.
TL;DR: In this article, a simulation of the evolution of the two-dimensional soap froth, with initial configurations of 500 cells, provides strong evidence that the asymptotic value of μ2 the second moment of the distribution of cell sides, lies in the range 1·42 ± 0.05 for the standard idealized model.
Abstract: A computer simulation of the evolution of the two-dimensional soap froth, with initial configurations of 500 cells, provides strong evidence that the asymptotic value of μ2 the second moment of the distribution of cell sides, lies in the range 1·42 ±0.05 for the standard idealized model of soap froth.
TL;DR: In this paper, millimetre-sized grains of F-type icosahedral phase are formed in ingots of Al-Cu-Fe alloy, for which the nominal composition is Al-20.41 at.% Cu-13.27 at.%.
Abstract: Millimetre-sized grains of F-type icosahedral phase are formed in ingots of Al—Cu—Fe alloy, for which the nominal composition is Al—20.41 at.% Cu—13.27 at.% Fe. These grains are located in the upper half part of the ingots. The lower half does not include the icosahedral phase but ordinary crystalline phases. In order to form these grains, the following heat treatment was applied The specimens were kept at 1143 K for 12 h and slowly cooled to 1095 K with the cooling rate 2 or 4 K h −1. They were successively annealed at this temperature for several tens of hours and then quenched in water and subsequently in liquid nitrogen. This quenching is essential in order to prevent structural transformation from the icosahedral phase to non-icosahedral approximate structure. Electron diffraction experiments as well as X-ray hue diffraction confirmed that the millimetre-size grains correspond to ‘single quasicrystals’, and that deviations from the perfect icosahedral symmetry are small.
TL;DR: In this paper, a small fraction of relatively insoluble gas effectively inhibits rapid coarsening of a liquid/gas foam, which is of particular practical importance in the brewing industry.
Abstract: A small fraction of relatively insoluble gas effectively inhibits rapid coarsening of a liquid/gas foam. This effect, which is of particular practical importance in the brewing industry, is explained and simulated. Below a certain critical concentration of the insoluble gas, an inhomogeneous structure is formed. The critical concentration is determined by the ratio of cell pressure differences to total pressure.
TL;DR: In this paper, misfit dislocations at the interface of a thin epitaxial layer of In0-18Ga0-82As on GaAs have been studied by glancing incidence double-crystal synchrotron X-ray topography.
Abstract: Misfit dislocations at the interface of a thin epitaxial layer of In0-18Ga0-82As on GaAs have been studied by glancing incidence double-crystal synchrotron X-ray topography. The misfit dislocation density increases dramatically as the layer thickness increases from 17 to 18nm across the sample. Clear evidence for the nucleation of misfit dislocations at threading dislocations from the substrate is presented.
TL;DR: In this paper, the formation of the icosahedral phase (i-phase) in rapidly quenched Ti68-xCr32Six alloys, where 69 ≤ x ≤ 18.
Abstract: We report, for the first time, the formation of the icosahedral phase (i-phase) in rapidly quenched Ti68-xCr32Six alloys, where 69 ≤ x ≤ 18. The i-phase occurs at a composition for which there are no structurally similar crystalline phases. Transmission electron microscopy (TEM) and X-ray diffraction measurements are presented. The TEM diffraction patterns show strong localized diffuse scattering and spot anisotropy. These features are compared with similar ones found in Al83Cr17 and Ti61Mn37Si2 icosahedral phases. No evidence for an enthalpic signature of phase transformation was found upon heating to the melting temperature, indicating that the i-phase is at least strongly metastable. The first estimates of the density are reported.
TL;DR: In this article, a mechanism is proposed to explain the change in the ductility of Ni-rich Ni3Al on addition of small amounts of boron, which is argued that grain boundary segregation of B is not the cause of the increase in ductility.
Abstract: A mechanism is proposed to explain the change in the ductility of Ni-rich Ni3Al on addition of small amounts of boron. It is argued that grain boundary segregation of B is not the cause of the increase in ductility. Instead, Ni–B bonds and antisite Ni defects reduce the directionality of the bonding in ordered Ni3Al and enhance the ductility in the interior of grains so that deformation can be easily transmitted from one grain to another. Experimental evidence for this model is presented. An estimate of the concentration of B to achieve maximum ductility is made and the estimate agrees well with the experimental value.
TL;DR: In this article, it was shown that two-phase titanium aluminides with a TiAl/Ti3Al twophase microstructure exhibit mechanical properties which are superior to those for either single-phase TiAl or Ti3Al.
Abstract: Titanium aluminides with a TiAl/Ti3Al two-phase microstructure exhibit mechanical properties which are superior to those for either single-phase TiAl or Ti3Al. The TiAl lamella in the two-phase microstructures show differently oriented TiAl regions which are frequently observed to coexist within the same TiAl lamella. The atomic arrangements of these regions are similar to antiphase domains/{011} twins of TiAl. However, the mechanism by which these regions are formed cannot be derived from the TiAl phase alone. This study suggests that, for two-phase titanium aluminides, Ti3Al has a strong influence on the development of the TiAl microstructure.
TL;DR: In this article, a defect structure produced by fast-neutron irradiation of YBa2Cu3 O7-x was investigated using transmission electron microscopy, and relations with changes in bulk properties were suggested.
Abstract: A defect structure produced by fast-neutron irradiation of YBa2Cu3 O7–x has been investigated using transmission electron microscopy. Correlations with changes in bulk properties are suggested.
TL;DR: In this paper, the electron states of copper-decorated twinned boundaries in n- and p-type silicon bicrystals are investigated by deep-level transient spectroscopy.
Abstract: The electron states of copper-decorated twinned boundaries in n- and p-type silicon bicrystals are investigated by deep-level transient spectroscopy. A correlation is established with the precipitate microstructures obtained under different conditions of copper precipitation. The analysis of the data supports a model for the trapping effects of the precipitates, regarded as metallic inclusions in rectifying contact with the silicon matrix.
TL;DR: In this article, electron-beam-induced solid phase epitaxy (SPE) has been obtained on cross-sections of implanted Si layers, by in situ irradiation in the electron microscope, with electrons of energies of 200, 250 and 300 keV, at both room and liquid-nitrogen temperature.
Abstract: Electron-beam-induced solid-phase epitaxy (SPE) has been obtained on cross-sections of implanted Si layers, by in situ irradiation in the electron microscope, with electrons of energies of 200, 250 and 300 keV, at both room and liquid-nitrogen temperature. The, absence of a transition from SPE to layer-by-layer amorphization (which is observed during ion-beam irradiation on decreasing the temperature below a certain critical value) and the athemal nature of the electron-induced crystallization process below room temperature, indicate that, although elastic displacement is the basic mechanism of both processes, the models which describe ion-beam-induced epitaxy in the temperature range 200≤T≤400°C cannot be extrapolated to explain the results of electron irradiation below room temperature.