Showing papers in "Philosophical Magazine in 1969"
TL;DR: In this article, a method for determining the position of dislocation lines accurately from dislocation images is reported and applied to the case of measuring the separation of partial dislocations.
Abstract: A method for determining the position of dislocation lines accurately from dislocation images is reported and applied to the case of measuring the separation of partial dislocations. A theoretical analysis of the method is given, and the predictions verified by many-beam computed image profiles. The separation of partial dislocations in Cu + 10at. % Al has been determined using the method. The separation was found to be 120±20 A. Other applications of the method are suggested.
608 citations
TL;DR: In this article, a direct experimental method has been used to determine the magnitudes of the ice-water vapour, icewater, and ice-ice (grain boundary) surface free energies, which were found to be 109, 33 and 65 erg/cm2, respectively, at 0°C.
Abstract: A direct experimental method has been used to determine the magnitudes of the ice-water vapour, ice-water, and ice-ice (grain boundary) surface free energies. The magnitudes were found to be 109, 33 and 65 erg/cm2, respectively, at 0°C. The experimental results indicate that the grain boundary surface free energy varies slightly with the mismatch between the grains but that variations with crystal orientation of the ice-water vapour and the ice-water surface free energies are small.
303 citations
TL;DR: In this article, the authors considered the nucleation of a shear dislocation loop from the interface of a misfitting spherical particle in a deformable matrix and calculated the equilibrium shapes and positions of the nucleated dislocation.
Abstract: The nucleation of a shear dislocation loop from the interface of a misfitting spherical particle in a deformable matrix is considered. The equilibrium shapes and positions of the nucleated dislocation have been calculated. There are two critical levels of misfit. The lower critical level, which is dependent on the particle size, must be reached before nucleation can occur. Above the upper critical level, which is virtually independent of the particle size, nucleation will always occur. Experimental measurements of the critical misfit show that the upper critical level describes the spontaneous generation of dislocations at coherent particles, whereas the lower level describes their generation at incoherent particles. Possible reasons for this are discussed.
225 citations
TL;DR: In this paper, the dislocation sub-structure and surface markings formed during the early stages of fatigue in copper single crystals at room temperature were studied and the merits of various proposed mechanisms for fatigue are discussed.
Abstract: The dislocation sub-structure and surface markings formed during the early stages of fatigue in copper single crystals at room temperature were studied. For the first few cycles the structure is similar to that associated with stage I of tensile deformation. A three-dimensional network of interconnected veins packed tightly with mainly primary edge dislocations then develops. The scale of the structure decreases with increasing strain. The surface observations show that in the first few cycles long straight primary slip lines are formed, but later the new slip markings are fine and short, their length corresponding to the inter-obstacle distance. The merits of various proposed mechanisms for fatigue are discussed.
210 citations
TL;DR: In this article, an idealized model was proposed to estimate the height of the current peak in order of magnitude in agreement with what has been observed, and the model gave an estimate of the maximum resistanceless current.
Abstract: As H c2 is approached the rigidity of the flux-line lattice falls to zero more rapidly than the pinning strength of inhomogeneities. This allows the flux lattice to conform more readily to the configuration that locks it to the inhomogeneous structure, so that the maximum resistanceless current is increased. An idealized model gives an estimate of the height of the current peak in order-of-magnitude agreement with what has been observed.
197 citations
TL;DR: In this paper, a general theory is developed to account for the mobility of a dislocation which is sessile in its low-energy form, and three distinct mechanisms for movement are possible, each controlling the mobility over a different stress range.
Abstract: A general theory is developed to account for the mobility of a dislocation which is sessile in its low-energy form. It is shown that three distinct mechanisms for movement are possible, each controlling the mobility over a different stress range. A specific core model is used to derive an expression for the mobility of screw dislocations in b.c.c. metals, and it is shown that the theory can account satisfactorily for the temperature and orientation dependence of the yield stress and slip geometry, and the stress dependence of the activation energy and volume in b.c.c. metals.
151 citations
147 citations
TL;DR: In this paper, the contribution of electrical resistivity caused by grain boundaries has been measured in polycrystalline specimens of high purity copper and high purity aluminium and the specific grain boundary resistivity appeared to be independent of the impurity content, depended to a small extent on the temperature in the range 4·2 to 77°K.
Abstract: The contribution to the electrical resistivity caused by grain boundaries has been measured in polycrystalline specimens of high purity copper and various grades of high purity aluminium. The specific grain boundary resistivity appeared to be independent of the impurity content, depended to a small extent on the temperature in the range 4·2 to 77°K, and at 4·2°K had the values (3·12 ± 0·18) × 10−12 ohm cm2 for copper and (2·45 ± 0·09) × 10−12 ohm cm2 for aluminium.
143 citations
TL;DR: In this article, a technique for producing planar dislocation sub-boundaries in thin-film bierystal specimens of gold under controlled conditions is described, where single crystal thin films of the desired orientations are first grown epitaxially on appropriate substrates from the vapour phase.
Abstract: A technique for producing any type of planar dislocation sub-boundary in thin-film bierystal specimens of gold under controlled conditions is described. Single crystal thin films of the desired orientations are first grown epitaxially on appropriate substrates from the vapour phase. Two films, while still on their substrates, are then welded together face-to-face at any desired mis-orientation to produce a thin-film bierystal containing a planar dislocation array at the welded interface. The substrates are then dissolved away to allow examination of the sub-boundary at normal incidence by transmission electron microscopy. The technique is applied to the study of pure twist sub-boundaries on (001) and (111) planes in gold. Features of the structures of these boundaries are described. Further possible applications are discussed.
142 citations
TL;DR: In this article, the authors measured the pressure at which dislocations are first generated at SiO2 and Al2O3 particles in a copper matrix at room temperature, and the maximum shear stress at the particle surface was about G/100 for 2000 A diameter particles.
Abstract: When a hydrostatic pressure is applied to a metal containing particles or inclusions of different elastic constant, the local shear stresses which develop can be large enough to cause the interface between the particle and the metal matrix to emit dislocations. We have measured the pressure (from which we calculate the local shear stresses) at which dislocations are first generated at SiO2 and Al2O3 particles in a copper matrix at room temperature. The critical pressure, and thus shear stress, depends on the particle size. The maximum shear stress at the particle surface was about G/100 for 2000 A diameter particles, rising to G/40 for 1000 A diameter particles, where G is the shear modulus of the metal matrix.
127 citations
TL;DR: In this article, the optical properties of seven liquid metals, in the frequency range between 0·77 and 5·0 ev and at temperatures up to 1600°c, were measured by a polarimetric method.
Abstract: Measurements have been made, by a polarimetric method, of the optical properties of seven liquid metals, in the frequency range between 0·77 and 5·0 ev and at temperatures up to 1600°c. For Al the results show no sign of the absorption peak at about 1·5 ev which is observed for the solid and are in rough agreement with the Drude free-electron curves; deviations from these curves may be due to surface contamination, though an attempt to explain them quantitatively in this way is not successful. For Cu, Ag and Au the results at low frequencies suggest that the effective density of conduction electrons, n∗, rises slightly on melting, as is to be expected if the structure of the conduction band is smoothed out; the absorption edges due to excitation of electrons from the d band, however, are scarcely changed at all. For the transition metals Fe, Co and Ni, which lie next to one another in the Periodic Table, the absorption spectra show little in the way of structure; the most notable feature of the r...
TL;DR: In this article, the authors investigated the mechanism causing extrinsic faults to grow into a silicon wafer during thermal oxidation using annealing experiments on thin electron microscope foils.
Abstract: The mechanism causing extrinsic faults to grow into a silicon wafer during thermal oxidation has been investigated by annealing experiments on thin electron microscope foils. The defects grow on annealing in air at 1100°C and shrink on annealing in vacuo at the same temperature; this behaviour is explained in terms of the diffusion of vacancies between defect and surface. The sense of this flow is dependent on the vacancy concentration in equilibrium with the surface which is reduced to approximately 0·8 of the bulk equilibrium value because the vacancies are annihilated by the inward-growing oxide. During oxidation the faults emit vacancies to the surface, causing fault growth, whereas the vacancy flow is reversed on annealing in vacuo. The activation energy for fault shrinkage has been determined to be 2·1 ev, which is consistent with pipe diffusion along the core of the bonding Frank dislocation. The shrinkage rate of these faults at constant temperature is proportional to the equilibrium vaca...
TL;DR: In this article, a study of the stress-strain behavior, secondary slip geometry and slip line morphology of niobium single crystals purified by annealing in ultra-high vacuum was made.
Abstract: A study has been made of the stress-strain behaviour, secondary slip geometry and slip line morphology of niobium single crystals purified by annealing in ultra-high vacuum. Within favourable limits of orientation, temperature and strain rate the stress—strain curves exhibit well-defined three-stage hardening. The principal secondary slip system is not usually that bearing the highest proportion of the applied stress and this anomalous behaviour can account for the orientation dependence of overshoot. For temperatures below 250°K the stress-strain curve is non-linear and the initial hardening rate very high. The slip lines observed on crystals deformed at low temperatures belong to a {110} ⟨111⟩ system which, in extreme orientations, is only the sixth most highly stressed system. The slip appears as crystallographic lamellae, in contrast to the wavy slip observed for all other systems.
TL;DR: In this article, the authors made careful measurements of the stress-strain curves of tungsten wires of 10 μ and 20 μ diameter and of aligned composites of the same wires in copper at strains greater than the yield strain of the copper.
Abstract: Careful measurements have been made of the stress-strain curves of tungsten wires of 10 μ and 20 μ diameter and of aligned composites of the same wires in copper At strains greater than the yield strain of the copper the apparent stresses in the matrix, evaluated from the law of mixtures, are very high The effect appears to be due to the mutual constraint of the two phases caused by their different lateral contractions, since it disappears when the fibres yield and hence the transverse contractions become the same The experimental results are compared with those predicted for a completely plastic matrix and the stresses are found to be higher than calculated This is interpreted to mean that the yield of the copper is gradual, and that during stage II of the stress-strain curve of the composites an appreciable portion of the copper continues to deform elastically
TL;DR: In this paper, the geometry of the deformation produced by the climb of edge dislocations is defined and the conditions under which dislocation climb, by itself or in conjunction with glide, can produce a general change of shape.
Abstract: The geometry of the deformation produced by the climb of edge dislocations is defined and the conditions are determined under which dislocation climb, by itself or in conjunction with glide, can produce a general change of shape. The results are important in understanding the high-temperature deformation (T > 0.5Tm) of single crystals and polycrystals of materials with fewer than five independent slip systems. Some of these are discussed in detail. In particular the high-temperature creep of sapphire single crystals pulled parallel to (0001) is shown to be quantitatively consistent with Nabarro (Phil. Mag., 16, 231, 1967) creep by climbing dislocations.
TL;DR: In this article, aqueous solutions of citric acid kept quiescent at a constant temperature were found to develop concentration gradients with the highest concentrations in the lower regions.
Abstract: Supersaturated aqueous solutions of citric acid kept quiescent at a constant temperature are found to develop concentration gradients with the highest concentrations in the lower regions. This unusual behaviour is considered as supporting evidence for the existence of molecular clusters in solutions.
TL;DR: In this article, an analysis of electron energy loss spectra of some 12 transition metal dichalcogenides has given useful information towards the understanding of the band structures of these layer-type solids.
Abstract: An analysis of electron energy loss spectra of some 12 transition metal dichalcogenides has given useful information towards the understanding of the band structures of these layer-type solids. Using the experience gained from studying graphite in which the valence band consists of two groups, namely π and σ bands, a similar band scheme has also been established for the transition metal dichalcogenides. The plasma resonance energies of π electrons are discussed in terms of the screening by σ electrons and the finite oscillator coupling between the two valence bands. Single electron interband excitations have been observed, which are expected to form the basis for further detailed study into the band structures of these materials. Many atomic-like excitations associated with the core d and core f electrons have also been observed.
TL;DR: In this article, it is shown that the dislocation network at the interface can give incomplete compensation of the mismatch of an island with the substrate causing a periodic variation of the strain due to the introduction of successive misfit dislocations as the island grows.
Abstract: Epitaxial tin islands are grown by evaporation onto tin telluride substrates in ultra-high vacuum. The strain within an island, measured by moire fringe techniques, shows a general inverse variation with size rising to over 2% in islands 100 A wide. It is shown that the dislocation network at the interface can give incomplete compensation of the mismatch of an island with the substrate causing a periodic variation of the strain due to the introduction of successive misfit dislocations as the island grows. There is no evidence for the existence of a barrier against the nucleation of misfit dislocations at the edge of an island. A simplified model to describe the variation of strain during growth is set out and compared with the more general theory developed by van der Merwe.
TL;DR: In this paper, an electron microscope investigation of slightly reduced rutile (TiOx x= 1·986, 1·94) suggests the existence of a new family of ordered phases, probably TinO 2n-1 (n=15,∼36), derived from rutiles by crystallographic shear on planes parallel to {132}.
Abstract: An electron microscope investigation of slightly reduced rutile (TiOx x= 1·986, 1·94) suggests the existence of a new family of ordered phases, probably TinO 2n-1 (n=15,…∼36), derived from rutile by crystallographic shear on planes parallel to {132}. Still smaller degrees of reduction (x= 1·995) introduce disordered shear planes parallel to {132} and {101}. Some comments on the mechanism of reduction are also included.
TL;DR: In this article, the authors investigated the mechanical properties, dislocation configurations and densities of single crystals of copper deformed in the strain-rate range 10−4 to 104 sec−1.
Abstract: The mechanical properties, dislocation configurations and densities have been investigated in single crystals of copper deformed in the strain-rate range 10−4 to 104 sec−1. It was found that the flow stress of copper exhibits two regions of strain-rate sensitivity. Below strain rates of 103 sec−1 the flow stress was relatively insensitive to strain rate but above 103 sec−1 the flow stress is a sensitive linear function of the strain rate. However, the dislocation density and configurations versus strain were found to be almost independent of strain rate throughout the range 10−4 to 104 sec−1. There was a straight line relationship between the flow stress and the square root of the dislocation density at all strain rates. However, at a strain rate of 6·5 X 103 sec−1 there is a positive intercept τ0 on the stress axis at zero dislocation density. The term τ0 can be related to the damping process retarding dislocation motion.
TL;DR: In this article, the effect of thermal diffuse scattering, single electron excitations and plasmon excitation on the quality of electron microscope images of crystals is discussed and the significance of the results is discussed with reference to electron microscopy of thick crystals, direct resolution of atomic positions and out-of-focus images.
Abstract: Observations of the effect of thermal diffuse scattering, single electron excitations and plasmon excitation on the quality of electron microscope images of crystals are presented and discussed. Preservation of image contrast after thermal diffuse scattering seems to be poor in practice but only a small fraction of the electrons accepted by the aperture are affected in this way. To a first approximation, contrast appears to be preserved after single electron excitation or plasmon excitation especially at small scattering angles where no evidence of interband transitions of the fast electron has been found. At larger scattering angles some loss of contrast occurs so that the size of the aperture can be important. The significance of the results is discussed with reference to electron microscopy of thick crystals, direct resolution of atomic positions and out-of-focus images.
TL;DR: The results obtained from experiments on the mechanical properties of single crystals of ice doped with several impurities are presented in this article, and a possible explanation of the results is offered.
Abstract: The results obtained from experiments on the mechanical properties of single crystals of ice doped with several impurities are presented. At −70°C, small amounts of dissolved hydrogen flouride (HF) have a pronounced softening effect, ammonia (NH3) produces a slight hardening, and ammonium fluoride (NH4F) has no apparent effect. These results are discussed in terms of Glen's (1968) theory of dislocation movement in ice crystals and a possible explanation of the results is offered.
TL;DR: In this article, the formation of second phases during the ion implantation of thin metallic films is discussed in general terms, and the effects of radiation-enhanced diffusion and surface sputtering on precipitate growth and distribution, and morphology and orientation relationships of the precipitates concerned, which include CuAl2 and AlSb and Pb in both solid and liquid states.
Abstract: The formation of precipitates of second phases during the ion implantation of thin metallic films is discussed in general terms. Observations from an electron microscope investigation of precipitation behaviour during the implantation of aluminium specimens with 70-80 kev Cu+, Sb+ and Pb+ ions are described in detail. Special reference is made to the effects of radiation-enhanced diffusion and surface sputtering on precipitate growth and distribution, and to the morphology and orientation relationships of the precipitates concerned, which include CuAl2 and AlSb, and Pb in both solid and liquid states.
TL;DR: In this paper, a simple mechanism for removing dislocation loops in face-centred cubic crystals is proposed, which involves the formation of a junction around half the loop followed by the gliding together and coalescence of this with the other half.
Abstract: Dislocation loops in irradiated or quenched metal crystals are swept out by the glide dislocations during deformation, leaving cleared channels. The removal of prismatic loops with the same Burgers vector as the glide dislocations can occur by direct recombination (Saada and Washburn 1963) but the annihilation of loops with Burgers vector out of the glide plane has not been explained. A simple mechanism is proposed for their removal in face-centred cubic crystals. This involves the formation of a junction around half the loop followed by the gliding together and coalescence of this with the other half. Both steps are glissile and energetically favourable, and give complete annihilation of the loop at the expense of a change in shape of the glide dislocation. Faulted loops may also be removed in this way if they first unfault in the presence of a glide dislocation. The mechanism is not diffusion controlled, but requires a high loop mobility and may be affected by impurity pinning.
TL;DR: In this paper, the structure of Al-CuAl2 eutectic alloy has been examined by transmission electron microscopy and the orientations of phases and of preferred lamellar interfaces have been determined and how small deviations from these preferred orientations are accommodated by tilt and/or twist boundaries.
Abstract: The structure of Al–CuAl2 eutectic alloy has been examined by transmission electron microscopy. The orientations of phases and of preferred lamellar interfaces have been determined and it is shown how small deviations from these preferred orientations are accommodated by tilt and/or twist boundaries. The dislocation structures of sub-boundaries and of the semicoherent phase boundaries are illustrated. The hardness of the eutectic material is more sensitive to the rate of cooling in the solid state than it is to the lamellar spacing and the dislocation distribution in quenched and slowly cooled alloy is compared.
TL;DR: In this article, a simple theory of homogeneous nucleation of interstitial clusters is developed which fits the experimental data on copper and is used to predict the time dependence of cluster growth, the width of the denuded zone close to interfaces, the apparent variation in threshold energy for cluster formation with foil thickness and temperature.
Abstract: The electron beam in an electron microscope produces lattice displacements when the accelerating voltage exceeds a critical value which depends on the specimen material. The available data on threshold energies and the rates of point defect production in the common elements are reviewed. It has been observed that under certain conditions defect clusters nucleate when point defects are formed. A simple theory of homogeneous nucleation of interstitial clusters is developed which fits the experimental data on copper. The theory is used to predict the time dependence of cluster growth, the width of the denuded zone close to interfaces, the apparent variation in threshold energy for cluster formation with foil thickness and temperature, the interstitial and vacancy concentrations and the enhanced diffusion which they produce. It is concluded that it is possible to avoid loop formation by a suitable choice of conditions, in particular by increasing the temperature and restricting the foil thickness. It...
TL;DR: In this article, the superlattice and domain structures exhibited by the ordered compound V6C5 are disrupted by electron microscope beam bombardment, and the effect is attributed to the disordering of the carbon sublattices that results from displacement of the atoms by the impinging electrons.
Abstract: The superlattice and domain structures exhibited by the ordered compound V6C5 are disrupted by electron microscope beam bombardment. The effect is attributed to the disordering of the carbon sublattice that results from displacement of the carbon atoms by the impinging electrons. Studies have been made of the disordering rate under electron bombardment at energies from 33 kev to 100 kev, using a Faraday cup to measure superlattice spot intensities. The results are compared with the predictions of a simple theory for the damage process, from which it is concluded that the displacement energy of carbon atoms in V6C5 has the surprisingly low value of 5·4 ev.
TL;DR: In this paper, the structures of dislocations on the primary and secondary slip systems in crystal with the fluorite structure are investigated and it is shown that the edge dislocation on the secondary system can adopt either a charged or a jogged configuration; the most likely form (for calcium fluoride) is intermediate between these extremes, i.e. a series of adjacent like charges terminating in a ‘half’ jog and followed by a series opposite like charges, etc.
Abstract: The structures of dislocations on the primary and secondary slip systems in crystals with the fluorite structure are investigated. It is shown that the edge dislocation on the primary system can adopt either a charged or a jogged configuration; the most likely form (for calcium fluoride) is intermediate between these extremes, i.e. a series of adjacent like charges terminating in a ’half’ jog and followed by a series of opposite like charges, etc. The screw dislocation and the edge dislocation on the secondary system are uncharged. The velocities of dislocations are measured and the screw dislocations are found to move faster than the edge. This observation is associated with the unusual nature of the edge dislocation.
TL;DR: In this paper, the characteristics of serrated flow in gold-indium solid solution alloys are studied over a range of strain rates and temperatures. But the authors focus on two types of serrations, the general Type A locking serrations and the Type C unlocking serrations.
Abstract: The characteristics of serrated flow in gold-indium solid solution alloys are studied over a range of strain rates and temperatures. Two types of serrations are distinguished, the general Type A locking serrations which are similar to those observed by Russell (1963) and the Type C unlocking serrations. The Type C serrations are restricted to a narrow temperature range at the upper temperature limit for serrated flow. The activation energy of the Type A serrations is 0·77 ev. The activation energy is considered to be the same as that for the exchange of an indium atom with a vacancy in gold. By using pre-strain experiments the exponent of strain (m+β) in the Ham—Jaffrey equation is found to be independent of the indium concentration, but dependent on the stage of work-hardening in which the serrations commence. (m+β) values of 2·14 and ≃3 are determined for Stages II and III respectively. The difference in these values is thought to result from the different rates of vacancy production. For (m+β)...
TL;DR: In this paper, the inelastic scattering of fast electrons caused by excitation of atomic electrons is considered, taking account of the Bloch wave nature of the fast electrons within the crystal.
Abstract: The inelastic scattering of fast electrons caused by excitation of atomic electrons is considered, taking account of the Bloch wave nature of the fast electrons within the crystal. It is shown that the interference, or contrast, produced by small-angle inelastic scattering is similar to that produced by elastically scattered and plasmon scattered electrons. The result explains recent electron microscope observations of contrast in crystals obtained by energy analysis techniques.