Showing papers in "Philosophical Magazine in 2003"

Development of new interatomic potentials appropriate for crystalline and liquid iron

Abstract: Two procedures were developed to fit interatomic potentials of the embedded-atom method (EAM) form and applied to determine a potential which describes crystalline and liquid iron. While both procedures use perfect crystal and crystal defect data, the first procedure also employs the first-principles forces in a model liquid and the second procedure uses experimental liquid structure factor data. These additional types of information were incorporated to ensure more reasonable descriptions of atomic interactions at small separations than is provided using standard approaches, such as fitting to the universal binding energy relation. The new potentials (provided herein) are, on average, in better agreement with the experimental or first-principles lattice parameter, elastic constants, point-defect energies, bcc–fcc transformation energy, liquid density, liquid structure factor, melting temperature and other properties than other existing EAM iron potentials.

The influence of efficient atomic packing on the constitution of metallic glasses

Abstract: Efficient atomic packing is shown to be a fundamental consideration in the formation of metallic glasses. A simple concept of packing efficiency, based on atom packing in the first coordination shell of solute-centred clusters, is proposed and developed. This model leads to the prediction that specific radius ratios, defined as the radius of the solute atom divided by the radius of the solvent atom, are preferred in the constitution of metallic glasses. Analysis of a large number of binary and complex metallic glasses shows that these specific critical radius ratios R* are indeed preferred in known metallic glasses. The predictions of this model extend previous proposals to describe the influence of topology on the formation of metallic glasses. Although this model represents a simple idealization, the strong agreement with published metallic glasses suggests that efficient atomic packing, enabled by solute-centred clusters, forms a fundamental consideration in the constitution of metallic glasses.

Effects of pressure on high-temperature dislocation creep in olivine

Abstract: Effects of pressure on high-temperature, dislocation creep in olivine ((Mg, Fe) 2 SiO 4 ) aggregates have been determined under both water-poor ('dry') and water-saturated ('wet') conditions. New experimental data were obtained at pressures of 1-2 GPa under 'dry' and 'wet' conditions using a newly developed high-resolution dislocation density measurement technique to estimate the creep strength. These data are compared with previous data at lower and higher pressures to determine the pressure dependence of high-temperature dislocation creep in olivine aggregates. We find that the creep strength †under 'dry' conditions increases monotonically with increasing pressure, whereas the creep strength under 'wet' conditions changes with pressure in a non-monotonic fashion: it first decreases rapidly with increasing pressure and then becomes less sensitive to pressure at above 1 GPa. Such behaviour can be described by the following formula: where the subscripts d and w refer to parameters for 'dry' and 'wet' condi...

Density functional theory studies of screw dislocation core structures in bcc metals

Abstract: The core structures of d111 ¢screw dislocations in bcc metals are studied using density functional theory in the local-density approximation. For Mo and Fe, direct calculations of the core structures show the cores to be symmetric with respect to 180° rotations around an axis perpendicular to the dislocation line. The magnetic moment in the Fe core is shown to be reduced relative to the bulk value. Calculations of nsurfaces and the elastic constants B , C ' and c 44 are reported for Fe and all group VB and VIB metals. Using a criterion suggested by Vitek and Duesbery the calculations point to symmetric core structures for all the studied metals.

One-dimensional atomic transport by clusters of self-interstitial atoms in iron and copper

Abstract: Atomic-scale computer simulation has been used to study the thermally activated atomic transport of self-interstitial atoms (SIAs) in the form of planar clusters in pure Cu and f-Fe. There is strong evidence that such clusters are commonly formed in metals during irradiation with high-energy particles and play an important role in accumulation and spatial distribution of surviving defects. An extensive study of the mobility of SIA clusters containing two to 331 interstitials has been carried out using the molecular dynamics simulation technique for the temperature range from 180 to 1200 K. The results obtained show that clusters larger than three to four SIAs are one-dimensionally mobile in both Cu and Fe. Large clusters of more than 100 SIAs in Cu and 300 SIAs in Fe have significantly reduced mobility. The problem of describing one-dimensional (1D) motion in three-dimensional space is discussed. An attempt is made to describe the mobility of SIA clusters within the approximation of 1D diffusion. For clus...

Periodic image effects in dislocation modelling

Abstract: The use of periodic boundary conditions for modelling crystal dislocations is predicated on one’s ability to handle the inevitable image effects. This communication deals with an often overlooked mathematical subtlety involved indealin g with the periodic dislocationarrays, that is conditional convergence of the lattice sums of image fields. By analysing the origin of conditional convergence and the numerical artefacts associated with it, we establish a mathematically consistent and numerically efficient procedure for regularization of the lattice sums and the corresponding image fields. The regularized solutions are free from the artefacts caused by conditional convergence and regain periodicity and translational invariance of the periodic supercells. Unlike the other existing methods, our approach is applicable to general anisotropic elasticity and arbitrary dislocation arrangements. The capabilities of this general methodology are demonstrated by application to a variety of situations encountered in atomistic and continuum modelling of crystal dislocations. The applications include introduction of dislocations in the periodic supercell for subsequent atomistic simulations, atomistic calculations of the core energies and the Peierls stress and continuum dislocation dynamics simulations in three dimensions.

Multiscale modelling of nanomechanics and micromechanics: an overview

Abstract: Recent advances in analytical and computational modelling frameworks to describe the mechanics of materials on scales ranging from the atomistic, through the microstructure or transitional, and up to the continuum are reviewed. It is shown that multiscale modelling of materials approaches relies on a systematic reduction in the degrees of freedom on the natural length scales that can be identified in the material. Connections between such scales are currently achieved either by a parametrization or by a ‘zoom-out’ or ‘coarse-graining’ procedure. Issues related to the links between the atomistic scale, nanoscale, microscale and macroscale are discussed, and the parameters required for the information to be transferred between one scale and an upper scale are identified. It is also shown that seamless coupling between length scales has not yet been achieved as a result of two main challenges: firstly, the computational complexity of seamlessly coupled simulations via the coarse-graining approach and, second...

In-situ transmission electron microscopy observations and molecular dynamics simulations of dislocation-defect interactions in ion-irradiated copper

Abstract: An in-situ transmission electron microscopy straining technique has been used to investigate the dynamics of dislocation-defect interactions in ion-irradiated copper and the subsequent formation of defect-free channels. Defect removal frequently required interaction with multiple dislocations, although screw dislocations were more efficient at annihilating defects than edge dislocations were. The defect pinning strength was determined from the dislocation curvature prior to breakaway and exhibited values ranging from 15 to 175 MPa. Pre-existing dislocations percolated through the defect field but did not show long-range motion, indicating that they are not responsible for creating the defect-free channels and have a limited contribution to the total plasticity. Defect-free channels were associated with the movement of many dislocations, which originated from grain boundaries or regions of high stress concentration such as at a crack tip. These experimental results are compared with atomistic simulations o...

A new empirical potential for simulating the formation of defects and their mobility in uranium dioxide

Abstract: An empirical interaction potential has been developed primarily to simulate displacement cascades in a UO2 matrix. After a bibliographical survey revealed that the potentials discussed in the literature are not entirely suitable for describing point defects and their migration, we decided to adapt the most representative of the existing potentials. Our objective was specifically to reproduce transport phenomena, which are fundamental to a satisfactory description of atomic displacement cascades. The modified potential more correctly describes the experimental energies of formation and migration of point defects (interstitials, vacancies, etc.) as well as the oxygen diffusion coefficients in the superionic phase. Thermal expansion, heat capacity and enthalpy are also better modelled over a wide temperature range. The new potential is much more transferable than earlier potentials fitted mainly to the properties of perfect crystals. The initial simulated atomic displacement cascades showed that this potenti...

Benchmark values for the Soret, thermal diffusion and diffusion coefficients of three binary organic liquid mixtures

Abstract: With the aim of providing reliable benchmark values, we have measured the Soret, diffusion and thermal diffusion coefficients of the three binary mixtures of dodecane, isobutylbenzene and 1,2,3,4 tetrahydronaphthalene for a concentration of 50 wt% at a temperature of 25°C. The experimental techniques applied by the five participating laboratories are transient holographic gratings, annular and parallelepipedic thermogravitational columns, and vertical parallelepipedic columns with velocity amplitude determination by laser doppler velocimetry. The systems have also been studied in a annular thermogravitational column filled with a porous medium in the gap. There is a good agreement between the different experiments with deviations of the order of a few per cent in most cases (8.5% at most). The numerical values are tabulated in the paper.

Nanostructures and deformation mechanisms in a cryogenically ball-milled Al-Mg alloy

Abstract: An Al-7.6 at.% Mg alloy was ball milled in liquid N2 for 8 h and its microstructures were investigated using transmission electron microscopy. Electron diffraction confirmed that the resulting powder is a supersaturated Al-Mg solid solution with an fcc structure. Three typical nanostructures with different grain-size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small-angle subboundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10-30 nm. In very small crystallites with dimensions less than 10 nm, twinning becomes an important deformation mechanism. The reasons for the different deformation mechanisms were discussed. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated ...

Atomic modelling of strengthening mechanisms due to voids and copper precipitates in α-iron

Abstract: Recently a model has been developed by Osetsky and Bacon to study edge dislocations moving over large distances on the atomic scale. It permits investigation of motion of a dislocation under different conditions of applied shear stress with constant or variable strain rate and temperature, and in the presence of obstacles. In this paper we apply the model to study the motion of an infinite straight but flexible edge dislocation through a row of either voids or coherent copper precipitates in bcc iron. Stress–strain curves, energy barrier profile and strength characteristics of obstacles and other dislocation configuration information have been obtained from the modelling and compared with continuum treatments. Some specific atomic-scale mechanisms associated with strengthening due to voids and precipitates over a range of size have been observed and discussed.

Cyclic deformation of polycrystalline Cu films

Abstract: Fatigue impairs the reliability of macroscopic metallic components utilized in a variety of technological applications. However, the fatigue behaviour of thin metal films and small-scale components used in microelectronics and mechanical microdevices has yet to be explored in detail. The fatigue behaviour in submicrometre thin films is likely to differ from that in bulk material, since the volume necessary for the formation of dislocation structures typical of cyclic deformation in bulk material is larger than that available in thin films. The thin-film dimensions and microstructure, therefore, affect the microscopic processes responsible for fatigue. The fatigue behaviour of Cu films 0.4, 0.8 and 3.0 µm thick on polyimide substrates was investigated. The specimens were fatigued at a total strain amplitude of 0.5% using an electromechanical tensile-testing machine. This work focuses on the characterization of fatigue mechanisms and the resulting fatigue damage of thin Cu films. Extrusions similar to those...

The transition from localized to homogeneous plasticity during nanoindentation of an amorphous metal

Abstract: Using nanoindentation, we examine the fundamental nature of plasticity in a bulk amorphous metal. We find that the mechanics of plasticity depend strongly on the indentation loading rate, with low rates promoting discretization of plasticity into rapid bursts. For sufficiently slow indentations, we find that plastic deformation becomes completely discretized in a series of isolated yielding events. As the loading rate is increased, a transition from discrete to continuous yielding is observed. These results are fundamentally different from the classical expectations for metallic glasses, in which the transition from discrete to continuous yielding occurs upon a decrease in deformation rate. The present experimental results are analysed with reference to the theoretical ideal-plastic strain field beneath an indenter and rationalized on the basis of mechanistic models of glass plasticity.

Multiple-step martensitic transformations in Ni-rich NiTi alloys--an in-situ transmission electron microscopy investigation

Abstract: Multiple-step martensitic transformations in Ni-rich NiTi shape memory alloys have so far been rationalized on the basis of dislocation stress fields, coherency stress fields around Ni 4 Ti 3 precipitates and evolving Ni concentrations between precipitates during ageing. The primary objective of the present paper is to show that such transformations can also occur owing to heterogeneous microstructures that form during ageing of solution annealed defect-free materials. These microstructures are characterized by Ni 4 Ti 3 grain-boundary precipitation and by precipitate-free grain interiors. Two microstructures which give rise to two and three distinct differential scanning calorimetry (DSC) peaks on cooling from the B2 regime are subjected to in-situ cooling and heating cycles in the transmission electron microscope. Martensitic transformations are directly studied and the observations provide a new explanation for multiple-step martensitic transformations in Ni-rich NiTi alloys. Most importantly the resul...

Connectivity and percolation in simulated grain-boundary networks

Abstract: Random percolation theory is a common basis for modelling intergranular phenomena such as cracking, corrosion or diffusion. However, crystallographic constraints in real microstructures dictate that grain boundaries are not assembled at random. In this work a Monte Carlo method is used to construct physically realistic networks composed of high-angle grain boundaries that are susceptible to intergranular attack, as well as twin-variant boundaries that are damage resistant. When crystallographic constraints are enforced, the simulated networks exhibit triple-junction distributions that agree with experiment and reveal the non-random nature of grain-boundary connectivity. The percolation threshold has been determined for several constrained boundary networks and is substantially different from the classical result of percolation theory; compared with a randomly assembled network, about 50-75% more resistant boundaries are required to break up the network of susceptible boundaries. Triple-junction distributi...

Assessment of the fatigue crack closure phenomenon in damage-tolerant aluminium alloy by in-situ high-resolution synchrotron X-ray microtomography

Abstract: Synchrotron X-ray microtomography has been utilized for the in-situ observation of steady-state plane-strain fatigue crack growth. A high-resolution experimental configuration and phase contrast imaging technique have enabled the reconstruction of crack images with an isotropic voxel with a 0.7 µm edge. The details of a crack are readily observed, together with evidence of the incidence and mechanical influence of closure. After preliminary investigations of the achievable accuracy and reproducibility, a variety of measurement methods are used to quantify crack-opening displacement (COD) and closure from the tomography data. Utilization of the physical displacements of microstructural features is proposed to obtain detailed COD data, and its feasibility is confirmed. Loss of fracture surface contact occurs gradually up to the maximum load. This is significantly different from tendencies reported where a single definable opening level is essentially assumed to exist. The closure behaviour is found to be at...

Strain relaxation in the epitaxy of La2/3Sr1/3MnO3 grown by pulsed-laser deposition on SrTiO3(001)

Abstract: With a Curie point at 370 K, the half-metal (La0.7Sr0.3)MnO3 (LSMO) is one of the most interesting candidates for electronic devices based on tunnel magnetoresistance. SrTiO3 (STO) is up to now the best substrate for the epitaxy of suitable thin films of LSMO. The pseudocubic unit cell of rhombohedral LSMO has a parameter a LSMO such that (a STO − a LSMO)/a LSMO = + 0.83% (where a STO is the parameter of cubic STO) and an angle of 90.26°. As strained growth is tetragonal, relaxation implies recovery of both the pseudocubic parameter and of the original angle. In the LSMO layers that we prepare by pulsed-laser deposition, we show that these two processes are quite independent. The angular distortion is partially recovered by twinning in films 25 nm thick, while recovery of the parameter never occurs in the thickness range that we explored (up to 432 nm). A relaxation, however, takes place above a thickness of 100 nm, associated with a transition from two-dimensional to three-dimensional columnar growth. It...

Precise determination of the Soret, thermal diffusion and mass diffusion coefficients of binary mixtures of dodecane, isobutylbenzene and 1,2,3,4-tetrahydronaphthalene by a holographic grating technique

Abstract: Within the framework of an international benchmark test, the Soret coefficient S T , thermal diffusion coefficient D T and mutual mass diffusion coefficient D of the three binary mixtures of dodeca...

Atomistic simulation of dislocation emission in nanosized grain boundaries

Abstract: The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB) in nanocrystalline metals. It is shown that, in a 12 nm grain-size sample, GBs containing grain-boundary dislocations (GBDs) can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free-volume migration. As in previous work, the nucleation occurs at a GBD, which, upon nucleation and propagation, is removed. In the present case, free-volume migration occurs away from the nucleation region both before and after the nucleation event.

Radiation damage near grain boundaries

Abstract: Large-scale molecular dynamics of cascade production of the primary damage state are performed in nanocrystalline nickel with an average grain diameter of 12 nm and primary knock-on atom kinetic energies ranging from 5 to 30 keV. The role of the grain boundary during the cascade production of irradiated NC Ni is discussed in terms of grain-boundary structure. It is shown that regions of misfit in the grain boundaries can absorb self-interstitials and that stacking-fault tetrahedra are formed in the neighbourhood of the grain boundary.

Microstructure and mechanical properties of a partially crystallized La-based bulk metallic glass

Abstract: The evolution of microstructure upon partial crystallization and its influence on the mechanical properties such as hardness, elastic modulus and viscosity in a La 55 Al 25 Cu 10 Ni 5 Co 5 bulk metallic glass alloy are studied. Specimens with various volume fractions of crystalline phases were obtained by annealing the as-cast amorphous alloy above its glass transition temperature and were characterized by transmission electron microscopy. Microscopic examination of the heat-treated samples shows short-range-ordered domains prior to nanocrystallization within the amorphous matrix, followed by the growth and impingement of the crystallites. Whereas the hardness of the annealed samples increases linearly with increasing crystallinity, the elastic modulus and the viscosity both increase abruptly when the crystalline volume fraction is about 40 vol.%, with a only minor variation on either side of this range. The sudden rises in the modulus and viscosity are similar to those in the literature data on the fract...

Intrinsic nanoscale phase separation of bulk As2S3 glass

Abstract: Raman scattering on bulk AsxS1� x glasses shows that vibrational modes of As4S4 monomer first appear near x ¼ 0.38, and their concentration increases precipitously with increasing x, suggesting that the stoichiometric glass (x ¼ 0.40) is intrinsically phase separated into small As-rich (As4S4) and large S-rich clusters. Support for the Raman-active vibrational modes of the orpiment-like and realgar-like nanophases is provided by ab-initio density functional theory calculations on appropriate clusters. Nanoscale phase separation provides a basis for understanding the global maximum in the glass transition temperature Tg near x ¼ 0.40, and the departure from Arrhenius temperature activation of As2S3 melt viscosities.

Coherent motion of interstitial defects in a crystalline material

Abstract: Thermally activated Brownian motion of interstitial defects is one of the factors driving the evolution of microstructure of crystalline metals under irradiation. In the limit of relatively small system size the motion of defects can be followed on the atomistic scale by using molecular dynamics. However, understanding the kinetics of evolution of microstructure requires investigating how defects migrate and interact on a scale which is substantially greater than that accessible to molecular dynamics. This paper shows how mobile interstitial defects can be described by quasiparticle solutions of the multistring Frenkel–Kontorova (MSFK) model, which prove the equivalence between the crowdion and the glissile dislocation loop representations of small interstitial clusters. An exact solution of the MSFK model is found for the case of an infinite straight edge dislocation. This solution illustrates the fundamental link between the concepts of a crowdion and a dislocation in a crystalline material.

The importance of being curved: bowing dislocations in a continuum description

Abstract: Evolution equations for scalar density and orientation of fields of curved dislocations formulated in the framework of the continuum theory of moving dislocations serve as the starting point for development of a non-local dislocation-based constitutive relation for crystal plasticity, on the length scale intermediate between the phenomenological hardening laws of strain-gradient crystal plasticity and the explicit treatment of three-dimensional discrete dislocation dynamics. The key features of the proposed approach are the refined averaging in the continuum theory based on separation of single-valued dislocation fields, and the accounting for the line energy of the bowed dislocations which renders the theory non-local.

Deformation structures developing on fine scales

Abstract: Quantitative measurement and analysis of structural parameters has shown for a variety of metals and processes that the microstructural evolution follows a universal path of grain subdivision down to the nanoscale. This behaviour has allowed an analysis of the formation and evolution of graded nanoscale structures produced by sliding. Transmission electron microscopy studies and scaling analyses of such structures show the dominating role of dislocations in the development deformation microstructures on multiple length scales. The crucial role of dislocations has been documented by high-resolution electron microscopy analysis, which has revealed the presence of a large number of glide dislocations in layers between geometrically necessary boundaries with individual spacing as fine as 5 nm.

Dislocation–impenetrable precipitate interaction: a three-dimensional discrete dislocation dynamics analysis

Abstract: In an attempt to better understand the effect of the difference between the shear moduli of the particle and matrix on the flow stress and the work hardening, a numerical approach based on discrete dislocation simulations is developed in which the image stress caused by a second phase impenetrable particle on dislocations is implemented. Glide of a dislocation line of initially screw type through a channel between two spherical particles of shear modulus G p is simulated. Shear stress is applied incrementally on the slip plane and the equilibrium position of the dislocation line is calculated for the given applied stress. It is found that the flow stress at which the dislocation bypasses the obstacles by bowing between a pair of particles varies as , where G m is the shear modulus of the matrix and ΔG is the difference between shear moduli. α is found to be less than 1 and the effect of ΔG is amplified as the radius of the spherical particles increases. The stress increment required to force a dislocation...

The determination of stresses in thin films; modelling elastic grain interaction

Abstract: X-ray diffraction is frequently employed for the analysis of mechanical stresses in polycrystalline specimens. To this end, suitable so-called diffraction elastic constants are needed for determining the components of the mechanical stress tensor from measured lattice strains. These diffraction elastic constants depend on the single-crystal elastic constants of the material considered and the so-called grain interaction, describing the distribution of stresses and strains over the crystallographically differently oriented crystallites composing the specimen. Well-known grain interaction models, as due to Voigt, to Reuss, to Neerfeld and Hill and to Eshelby and Kroner, may be applied to bulk specimens, but they are generally not suitable for thin films. In this paper, an average 'effective' grain interaction model is proposed that consists of a linear combination of basic extreme models including new models specially suited to thin films. Experimental verification has been achieved by X-ray diffraction str...

Microstructural aspects of the hcp-fcc allotropic phase transformation induced in cobalt by ball milling

Abstract: A detailed correlation between microstructure evolution and allotropic phase transformations occurring in Co when subjected to ball milling has been carried out. After short-term milling, the starting mixture of hcp + fcc Co develops into an almost pure hcp phase. However, for longer milling times, plastic deformation introduces large amounts of stacking faults, especially of twin type, in the hcp structure. As a consequence, some of the hcp Co is converted back into fcc and the hcp unit cell is progressively anisotropically distorted. After long-term milling, a steady 'pseudo-equilibrium' state is observed, where all microstructural parameters, including the fcc percentage, tend to level off. However, the milling intensity can still be adjusted to increase further the stacking-fault probability and, consequently, the amount of fcc Co in the milled powders. The results imply that the stacking-fault formation, rather than the local temperature rise or crystallite size reduction associated with the milling ...

Ab initio calculation of tensile strength in iron

Abstract: A tensile test in ferromagnetic iron for loading in [001] and [111] directions is simulated by ab initio electronic structure calculations using all-electron full-potential linearized augmented-plane-wave method within the generalized gradient approximation. The theoretical tensile strengths and Young's moduli of ferromagnetic iron are determined and compared with those of other materials. The magnetic and elastic behaviours of iron under uniaxial tensile loading are discussed in detail and compared with the results for isotropic tension (i.e. for negative hydrostatic pressure). Marked anisotropy of theoretical tensile strength in [001] and [111] direction is explained in terms of higher-symmetry structures present or absent along the deformation paths.