Showing papers in "Philosophical Magazine Letters in 1995"
TL;DR: In this article, the deformation of icosahedral Al-Pd-Mn single quasicrystals has been studied by in situ straining experiments in a high-voltage electron microscope at elevated temperatures.
Abstract: The plastic deformation of icosahedral Al-Pd-Mn single quasicrystals has been studied by in situ straining experiments in a high-voltage electron microscope at elevated temperatures. The results provide the first direct evidence for dislocation motion in quasicrystals. The dislocation velocity, for an applied stress of 390 MPa, was determined as 7×10−7ms−1. It was found that the dislocation motion takes place in planes which are perpendicular to threefold and fivefold lattice directions.
105 citations
TL;DR: In this article, a new description of the growth mechanism for the islands in submonolayer film coverage is presented which goes beyond the usual mean-field approximations, and it is shown that the scaling behaviour observed during heterogeneous growth simulations is explained by the existence of capture zones for each island which remain unchanged throughout the evolution.
Abstract: A new description of the growth mechanism for the islands in submonolayer film coverage is presented which goes beyond the usual mean-field approximations. We show that the scaling behaviour observed during heterogeneous growth simulations is explained by the existence of capture zones for each island which remain unchanged throughout the evolution. These capture zones are in fact the Voronoi polygons for the island centres and so represent the departure of the local island environment from the average. The relevance and range of applicability of this description for homogeneous nucleation systems are discussed.
89 citations
TL;DR: In this paper, 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.
Abstract: 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]β transformation dislocations (structural ledges) on (112)β∥[1100)α planes is accommodated by a set of glissile dislocation loops existing on every sixth (1100)α plane 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]α J 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 ...
85 citations
TL;DR: In this paper, six-dimensional dislocation Burgers vectors were determined employing the convergent-beam electron diffraction technique and their moduli were 0·113, 0·183 and 0·296nm.
Abstract: Al[sbnd]Pd[sbnd]Mn single-quasicrystals have been plastically deformed at temperatures between 730 and 800°C up to different strain values and analysed in the transmission electron microscope. Dislocations were created during deformation. The dislocation density ranged between 1·7 × 107 cm−2 in undeformed samples and 7·8 × 108cm−2in material deformed at 732°C. Six-dimensional dislocation Burgers vectors were determined employing the convergent-beam electron diffraction technique. 87% of these Burgers vectors were oriented parallel to twofold directions in three-dimensional physical space. Their moduli were 0·113, 0·183 and 0·296nm. The ratio of the phason to the phonon component of the Burgers vectors was found to increase with increasing strain. A variety of slip systems was observed. In most cases the respective slip plane normals were parallel to fivefold and threefold directions.
81 citations
TL;DR: In this article, it was shown that the second-electron affinity of an in-crystal O2 ion depends on both the chemical composition and the nuclear geometry of the oxide.
Abstract: The second-electron affinity of the oxygen atom is often required in modelling the behaviour of ceramics. It is usually taken to be about 8 eV, independent of the structure of the oxide being modelled. We show that this is not so. Quantum-mechanical calculations show that the true second-electron affinity of an in-crystal O2 - ion depends on both the chemical composition and the nuclear geometry. Moreover, for a binary oxide at its equilibrium geometry, this true second-electron affinity is about 40% greater than 8 eV. This discrepancy between the true second-electron affinity and the value of 8 eV derived from semi-empirical potentials determined by fitting to experimental crystal data arises from the structural dependence of the true value. In a semi-empirical treatment, this dependence becomes absorbed into the quantity regarded as the short-range cation-oxide interaction with the consequence that the resulting second-electron affinity is smaller than the true value. A further consequence is t...
70 citations
TL;DR: In this article, the authors performed deformation experiments on icosahedral Al-Pd-Mn single-quasicrystals to determine the thermodynamic activation parameters of the deformation process.
Abstract: Plastic deformation experiments were performed on icosahedral Al-Pd-Mn single-quasicrystals to determine the thermodynamic activation parameters of the deformation process. The stress exponent and the strain-rate sensitivity of the flow stress were obtained by means of stress relaxation experiments. The activation enthalpy of the deformation process was measured by temperature change experiments. In the range from 730 to 800°C a nearly constant value of about 7 eV was determined.
68 citations
TL;DR: In this article, the authors investigated the temperature dependence of grain boundaries with angles of misorientation of 38·2° and 40·5° in bicrystals of both pure (99·999%) Al and the same Al doped with 10ppmGa in the temperature regime between 400 and 580°C.
Abstract: The temperature dependence of 〈111〉 tilt grain boundaries with angles of misorientation of 38·2° and 40·5° was investigated in bicrystals of both pure (99·999%) Al and the same Al doped with 10ppmGa in the temperature regime between 400 and 580°C. The grain boundary mobility for the investigated grain boundaries over the entire investigated temperature range is enhanced by minor additions of Ga. Both the activation enthalpy and the pre-exponential factor are affected by Ga doping. The orientation dependence of grain boundary mobility is strongly reduced but not completely removed. The drastic rise in grain boundary mobility by the addition of Ga to pure Al is interpreted as a consequence of a change in the boundary structure and the mechanism of boundary migration owing to a pre-wetting phase transition and formation of a liquid (or quasiliquid) Ga-rich layer on the grain boundary.
67 citations
TL;DR: In this article, the Stroh sextic formalism was extended to a ten-dimensional formalism and it was shown that the Green function for an elliptic hole or rigid inclusion in an anisotropic elastic medium can be modified easily.
Abstract: By extending the Stroh sextic formalism to a ten-dimensional formalism, it is shown that the Green function for an elliptic hole or rigid inclusion in an anisotropic elastic medium can be modified easily for an anisotropic medium with piezoelectric, piezomagnetic and magnetoelectric coupling.
66 citations
TL;DR: In this paper, high-resolution electron microscopy has been used to study the structure of copper-rich precipitates in an Fe-1.5wt% Cu alloy irradiated at 295°C with 2.5 MeV electrons to a dose of 3.4 × 10−2 (displacements per atom (dpa)).
Abstract: High-resolution electron microscopy has been used to study the structure of copper-rich precipitates in an Fe-1.5wt% Cu alloy irradiated at 295°C with 2.5 MeV electrons to a dose of 3.1 × 1023 m−2 (1.4 × 10−2 (displacements per atom (dpa)) at a dose rate of 4.1 × 1017 m−2 s−1 (2 × 10−9 dpa s−1). Most of if not all the precipitates smaller than about 8 nm in diameter were found to have a twinned 9R structure similar to that seen in thermally aged model alloys. Some precipitates larger than about 8 nm in diameter appeared to have transformed wholly or partly to 3R or f.c.c. The results confirm that the usual assumption that the main effect of irradiation is to enhance the diffusion of copper, and hence to accelerate the diffusion kinetics, is substantially correct. The smaller size at which the 9R-3R-f.c.c. transformation seems to occur under irradiation may be due to incorporation of vacancies in the precipitates or to nucleation of small dislocation loops at the precipitate-matrix interface, eith...
54 citations
TL;DR: In this paper, the field around a line defect along the axis of an angularly inhomogeneous, elastically anisotropic medium with piezoelectric, piezomagnetic and magneto-lectric coupling has a surprisingly simple form.
Abstract: The field around a line defect along the axis of an angularly inhomogeneous, elastically anisotropic medium with piezoelectric, piezomagnetic and magnetoelectric coupling has a surprisingly simple form.
51 citations
TL;DR: In this paper, a large single quasicrystals of the icosahedral Al-Pd-Mn phase have been grown using the Czochralski technique.
Abstract: Large single quasicrystals of the icosahedral Al-Pd-Mn phase have been grown using the Czochralski technique Different experimental conditions which favour the growth of a large single grain have been tried The best single grain obtained is 3 cm3 in volume and has an overall mosaic spread of 006°, revealing the high quality of the sample This represents an increase in volume of one order of magnitude, together with a decrease in the mosaic spread of the same amount when compared with our previous results using the Bridgman technique The quasicrystallinity of the samples has been checked by various methods
TL;DR: In this paper, the interfacial structure of [1011] twins obtained using atomic-scale computer simulation by Serra, Pond and Bacon in 1991 is shown to be qualitatively consistent with high-resolution transmission electron micrographs of such twins in Ti obtained by Kasukabe, Yamada, Peng and Bursil in 1993.
Abstract: The interfacial structure of [1011] twins obtained using atomic-scale computer simulation by Serra, Pond and Bacon in 1991 is shown to be qualitatively consistent with high-resolution transmission electron micrographs of such twins in Ti obtained by Kasukabe, Yamada, Peng and Bursil in 1993. In addition, images of two twinning dislocations are analysed using a circuit mapping method to determine their Burgers vectors and to demonstrate that they correspond to defects studied previously using computer simulation by Serra, Bacon and Pond in 1988 and by Serra, Pond and Bacon in 1991.
TL;DR: The first report of an oxygen-stabilized quasicrystal is presented in this article, where oxygen is shown to be essential for the formation of the icosahedral and 1/1 approximant phases in Ti-Mn-Si and Ti-Cr-Si alloys.
Abstract: The first report of an oxygen-stabilized quasicrystal is presented. Oxygen is shown to be essential for the formation of the icosahedral and 1/1 approximant phases in Ti-Mn-Si and Ti-Cr-Si alloys. Recent X-ray and neutron diffraction structural studies by us of the 1/1 approximant phase reveal specific sites for the oxygen in the Mackay icosahedral clusters. On the basis of these data, possible explanations for the role of oxygen in quasicrystal formation are considered. These include firstly the creation of a nucleation barrier for competing crystalline phases, secondly the stabilization of the fundamental Mackay icosahedral cluster and thirdly an adjustment of the electron-to-atom ratio to shift the Fermi surface into a minimum in the density of states.
TL;DR: In this paper, the difference between the heat capacities of chemically disordered and Ll2-ordered Cu3Au was measured by calorimetry from 70 to 300 K, by comparing these measured results with a harmonic heat capacity calculated with a Born-von Kaman model.
Abstract: The difference between the heat capacities of chemically disordered and Ll2-ordered Cu3Au was measured by calorimetry from 70 to 300 K. By comparing these measured results with a harmonic heat capacity calculated with a Born-von Kaman model, we estimate the difference in vibrational entropy between disordered and ordered Cu3Au to be (0·14 ± 0·05)k B atom −1 at high temperatures. The smaller vibrational entropy of the ordered alloy originates from features of its optical modes, features which seem related to the different sizes of Cu and Au atoms.
TL;DR: In this paper, a phase transformation from haematite α-Fe2O3 (antiferromagnetic) to maghemite γ-Fe 2 O3 (ferrimagnetic) by the mechanical action of grinding is reported.
Abstract: The observation of a direct phase transformation from haematite α-Fe2O3 (antiferromagnetic) to maghemite γ-Fe2O3 (ferrimagnetic) by the mechanical action of grinding is reported. A simple grinding process is used; it is characterized by the presence of a shearing component exerted on the α-Fe2O3 particles. All crystallographic and magnetic measurements show existence of a phase transformation a - γ. The interpretation is based on a mechanism using two crystallographic concepts: the orientation relationship between the two phases, and movement of the oxygen planes due to a periodic shearing sequence. The results are also consistent with the phenomena related to the inverse transformation γ → α.
TL;DR: In this paper, a focused-ion-beam system has been applied to prepare a thin foil specimen of silicon which contained cracks in the plane of the foil, and it was possible to observe a much larger area at and near a crack tip than has been hitherto possible.
Abstract: A focused-ion-beam system has been applied to prepare a thin foil specimen of silicon which contained cracks in the plane of the foil. It was possible to observe a much larger area at and near a crack tip than has been hitherto possible. The crack formation around a Vickers indent appears to be much more complicated than has been supposed.
TL;DR: In this article, the formation of antiphase-boundary tubes by direct annihilation of screw segments is questioned and transmission electron microscopy evidence is given of mechanisms of annihilation that take place in the course of room-temperature plastic deformation of Ni3Al.
Abstract: Transmission electron microscopy evidence is given of mechanisms. of annihilation that take place in the course of the room-temperature plastic deformation of Ni3Al. Under a load orientation that favours the operation of a single-slip system, the short dipoles that are present in foils sectioned parallel to the slip plane are identified as elongated closed superdislocation loops instead of dipoles terminating at free surfaces. The fact that dipoles are often aligned in rows such that one extremity of a given dipole corresponds with the closest extremity of the next dipole in the screw orientation is explained in terms of the cross-slip properties of Ni3Al-based alloys. The presence of two distinct categories of dipole rows is interpreted in terms of two different approach mechanisms. The formation of antiphase-boundary tubes-by direct annihilation of screw segments is questioned.
TL;DR: In this article, the structural changes in MgAl2O4 and MgO·3A12O3 spinel single crystals following 400 keV Xe2+ irradiation at 100 K to a dose of 1 × 1016 ions cm−2 were studied.
Abstract: We have studied the structural changes in MgAl2O4 and MgO·3A12O3 spinel single crystals following 400 keV Xe2+ irradiation at 100 K to a dose of 1 × 1016 ions 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.
TL;DR: In this paper, a total energy study on the NbCr2 Laves phase using first-principles electronic structure calculations based on the full-potential linear muffin-tin orbital method was performed.
Abstract: A total energy study has been performed on the NbCr2 Laves 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.
TL;DR: In this article, the orientation-phase-space-averaged excess volume associated with grain boundaries in nanocrystalline Pd has been deduced from a rule-of-mixtures analysis of density and thermal expansion coefficient measurements.
Abstract: The orientation-phase-space-averaged excess volume associated with grain boundaries in nanocrystalline Pd has been deduced from a rule-of-mixtures analysis of density and thermal expansion coefficient measurements. The atomic density is found to fluctuate along the grain-boundary core with an amplitude that could be reduced by low-temperature annealing of the sample. The scaling relation between resistivity and crystallite size changes significantly below approximately 7nm, suggesting that, in this limit, the intrinsic nature of a grain boundary becomes coupled to its spatial extent.
TL;DR: In this article, a two-body band theory approach was proposed to treat correlated f-electron systems, and the hybridization and exchange Coulomb interaction effects were treated on an equal footing, which provided a non-magnetic singlet state competing side by side with other, magnetic states.
Abstract: We propose a new two-electron band theory approach to treat correlated f-electron systems. This involves: first, an exact two-body treatment of the local correlations for a ‘solid-state atom’, i.e. non-spherical local environment of a cerium (or other pertinent) nucleus; then, turning on the interactions giving extended two-electron states (bands). This approach treats the hybridization and exchange Coulomb interaction effects within this two-body treatment on an equal footing. Without invoking the concept of Kondo resonance, it provides a non-magnetic singlet state competing, side by side with other, magnetic states. It explains the wide diversity in f magnetism and the heavy fermion phenomenology, including the specific heat and susceptibility, and most significant, provides an absolute prediction of the Wilson ratio, reproducing the experimental scale and trend in change of the Wilson (low-temperature susceptibility to specific heat) ratio on going from non-magnetic to magnetic heavy fermion m...
TL;DR: In this paper, a fine metastable precipitate (designated X) formed as platelets on the [111]α planes of the alloy Al-2.5 wt% Cu-1.5 Wt% Mg-0.5Wt%Ag, aged at 513 K. Microdiffraction patterns observed from individual platelets indicated that they have a hexagonal structure with the unit-cell parameters a = 0.496-nm and c = 1.375 nm, and an orientation relationship of (0001)× ||(111)α and [
Abstract: Electron microdiffraction experiments have been used to study a fine metastable precipitate (designated X) that forms as platelets on the [111]α planes of the alloy Al-2.5 wt% Cu-1.5 wt% Mg-0.5 wt%Ag, aged at 513 K. Microdiffraction patterns observed from individual platelets indicated that they have a hexagonal structure with the unit-cell parameters a = 0.496-nm and c = 1.375 nm, and an orientation relationship of (0001)× ||(111)α and [1010]× ||[110]α. Although earlier work by Bergman, Waugh and Pauling on similar alloys suggested that the platelets had a b.c.c. T-phase structure, the present results indicate that X phase is not in fact an isomorph of the T-phase that has been observed in the ternary Al-Zn-Mg and Al-Cu-Mg alloy systems.
TL;DR: In this paper, the α → γ phase transformation, which occurs during quenching and produces extremely fine α2-γ lamellae, is discussed in relation to the analysis of the α 2-γ interface structure.
Abstract: The α → γ phase transformation, which occurs during quenching and produces extremely fine α2-γ lamellae, is discussed in relation to the analysis of the α2-γ interface structure. In a high-resolution image obtained from the growth edge of a γ plate in the α2 matrix, steps that were always found to be (0002)α2 or (111)γ planes high were observed at the α2-γ interface. This indicates that the α → γ phase transformation is performed by the migration of (a/3) 〈1100〉hcp Shockley partial dislocations on alternate 〈1100〉hcp planes to change the stacking sequence AB… to ABC….Chemical analysis by analytical transmission electron microscopy with energy-dispersive X-rays showed that the α2 and γ plates have the same composition, so that the α2→γ phase transformation which occurred during quenching can be interpreted as being composition invariant when these results are taken into consideration, it is suggested that the transformation is marterisitic.
TL;DR: In this paper, the elasticity theory of dislocations in quasicrystals has been extended to a twelve-dimensional formalism, and the elastic fields induced by a straight dislocation line are given in terms of integral representations.
Abstract: The six-dimensional framework developed by Stroh to treat the elasticity theory of dislocations in crystals has been extended to a twelve-dimensional formalism in quasicrystals. The elastic fields induced by a straight dislocation line in quasicrystals are given in terms of integral representations, which may be suitable for numerical calculation.
TL;DR: In this article, a model of the required configuration of atomic surfaces (acceptance regions) is discussed by using the six-dimensional diamond lattice, which has a lattice parameter aF2 = 2aF.
Abstract: The low-temperature icosahedral phase, which forms below approximately 700°C near the alloy composition Al71Pd21Mn8 has been investigated by means of electron diffraction. Precise analysis of diffraction patterns indicates that the Bravais lattice of the low-temperature phase is P type, different from the formerly proposed F type. The six-dimensional lattice parameter of this new P-type lattice is determined to be aP = 20.881 A by powder X-ray diffraction and is related to the high-temperature F-type iscosahedral phase by aP = τaF , where τ and aF denote the golden mean and the six-dimensional lattice parameter respectively of the high-temperature phase. The low-temperature transformation can be understood by a small breaking of F symmetry caused by superlattice ordering. A model of the required configuration of atomic surfaces (acceptance regions) is discussed by using the six-dimensional ‘diamond’ lattice. This diamond lattice has a lattice parameter aF2 = 2aF and is related to the above P-latt...
TL;DR: In this paper, it was found that a new type of defect structure on the {001}-type plane is formed in germanium by deuteron bombardment, and the defect is of interstitial type with a double period of the {1} plane in the {110} projection.
Abstract: It is found that a new type of defect structure on the {001}-type plane is formed in germanium by deuteron bombardment. High-resolution electron microscopy observation revealed that the defect is of interstitial type with a double period of the {001} plane in the {110} projection. The structure is formally constructed by connecting two {100} 2 × 1 surfaces, rotated by 90[ddot] to each other, in which no dangling bond is involved.
TL;DR: In this article, the structures of decagonal phases in binary Al-Ni and Al-Co systems and ternary Al70Ni x TM1−x(TM = Co, Rh or Ir) alloys have been studied by electron diffraction.
Abstract: The structures of decagonal phases in binary Al-Ni and Al-Co systems and ternary Al70Ni x TM1−x(TM = Co, Rh or Ir) alloys have been studied by electron diffraction. In the Al-Co system, the periodicity of the decagonal phase is 1.6 nm in Al80Co20, and 0.8 nm in Al76.5Co23.5 and Al72Co28, suggesting that the stacking rule between Al and Co atoms is different along the c axis for the three alloys. In the ternary systems, diffuse streaks and periodicity systematically vary with the atom substitution of Co for Ni and Ir for Ni, indicating that Ni, Co and Ir atoms play different roles in the decagonal phase and exhibit various sorts of chemical order with Al atoms.
TL;DR: An orthorhombic phase with 1.23 nm, b = 1.24 nm and c = 3.07 nm has been found in the Al12Fe2Cr alloy as mentioned in this paper.
Abstract: An orthorhombic phase with a = 1.23 nm, b = 1.24 nm and c = 3.07nm has been found in the Al12Fe2Cr alloy. The distribution of strong diffraction spots in the [010], [100] and [001] electron diffraction patterns (EDPs) corresponds to the tenfold, the 2D twofold, and the 2P twofold EDPs respectively of a decagonal quasicrystal. Therefore this orthorhombic phase is called a decagonal approximant. However, unlike other orthorhombic approximants, its parameter c = 3.07 nm does not agree with any of the values obtained by substituting a rational ratio of two consecutive Fibonacci numbers, such as 1/1, 2/1, 3/2, 5/3, etc., for the irrational τ in one of the quasiperiodic directions in a decagonal quasicrystal. Its [010] high-resolution electron microscopy image consists of a network of 72° and 36° rhombi whose vertices are surrounded by a decagon of image points. The ratio of the thick to thin rhombi is 4 to 1 rather than a Fibonacci ratio. This is a new type of decagonal approximant. On ordering, a C-c...
Abstract: The results of atomistic simulations based on the Born model of solids show that the crystalline-to-amorphous phase transition in α-quartz is the result of a dynamical instability. This work illustrates the potential of atomistic simulation to identify the atomic mechanisms for phase transformations. On applying high pressure to α-quartz, an acoustic vibrational mode became unstable, indicating supercell formation of size 3 × 3 × 1. Further increase in pressure resulted in the collapse of the crystalline structure to an amorphous solid displaying variations in bond length, bond angle and coordination number.
TL;DR: In this paper, anisotropy in the wetting behavior of a silicate liquid in contact with the faceted sapphire surface has been demonstrated using a simple specimen geometry.
Abstract: Anisotropy in the wetting behaviour of a silicate liquid in contact with the faceted sapphire {1010} surface has been demonstrated using a simple specimen geometry. The shape of the glass pockets is influenced by the asymmetric faceting of the α-Al2O3 {1010} surface. The glass remains preferentially in contact with the {1010} facets in α-Al2O3. Using this approach, which allows control over temperature and composition of the glass, a sequential study of the mechanisms of solution-precipitation and facet growth is possible.