Showing papers in "Philosophical Magazine in 2011"
TL;DR: In this paper, a review of recent approaches to understand crack dynamics using the phase field method is presented. And the relevance of different models, which stem from continuum field descriptions of brittle materials and crystals, to address questions concerning crack path selection and branching instabilities, as well as models that are based on mesoscale concepts for crack tip.
Abstract: Fracture is a fundamental mechanism of materials failure. Propagating cracks can exhibit a rich dynamical behavior controlled by a subtle interplay between microscopic failure processes in the crack tip region and macroscopic elasticity. We review recent approaches to understand crack dynamics using the phase field method. This method, developed originally for phase transformations, has the well-known advantage of avoiding explicit front tracking by making material interfaces spatially diffuse. In a fracture context, this method is able to capture both the short-scale physics of failure and macroscopic linear elasticity within a self-consistent set of equations that can be simulated on experimentally relevant length and time scales. We discuss the relevance of different models, which stem from continuum field descriptions of brittle materials and crystals, to address questions concerning crack path selection and branching instabilities, as well as models that are based on mesoscale concepts for crack tip ...
150 citations
TL;DR: In this paper, the authors used the yield stress of a material to estimate the effect of size effects on the slip system in MgO and found that both the hard and soft slip system can be characterised individually.
Abstract: Microcompression has attracted considerable interest in the study of size effects, mainly in soft metals. Little data is available in the literature on experiments on materials with a higher bulk flow stress, although it has been shown that the technique can be successfully employed to suppress cracking due to the small specimen dimensions. Here, microcompressions on MgO were carried out to demonstrate the possibility of individually activating different slip systems. The yield stresses obtained in conjunction with transmission electron microscopy show that both the hard and soft slip system in MgO can be characterised individually. Microcompression is, therefore, a potential alternative to macroscopic testing of brittle materials under confining pressure or at high temperatures. To determine the influence of size on such measurements, results on the two slip systems in MgO and from the literature are compared. It is found that the bulk yield stress of a material might be used to estimate the effect of si...
132 citations
TL;DR: In this article, a ductile-brittle transition at a micropillar diameter of approximately 1 µm was predicted, consistent with experimental observations and the conditions under which such splitting can occur.
Abstract: Experiments have been carried out on how compressive failure of axis GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in all micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. However, GaAs micropillars with diameters less than approximately 1 µm did not split, nor was splitting observed where slip occurred on only one plane. The conditions under which such splitting can occur have been estimated by modifying an existing analysis. This predicts a ductile–brittle transition at a micropillar diameter of approximately 1 µm, consistent with experimental observations.
112 citations
TL;DR: In this paper, the effect of grain boundary impurities, especially phosphorous and oxygen, on the fracture resistance of the boundaries of tungsten materials was determined by analysing the fracture surfaces.
Abstract: Ten tungsten materials with different impurity concentrations and different microstructures have been investigated by Auger electron spectroscopy and scanning electron microscopy with respect to their fracture behaviour. For almost all samples, both inter- and transgranular fracture are observed, and the proportion of each type varies. Due to the difference in their impurity content and grain boundary area, a large variation in the grain boundary impurities can be expected. By analysing the fracture surfaces the effect of grain boundary impurities, especially phosphorous and oxygen, on the fracture resistance of the boundaries was determined. The results indicate that for the analysed tungsten materials, grain boundary impurities do not have a significant influence on the fracture resistance of the boundaries. Other factors such as the size and shape of the grains, the amount of deformation and therefore the density of dislocations within the grains have a greater impact on the fracture behaviour of tungsten.
97 citations
TL;DR: In this paper, the acceleration and deformation properties of face centered cubic (fcc) alloys in the presence of slip-coherent twin boundary (CTB) interactions are investigated.
Abstract: The energetics of slip–coherent twin boundary (CTB) interactions are established under tensile deformation in face centered cubic (fcc) copper with molecular dynamics simulations, exploring the entire stereographic triangle. The CTBs serve as effective barriers in some crystal orientations more than others, consistent with experimental observations. The resulting dislocation structures upon slip–twin reactions are identified in terms of Burgers vector analysis. Visualization of the dislocation transmission, lock formation, dislocation incorporation to twin boundaries, dislocation multiplication at the matrix–twin interface and twin translation, growth, and contraction behaviors cover the most significant reactions that can physically occur providing a deeper understanding of the mechanical behavior of fcc alloys in the presence of twin boundaries. The results make a distinction between deformation and annealing twins interacting with incident dislocations and point to the considerable role both types of t...
87 citations
TL;DR: In this article, the authors investigated the influence of stacking fault energy (SFE) on the evolution of microstructural homogeneity and found that uniform microstructures are more readily achieved in materials with high or low SFE than in materials having medium SFE.
Abstract: Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (dmin) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of dmin on the SFE decreases when the severity of the external loading conditions is increased.
81 citations
TL;DR: In this paper, a dislocation dynamics model for the coarsening of prismatic loops in fcc metals is presented. But this model assumes vacancy bulk diffusion and does not account for the velocity of the loops along their prismatic cylinders.
Abstract: Dislocation climb mobilities, assuming vacancy bulk diffusion, are derived and implemented in dislocation dynamics simulations to study the coarsening of vacancy prismatic loops in fcc metals. When loops cannot glide, comparison of the simulations with a coarsening model based on the line tension approximation shows good agreement. Dislocation dynamics simulations with both glide and climb are then performed. Allowing for glide of the loops along their prismatic cylinders leads to faster coarsening kinetics, as direct coalescence of the loops is now possible.
80 citations
TL;DR: Workhardening behavior of Mg single crystals oriented for basal slip was studied by means of tensile tests carried out at 4, 78 and 295 K as mentioned in this paper, where the crystals showed critical resolved shear stress values (CRSS) for a {0001} basal slip system in the range 1.5 MPa.
Abstract: Work-hardening behaviour of Mg single crystals oriented for basal slip was studied by means of tensile tests carried out at 4, 78 and 295 K. The crystals show critical resolved shear stress values (CRSS) for a {0001} basal slip system in the range 1–1.5 MPa. The samples exhibit two-stage work hardening characteristics consisting of a long easy glide stage and a stage of rapid hardening terminated by failure. The onset of the plastic flow up to the point of fracture is accompanied by a low work-hardening rate in the range 5 × 10−5–5 × 10−4 µ, corresponding to the hardening rate in Stage I of copper single crystals. The analysis of thermally activated glide parameters suggests that forest interactions are rate-controlling processes. The very low value of the activation distance found at 4 K, ∼0.047 b, is attributed to zero-point energy effects. The failure of crystals occurs well before their hardening capacity is exhausted by mechanisms which are characteristic of deformation temperature.
80 citations
TL;DR: In this paper, the plastic behavior of Co3(Al,W) polycrystals with the L12 structure has been investigated in compression from 77 to 1273 K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100
Abstract: The plastic behaviour of Co3(Al,W) polycrystals with the L12 structure has been investigated in compression from 77 to 1273 K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950 K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100 K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L12 compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co3(Al,W) and other L12 c...
79 citations
TL;DR: In this paper, the suitability of high temperature techniques was investigated by finite element analysis modelling to give a qualitative view of how the thermal picture develops under a diamond indenter without controlled heating of the diamond.
Abstract: The suitability of high temperature techniques was investigated by finite element analysis modelling to give a qualitative view of how the thermal picture develops under a diamond indenter without controlled heating of the diamond. In the case of a low-conductivity sample such as fused silica, the thermal gradient below the indenter tip is relatively diffuse, whereas with a high-conductivity sample such as gold, most of the sample is able to equilibrate at the set temperature, leading to a very steep thermal gradient in the volume of material that must accommodate the deformation. However, in both cases indentation is occurring in material that is at a lower, and unknown, temperature than the bulk sample. The results of the model are validated by comparing results obtained by heating the indenter either indirectly by contact with the sample or utilising a separate heater for the indenter (an isothermal contact method). Nanoindentation results are presented for experiments using a cubic boron nitride Berko...
77 citations
TL;DR: In this article, an Fe-Cr interatomic potential was developed to model high-Cr ferritic alloys, fitted to thermodynamic and point-defect properties obtained from density functional theory (DFT) calculations and experiments.
Abstract: We present an Fe–Cr interatomic potential to model high-Cr ferritic alloys. The potential is fitted to thermodynamic and point-defect properties obtained from density functional theory (DFT) calculations and experiments. The developed potential is also benchmarked against other potentials available in literature. It shows particularly good agreement with the DFT obtained mixing enthalpy of the random alloy, the formation energy of intermetallics and experimental excess vibrational entropy and phase diagram. In addition, DFT calculated point-defect properties, both interstitial and substitutional, are well reproduced, as is the screw dislocation core structure. As a first validation of the potential, we study the precipitation hardening of Fe–Cr alloys via static simulations of the interaction between Cr precipitates and screw dislocations. It is concluded that the description of the dislocation core modification near a precipitate might have a significant influence on the interaction mechanisms observed i...
TL;DR: Hardness and Young's modulus were measured by nanoindentation on a series of electrodeposited nanocrystalline nickel and nickel-iron alloys.
Abstract: Hardness and Young's modulus were measured by nanoindentation on a series of electrodeposited nanocrystalline nickel and nickel–iron alloys. Hardness values showed a transition from regular to inverse Hall–Petch behaviour, consistent with previous studies. There was no significant influence of grain size on the Young's modulus of nanocrystalline nickel and nickel–iron alloys with grain sizes greater than 20 nm. The Young's modulus values for nanocrystalline nickel and nickel–iron alloys for grain sizes less than 20 nm were slightly reduced when compared to their conventional (randomly oriented) polycrystalline counterparts. The observed trend with decreasing grain size was found to be consistent with composite model predictions that consider the influence of intercrystalline defects. However, there was some notable variability of the measured values when compared to the model predictions. Three theoretical relationships were used to characterise the anisotropic elastic behaviour of these materials. As a r...
TL;DR: In this paper, the authors performed uniaxial tensile testing of single crystalline electroplated copper nanopillars with diameters between 75 nm and 165 nm fabricated without the use of a focused ion beam (FIB).
Abstract: The results are presented of uniaxial tensile testing of single crystalline electroplated copper nanopillars with diameters between 75 nm and 165 nm fabricated without the use of a focused ion beam (FIB). The experiments were performed in an in situ nanomechanical instrument, SEMentor, and reveal that the pillars’ ultimate tensile strengths follow a similar power law dependence on diameter as reported for microcompression studies on fcc metals fabricated with and without FIB. Further, these pillars are characterized by limited or non-existent initial homogeneous deformation, immediately followed by necking in the top portion of the pillar. The particular deformation attributes are discussed in the context of hardening by dislocation starvation. Site-specific transmission electron microscopy microstructural analysis of as-fabricated nanopillars indicates the presence of scarce twin boundaries in some specimens. We comment on the potential for mechanical effects due to the presence of twins.
TL;DR: In this article, a model of micro-pillar plasticity controlled by the nucleation of dislocations at free surfaces was developed using methods of dislocation dynamics innovated by Johnston and Gilman 50 years ago.
Abstract: A model of micro-pillar plasticity controlled by the nucleation of dislocations at free surfaces was developed using methods of dislocation dynamics innovated by Johnston and Gilman 50 years ago. It is shown that the stress dependence of the rate of nucleation can be related to the dependence of the flow strength on the diameter of the pillar. The steady state flow stress in this model depends only on the kinetics of nucleation, whereas the transient deformation behavior depends on the initial dislocation density and the dislocation mobility, as well as the rate of nucleation. The model can describe not only the dependence of the strength on pillar diameter, but also recent experiments on fcc gold crystals, wherein pre-straining leads to softening and annealing leads to hardening. The model does not account for the stochastic, jerky nature of plastic flow in micro-pillars and is not meant to apply to pillars large enough to support substructure formation.
TL;DR: In this article, the authors propose constitutive models for polycrystalline aggregates with intergranular cavities and test them against full-field numerical simulations, where the dilatational effects associated with the presence of cavities must be accounted for.
Abstract: We propose constitutive models for polycrystalline aggregates with intergranular cavities and test them against full-field numerical simulations. Such conditions are prevalent in many engineering applications and failure of metallic components (e.g. HIPing and other forming processes, spallation under dynamic loading conditions, etc.), where the dilatational effects associated with the presence of cavities must be accounted for, and standard polycrystalline models for incompressible plasticity are not appropriate. On the other hand, it is not clear that the use of porous plasticity models with isotropic matrix behavior is relevant, particularly, when large deformations can lead to significant texture evolution and therefore to strong matrix anisotropy. Of course, finite strains can also lead to significant changes in the porosity and pore shape, resulting in additional anisotropy development. In this work, we make use of ‘variational linear-comparison’ homogenization methods to develop constitutive models...
TL;DR: A P-type icosahedral quasicrystal is formed in an Au-Al-Yb alloy with a six-dimensional lattice parameter a 6D = 7.448.
Abstract: A P-type icosahedral quasicrystal is formed in an Au–Al–Yb alloy with a six-dimensional lattice parameter a 6D = 7.448 A. The composition of the quasicrystal was analyzed to be Au51Al34Yb15. The quasicrystal is formed in as-cast alloys and is regarded as metastable with decomposition to other crystalline phases during annealing at 700°C. Among Tsai-type quasicrystals, this quasicrystal is situated just between the Zn–Sc group with a smaller a 6D and the larger Cd–Yb group. The intermediate valence of Yb recently observed in this quasicrystal may be due to this unique situation, namely a smaller major component in Au-Al-Yb than in Cd–Yb. The predominant phase in the annealed specimen is a 1/1 cubic approximant with lattice parameter a = 14.500 A belonging to the space group Im . This phase is stable at the composition Au51Al35Yb14 at 700°C. Rietveld structural analysis indicated that the crystal structure is a periodic arrangement of Tsai-type clusters with four Au–Al atoms at their centers. Chemical order...
TL;DR: In this article, the allotropic, martensitic phase transformation (hcp→fcc) in cobalt was investigated by differential scanning calorimetry (DSC) upon isochronal annealing at heating rates in the range from 10 to 40 K min−1.
Abstract: The allotropic, martensitic phase transformation (hcp → fcc) in cobalt was investigated by differential scanning calorimetry (DSC) upon isochronal annealing at heating rates in the range from 10 K min−1 to 40 K min−1. The microstructural evolution was traced by optical microscopy and X-ray diffractometry. The kinetics of the phase transformation from hcp to fcc Co upon isochronal annealing was described on the basis of a modular phase transformation model. Appropriate model descriptions for athermal nucleation and thermally activated, anisotropic interface controlled growth tailored to the martensitic phase transformation of Co were implemented into the modular model. Fitting of this model of phase transformation kinetics to simultaneously all isochronal DSC runs yielded values for the energy of the interface separating the hcp and fcc Co phase and the activation energy for growth.
TL;DR: In this article, cube corner nanoindentation was performed using a Hysitron Triboscope on a number of different bulk (single crystal silicon, fused silica, single crystal sapphire) and thin film materials (Si-DLC, aC, SnO2, ZnO:Al, ITO).
Abstract: Fracture toughness is an important technical material parameter, which, for bulk materials, can be determined by macroscopic crack extension experiments. For thin coatings, this classical method is not applicable. However, during the last 25 years, techniques have been developed which allow fracture toughness determination from indentation experiments by measuring crack lengths generated during controlled indentation. For its evaluation, a number of different models have been proposed which partly yield significant different results. In the present work, cube corner nanoindentation was performed using a Hysitron Triboscope on a number of different bulk (single crystal silicon, fused silica, single crystal sapphire) and thin film materials (Si-DLC, aC, SnO2, ZnO:Al, ITO). Crack lengths were measured by AFM-based methods and six different models, including a crack-energy-based and a thin film model, were used for evaluation. The models are validated by comparing mean deviations of experimental and literatur...
TL;DR: In this article, the authors highlight the use of a field-emission TEM, operated in scanning transmission electron microscopy mode, as a viable tool for defect analysis; both experimental and computational results are presented.
Abstract: Transmission electron microscopy (TEM) has been instrumental in advancing the field of crystalline defect analysis. Conventional TEM imaging techniques, such as bright field (BF), dark field (DF), and weak beam dark field (WBDF or g–3g) imaging, have been well-documented in the scientific literature, with simulation methods readily available for each. The present contribution highlights the use of a field-emission TEM, operated in scanning transmission electron microscopy mode, as a viable tool for defect analysis. Common techniques such as two-beam diffraction contrast and zone axis imaging are applied to defect analysis; both experimental and computational results are presented. Effects of experimental parameters such as camera length, beam divergence angle, and diffraction aperture placement are also discussed and illustrated by both experimental and computed micrographs of stacking faults.
TL;DR: In this article, the deformation twins in the Cu layers preferentially nucleate from the Cu(112)/Nb(112)-interface habit planes rather than the predominant Cu(111)//Nb (110) interface planes.
Abstract: We investigate deformation of pure Cu, pure Nb and 30 nm Cu/30 nm Nb nanolaminates induced by high strain rate shock loading. Abundant dislocation activities are observed in shocked pure Cu and Nb. In addition, a few deformation twins are found in the shocked pure Cu. In contrast, in shocked Cu/Nb nanolaminates, abundant deformation twins are found in the Cu layers, but only dislocations in the Nb layers. High resolution transmission electron microscopy reveals that the deformation twins in the Cu layers preferentially nucleate from the Cu(112)//Nb(112) interface habit planes rather than the predominant Cu(111)//Nb(110) interface planes. Our comparative study on the shock-induced plastic deformation of the pure metals (Cu and Nb) and the Cu/Nb nanolaminates underscores the critical role of heterogeneous phase interfaces in the dynamic deformation of multilayer materials.
TL;DR: In this paper, a new interatomic potential for α-Fe that is able to rationalize experimental flow stress data is proposed, which is based on the Peierls potential.
Abstract: In this work, we seek to develop a new interatomic potential for α-Fe that is able to rationalize experimental flow stress data. We generate a series of potentials with similar bulk and point defect properties, but exhibit different energetic landscapes for the Peierls potential. The family of potentials all possess a compact core structure, which we find necessitates a camel-hump shaped Peierls potential. Within this constraint, we analyze the relationships between the Peierls potential, the 3-D kink nucleation energetics, and the resulting shape of the kink structures for the screw dislocation. We find that one of our models, labeled MPG20, gives very good agreement with experimental flow stress data over the entire stress range considered.
TL;DR: In this article, the effect of 10% pre-ageing deformation on the early precipitation behavior in AA6060 Al-Mg-Si alloy was investigated by high-resolution transmission electron microscopy (HRTEM) in ⟨100⟩Al projections.
Abstract: The effect of 10% pre-ageing deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy aged 10 min at 190°C was investigated by high-resolution transmission electron microscopy (HRTEM) in ⟨100⟩Al projections. The precipitate nucleation was heterogeneous since all precipitates were found to grow on dislocation lines. The pre-ageing deformation suppresses growth of Gunier–Preston zones and β″ phase. The resulting precipitates are still largely coherent with the aluminium matrix. They appear with two main morphologies; one consists of independent, small cross-sections arising from needles with disordered β′ and B′ structures. The other morphology is a much more continuous decoration where precipitates have elongated and conjoined cross-sections and where a particular precipitate phase could not be determined. All precipitates in this work were found to contain a common near-hexagonal sub-cell (SC) with projected bases a = b ≈ 0.4 nm. This strongly indicates that they are built over the sam...
TL;DR: The dynamics of water in aqueous mixtures with various hydrophilic solutes can be probed over practically unrestricted temperature and frequency ranges, in contrast to bulk water where crystallization preempts such study.
Abstract: The dynamics of water in aqueous mixtures with various hydrophilic solutes can be probed over practically unrestricted temperature and frequency ranges, in contrast to bulk water where crystallization preempts such study. The characteristics of the dynamics of water and their trends observed in aqueous mixtures on varying the solutes and concentration of water, in conjunction with that of water confined in spaces of nanometer size, lead us to infer the fundamental traits of the dynamics of water. These include the universal secondary relaxation, here called the ν-relaxation, the low degree of intermolecular coupling/cooperativity, and the ‘strong’ character of the structural primary relaxation. The dynamics of hydration water in hydrated proteins at sufficiently high hydration levels are similar in every respect to that in aqueous mixtures. In particular, the ν-relaxation of hydration water has a relaxation time nearly the same as that of the ν-relaxation of aqueous mixtures above and below the glass tran...
TL;DR: In this paper, the influence of high hydrostatic pressure on the electrical resistance in the ab-plane in HoBa2Cu3O7−δ single crystals with oxygen deficiency was investigated.
Abstract: We investigate the influence of high hydrostatic pressure on the electrical resistance in the ab-plane in HoBa2Cu3O7– δ single crystals with oxygen deficiency. It is determined that the high-pressure-induced redistribution of labile oxygen enhances phase separation, which is accompanied by structural relaxation and ascending diffusion within the volume of the sample. This results in a significant displacement of the temperature intervals that correspond to metal-to-dielectric-type transitions. It is determined that the formation of the low-temperature phase can occur at the twin boundaries.
TL;DR: In this article, a two-dimensional (2D) lattice model with anisotropic resonant microstructures is introduced to represent this lattice system and two methods are proposed to derive the equivalent continuum.
Abstract: A two-dimensional (2D) lattice model with anisotropic resonant microstructures is found to provide an anisotropic band gap structure. A 2D continuum with anisotropic effective mass density is introduced to represent this lattice system. Two methods are proposed to derive the equivalent continuum. In the first method, the effective mass density of the equivalent continuum is obtained by matching the dispersion relations for harmonic waves propagating in the principal directions. The second approach employs an approximate estimation of the effective mass density by volume-averaging an effective mass that represents the resonant microstructure. For both equivalent continuum models, the effective mass density is frequency-dependent and may become negative in certain frequency ranges. Subsequently, the effective mass density of the equivalent continuum assumes the form of a second-order tensor. Thus, it suffices to determine the effective mass density tensor with respect to the principle directions. It is show...
TL;DR: In this article, three different topics in phase-field modelling of solidification are discussed, with particular emphasis on the limitations of the currently available modelling approaches, and it is pointed out that the standard procedure to incorporate fluctuations into the phase field approach cannot be used in a straightforward way for a quantitative description of nucleation.
Abstract: Three different topics in phase-field modelling of solidification are discussed, with particular emphasis on the limitations of the currently available modelling approaches. First, thin-interface limits of two-sided phase-field models are examined, and it is shown that the antitrapping current is, in general, not sufficient to remove all thin-interface effects. Second, orientation-field models for polycrystalline solidification are analysed, and it is shown that the standard relaxational equation of motion for the orientation field is incorrect in coherent polycrystalline matter. Third, it is pointed out that the standard procedure to incorporate fluctuations into the phase-field approach cannot be used in a straightforward way for a quantitative description of nucleation.
TL;DR: In this article, the deformation of gold nanowires of diameter 30-70 nm and nanoporous gold specimens with ligament diameter 5-10 nm, both produced by electrodeposition into anodised aluminium oxide templates, was studied.
Abstract: We present a study of the deformation of gold nanowires of diameter 30–70 nm and nanoporous gold specimens with ligament diameter 5–10 nm, both produced by electrodeposition into anodised aluminium oxide templates. The nanowires show extensive surface slip steps and low dislocation densities with a few perfect dislocation loops, Shockley partial dislocations and microtwins observed after deformation. Nanoporous specimens show deformation localised to the nodes between the ligaments of the foamed structure, with high densities of microtwins and Shockley partial dislocations in these regions. Similar dislocation structures are seen in larger nanowires deformed in bending. This is shown to be consistent with a strain gradient plasticity model for the deformation of nanoporous gold, with the strain gradient accommodated by geometrically necessary twins and partial dislocations.
TL;DR: In this paper, the generalized Eshelby conduction tensor fields and localization tensor field inside circular and spherical inhomogeneities are shown to remain uniform even in the presence of Kapitza's interface thermal resistance.
Abstract: Eshelby's results and formalism for an elastic circular or spherical inhomogeneity embedded in an elastic infinite matrix are extended to the thermal conduction phenomenon with a Kapitza interface thermal resistance between matrix and inclusions. Closed-form expressions are derived for the generalized Eshelby interior and exterior conduction tensor fields and localization tensor fields in the case where the matrix and inclusion phases have the most general anisotropy. Unlike the relevant results in elasticity, the generalized Eshelby conduction tensor fields and localization tensor fields inside circular and spherical inhomogeneities are shown to remain uniform even in the presence of Kapitza's interface thermal resistance. With the help of these results, the size-dependent overall thermal conduction properties of composites are estimated by using the dilute, Mori–Tanaka, self-consistent and generalized self-consistent models. The analytical estimates are finally compared with numerical results delivered ...
TL;DR: In this paper, a simple dynamical density functional theory, the phase-field crystal (PFC) model, was used to describe homogeneous and heterogeneous crystal nucleation in two-dimensional (2D) monodisperse colloidal systems.
Abstract: A simple dynamical density functional theory, the phase-field crystal (PFC) model, was used to describe homogeneous and heterogeneous crystal nucleation in two-dimensional (2D) monodisperse colloidal systems and crystal nucleation in highly compressed Fe liquid. External periodic potentials were used to approximate inert crystalline substrates in addressing heterogeneous nucleation. In agreement with experiments in 2D colloids, the PFC model predicts that in 2D supersaturated liquids, crystalline freezing starts with homogeneous crystal nucleation without the occurrence of the hexatic phase. At extreme supersaturations, crystal nucleation happens after the appearance of an amorphous precursor both in two and three dimensions. Contrary to expectations based on the classical nucleation theory, it is shown that corners are not necessarily favourable places for crystal nucleation. Finally, it is shown that by adding external potential terms to the free energy, the PFC theory can be used to model colloid patte...
TL;DR: In this article, the authors derived an inherent law that determines when metallic glasses might yield or fracture based on the short-range order structure in metallic glasses, and constructed a unified failure criterion which satisfactorily predicts the complex failure behavior observed in metallic glass, and showed that the shear-to-normal strength ratio alpha and the strength-differential factor beta have dual control over whether metallic glasses yield in a ductile manner or fracture in brittleness.
Abstract: Metallic glasses exhibit not only multiple failure modes but also differences in ultimate strength, plastic strain to fracture and asymmetric deviation of failure angles from 45 degrees between tension and compression. The available failure theories cannot fully characterize these phenomena and the underlying physics has not been completely clarified. Here, based on the short-range order structure in metallic glasses, we derive an inherent law that determines when metallic glasses might yield or fracture. A unified failure criterion is constructed which satisfactorily predicts the complex failure behavior observed in metallic glasses. We show that the shear-to-normal strength ratio alpha and the strength-differential factor beta, characterizing shearing resistance between atomic layers and shear-caused dilatation, respectively, have dual control over whether metallic glasses yield in a ductile manner or fracture in brittleness.