Showing papers in "Philosophical Magazine in 2009"

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu–Zr alloys

Abstract: We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35...

Development of an interatomic potential for the Ni-Al system

Abstract: We construct an interatomic potential for the Ni-Al system within the embedded-atom method formalism. The potential is based on previously developed accurate potentials for pure Ni and Al. The cross-interactions are fitted to experimental cohesive energy, lattice parameter and elastic constants of B2-NiAl, as well as to ab initio formation energies of several real or imaginary intermetallic compounds with different crystal structures and chemical compositions. The potential accurately reproduces a variety of physical properties of the NiAl and Ni3Al phases, and shows reasonable agreement with experimental and ab initio data for phase stability across the Ni-Al phase diagram. Most of the properties reproduced by the new potential were not involved in the fitting process, which demonstrates its excellent transferability. Advantages and certain weaknesses of the new potential in comparison with other existing potentials are discussed in detail. The potential is expected to be especially suitable for simulati...

Ternary Fe-Cu-Ni many-body potential to model reactor pressure vessel steels: First validation by simulated thermal annealing

Abstract: In recent years, the development of atomistic models dealing with microstructure evolution and subsequent mechanical property change in reactor pressure vessel steels has been recognised as an important complement to experiments. In this framework, a literature study has shown the necessity of many-body interatomic potentials for multi-component alloys. In this paper, we develop a ternary many-body Fe–Cu–Ni potential for this purpose. As a first validation, we used it to perform a simulated thermal annealing study of the Fe–Cu and Fe–Cu–Ni alloys. Good qualitative agreement with experiments is found, although fully quantitative comparison proved impossible, due to limitations in the used simulation techniques. These limitations are also briefly discussed.

Extrusions and intrusions in fatigued metals. Part 1. State of the art and history

Abstract: Current state and historical progress in experimental and theoretical studies of surface relief appertaining to persistent slip bands (PSBs) and leading to fatigue crack initiation in cyclically deformed metals is presented as a thorough critical overview. A comprehensive inventory of microscopic techniques used for this study is tabulated chronologically with emphasis to their applicability to polycrystals. The most relevant experimental characteristics concerning surface relief evolution, namely the form of extrusions and intrusions in single- and polycrystalline materials, are surveyed. Theoretical models and computational simulations of extrusion and intrusion formation and fatigue crack initiation are critically reviewed. †In 2003, it was 100 years since J.A. Ewing and J.C.W. Humfrey first documented the nature of early surface fatigue damage in Swedish iron in their paper The Fracture of Metals under Repeated Alternations of Stress published in the journal Philosophical Transactions of the Royal Soc...

Development of interatomic potentials appropriate for simulation of solid–liquid interface properties in Al–Mg alloys

Abstract: Different approaches are analyzed for construction of semi-empirical potentials for binary alloys, focusing specifically on the capability of these potentials to describe solid–liquid phase equilibria, as a pre-requisite to studies of solidification phenomena. Fitting ab initio compound data does not ensure correct reproduction of the dilute solid-solution formation energy, and explicit inclusion of this quantity in the potential development procedure does not guarantee that the potential will predict the correct solid–liquid phase diagram. Therefore, we conclude that fitting only to solid phase properties, as is done in most potential development procedures, generally is not sufficient to develop a semi-empirical potential suitable for the simulation of solidification. A method is proposed for the incorporation of data for liquid solution energies in the potential development procedure, and a new semi-empirical potential developed suitable for simulations of dilute alloys of Mg in Al. The potential corre...

Measurement of the Soret, diffusion, and thermal diffusion coefficients of three binary organic benchmark mixtures and of ethanol–water mixtures using a beam deflection technique

Abstract: We have measured the Soret (S T ), diffusion (D), and thermal diffusion (D T ) coefficients of three binary mixtures of dodecane (DD), isobutylbenzene (IB) and 1,2,3,4-tetrahydronaphthalene (TH) for a concentration of 50 wt% at a temperature of 25°C by means of an optical beam deflection cell. This relevant experimental technique was still missing from a recent benchmark campaign for the measurement of the Soret effect. The measured coefficients agree to within a few percent (10% for S T , D of TH/IB) with the proposed benchmark values. A detailed analysis of the measurement process of the beam deflection cell, which allows for an elegant extension to include temperature gradients within the windows, is given, and improved benchmark values are suggested. In addition, ethanol–water mixtures have been investigated very carefully over a broad concentration and temperature range. Comparison with data of Kolodner and Wiegand gives a generally good agreement with some systematic deviations. Contrary to theoreti...

Relationship between the parting limit for de-alloying and a particular geometric high-density site percolation threshold

Abstract: The parting limit or de-alloying threshold for electrolytic dissolution of the more reactive component from a homogeneous fcc binary alloy is usually between 50 and 60 at%. The system that has been most studied, dissolution of Ag from Ag–Au, shows a parting limit close to 55 at% Ag. Here, Kinetic Monte Carlo (KMC) simulations of ‘Ag–Au’ alloys and geometric percolation modeling are used to study the relationship between this parting limit and the high-density site percolation thresholds p c(m) for an fcc lattice, subject to the rule that atoms with coordination greater than nine are prevented from dissolution. The value of p c(9) is calculated from geometric considerations to be 59.97 ± 0.03%. In comparison, using KMC simulations with no surface diffusion and no dissolution allowed for ‘Ag’ atoms with more than nine total neighbors, the parting limit is found to be slightly lower (58.4 ± 0.1%). This slight discrepancy is explained by consideration of the local atomic configurations of ‘Ag’ atoms – a few o...

Deformation of micron-sized aluminium bi-crystal pillars

Abstract: The deformation of micron-sized single-crystals is jumpy and stochastic, and this may pose potential formability and reliability problems if components for future micro-machines are to be made from small metal volumes. In this work, micron-sized bi-crystal pillars were fabricated by focussed ion-beam milling from grain-boundary regions in coarse-grained polycrystalline aluminium. Each bi-crystal pillar contained a grain boundary intersecting its top surface, and was subjected to compression using a flat-ended nanoindenter tip. Their deformation was found to have smaller strain bursts, fewer periods of strain hardening at elastic-like rates, as well as greater work-hardening rate and flow stress, than single-crystal pillars of similar sizes. Transmission electron microscopy revealed severe dislocation accumulation in the deformed bi-crystal pillars, whereas the residual dislocation density remained low in single-crystal micro-pillars of similar dimensions after deformation to comparable strains. The result...

Direct observation of plasticity and quantitative hardness measurements in single crystal cyclotrimethylene trinitramine by nanoindentation

Abstract: Investigation of deformation beginning with elasticity and continuing through the elastic–plastic transition to incipient cracking has been conducted for (210), (021) and (001) oriented single crystals of the explosive cyclotrimethylene trinitramine, commonly known as “RDX”. Nanoindentation was performed with a conical tip over a range of loads. The resulting load–depth data exhibited distinct, reproducible, orientation-dependent load excursions demonstrating elastic–plastic transitions. Indent impressions were imaged by atomic force microscopy revealing deformation features consistent with slip on six planes. Impressions on the (210) and (001) planes showed deformation pile-up features associated with the zone axes of slip planes. Slip traces were evident on the (210) plane indicating slip on four planes and suggesting cross-slip. Height data, for impressions formed by progressively increasing loads, indicated one additional slip system consistent with (010) slip. All of the orientations exhibited cracki...

Pyramidal slip in magnesium: Dislocations and stacking fault on the {1011} plane

Abstract: Molecular dynamics (MD) simulations are presented to illustrate the pyramidal slip processes on the plane (also a twinning plane) in magnesium. The simulations reveal an interesting slip mode consisting of two separate partial dislocations that nucleate independently. These dislocations glide in a slightly corrugated fashion on the non-close-packed plane, each having both ⟨c⟩ and ⟨a⟩ components. The first dislocation glides with an effective Burgers vector ( is also the twinning vector η1), and the second dislocation with an effective Burgers vector . When operating separately, the first incomplete dislocation leads to a wide stacking fault on the plane. When the second dislocation follows and combines with the first dislocation at the surface, the net end result is equivalent to a ⟨c + a⟩ dislocation. However, inside the crystal there is no unit ⟨c + a⟩ dislocation per se or its splitting.

Extrusions and intrusions in fatigued metals. Part 2. AFM and EBSD study of the early growth of extrusions and intrusions in 316L steel fatigued at room temperature

Abstract: Early stages of surface relief evolution of persistent slip markings (PSMs), formed in areas where persistent slip bands (PSBs) intersect the free surface, in polycrystalline 316L stainless steel cycled with constant plastic strain amplitude were studied using atomic force microscopy (AFM) and electron backscattering diffraction (EBSD). Focused ion beam (FIB) technique was employed to obtain additional, more detailed information on the shape of PSMs. To reveal true qualitative and quantitative information on the simultaneous growth of intrusions and extrusions within individual PSMs, identical areas both on the specimen surface and on its inverse copy obtained via plastic replica were studied using AFM. Intrusions are preceded by extrusions regardless of orientation of individual grains of the polycrystal. The first intrusions appear largely around 1% of fatigue life at the moment when ‘static’ extrusions are developed. They appear predominantly but not exclusively at the side of extrusions where the emer...

Topological and statistical properties of a constrained Voronoi tessellation

Abstract: Voronoi tessellation has been used widely to approximate and model various cellular structures and stochastic patterns appearing in nature. In this work, we present an extended version of the Voronoi tessellation method that partitions the space with certain constraints commonly encountered in either experimental measurements or theoretical models, such as cell volume or size distribution. The new Voronoi method is implemented using an inverse Monte Carlo method. We calculate the topological and statistical properties of tessellated Voronoi cells in several model systems with cell volumes obeying lognormal and bimodal distributions. We also compare the results with those obtained using the conventional Poisson–Voronoi method. We observed systematic changes in the topological properties as well as deviations from some established topological relations as the parameters in the constraint were varied. The application of this constrained Voronoi method in microstructure modelling and characterization in poly-...

Transport and mechanical properties of Yb-filled skutterudites

Abstract: A series of Yb-filled skutterudites were produced and powder samples consolidated using spark plasma sintering (SPS). The effect of different heating cycles on the resulting transport properties of the consolidated samples was explored. Specifically, the effect of sample uniformity on the electrical and thermal transport properties was explored. In addition to the optimal Yb-filling fraction, other factors, such as heating profiles and sintering conditions, were found to play a pivotal role in the performance of these materials. Large quantities of Yb-filled skutterudite material can be generated with high purity and uniformity. Resonant ultrasound spectroscopy was used to determine the elastic modulus and Poisson's ratio. Fracture strength was measured for 12 specimens and, taken with information obtained from resonant ultrasound spectroscopy and from thermal expansion and thermal transport characteristics, the thermal shock resistance parameter was evaluated. These parameters will be important for the e...

On the refinement of carbide precipitates by cryotreatment in AISI D2 steel

Abstract: Refinement of carbide particles by cryotreatment is often proposed as a major factor for the improvement of wear resistance in tool steels. However, this proposition is not substantiated by experimental evidence. This has been examined in this report by (i) detailed micro-structural analyses of the nature, volume fraction, size, population density and distribution of carbide particles, (ii) XRD and EDX micro-analysis on the bulk samples and electrochemically extracted carbides, and (iii) measurement of hardness and wear rate of a series of differently cryotreated AISI D2 steel. The results conclusively establish that (i) cryotreatment, in comparison to conventional treatment, induces precipitation of finer carbides with higher volume fraction and more uniform distribution, and (ii) population density and the size of secondary carbide particles significantly increases with holding time up to a critical duration at 77 K in cryotreatment. The latter observation indicates the pioneering direction towards optimization of cryotreatment design for techno-economic benefit.

Molecular dynamics simulation of diffusion in supercooled Cu–Zr alloys

Abstract: Molecular dynamics (MD) simulations of diffusion in Cu–Zr alloys in their liquid and supercooled liquid states were performed using a recently developed Finnis–Sinclair many-body interatomic potential. To help assess how well the interatomic potential describes the energetics of the Cu–Zr system, the liquid structure determined by MD simulations was compared with wide-angle X-ray scattering measurements of the liquid structure for a Cu64.5Zr35.5 alloy. Diffusion was examined as a function of composition, pressure and temperature. The simulations reveal that the diffusion exhibits strong compositional dependence, with both species exhibiting minimum diffusivities at ∼70% Cu. Moreover, the MD simulations show that the activation volumes for Zr and Cu atoms exhibit a maximum near 70% Cu. Evidence is obtained that the glass transition temperature also changes strongly with composition, thereby contributing to the diffusion behaviour. The relationship between this minimum in diffusion and the apparent best gla...

Resonant ultrasound spectroscopy measurement of Young's modulus, shear modulus and Poisson's ratio as a function of porosity for alumina and hydroxyapatite

Abstract: Modulus–porosity relationships are critical for engineered bone tissue scaffold materials such as hydroxyapatite (HA), where porosity is essential to biological function. Resonant ultrasound spectroscopy (RUS) measurements revealed that the Young's modulus, E, and shear modulus, G, of both alumina and HA decrease monotonically with increasing volume fraction porosity, P, for 0.06 < P < 0.39 (alumina) and 0.05 < P < 0.51 (HA). Although the elastic moduli of porous materials have been measured by a number of different ultrasonic resonance techniques (of which the RUS technique is one example) and over the last decade the elastic moduli of many solids have been measured by the RUS technique, this study is the first systematic RUS study of porous materials. Comparison of E versus P data for alumina (which has been studied extensively) with literature data from several measurement techniques indicates the RUS technique is effective for modulus–porosity measurements. Another key result is that although the HA s...

Strain rates in molecular dynamics simulations of nanocrystalline metals

Abstract: To study deformation mechanisms using molecular dynamics, very high strain rates have to be applied. The effect of lowering the strain rate by two orders of magnitude on the deformation characteristics of nanocrystalline Al was investigated. For the highest strain rate, the onset of dislocation propagation is delayed, resulting in a stress overshoot. With decreasing strain rate the grain-averaged resolved shear stress reduces and cross-slip occurs more frequently. However, even at the lowest applied strain rate the grain boundary network can not accommodate all arriving dislocations, illustrating the challenge to determine the rate-limiting deformation mechanisms for experimental conditions.

Deformation behavior of silver submicrometer-pillars prepared by nanoimprinting

Abstract: Scaling in material properties is of great importance in microsystems and microelectronics where the device dimensions continuously shrink. Recently, compression tests of micron-sized pillars produced using a focused-ion beam (FIB) have become standard in the investigation of scaling effects. The influence of the fabrication process on the mechanical properties of the samples has, however, not been conclusively resolved. In this study, 130 silver pillars were fabricated using a novel embossing technique that does not pose the issues associated with FIB milling, i.e. surface amorphization and gallium contamination. Displacement-controlled compression tests on pillars with diameters of 130–3000 nm reveal for submicrometer samples an inverse proportionality of flow strength to diameter, which is associated with a wide strength distribution and a deformation taking place in large discrete strain bursts. The largest pillars show instead near bulk-like behavior. Unlike studies on other fcc materials, mechanical...

Void growth in bcc metals simulated with molecular dynamics using the Finnis–Sinclair potential

Abstract: The process of fracture in ductile metals involves the nucleation, growth, and linking of voids. This process takes place both at the low rates involved in typical engineering applications and at the high rates associated with dynamic fracture processes such as spallation. Here we study the growth of a void in a single crystal at high rates using molecular dynamics (MD) based on Finnis–Sinclair interatomic potentials for the body-centred cubic (bcc) metals V, Nb, Mo, Ta, and W. The use of the Finnis–Sinclair potential enables the study of plasticity associated with void growth at the atomic level at room temperature and strain rates from 109/s down to 106/s and systems as large as 128 million atoms. The atomistic systems are observed to undergo a transition from twinning at the higher end of this range to dislocation flow at the lower end. We analyse the simulations for the specific mechanisms of plasticity associated with void growth as dislocation loops are punched out to accommodate the growing void. W...

Amorphous silicon carbide photoelectrode for hydrogen production directly from water using sunlight

Abstract: Results on the use of amorphous silicon carbide (a-SiC:H) as a photoelectrode in an integrated ‘hybrid’ photoelectrochemical (PEC) cell to produce hydrogen directly from water using sunlight are presented. A high quality a-SiC:H thin film with bandgap >2.0eV was fabricated by RF-PECVD technique using a SiH4, H2 and CH4 gas mixture. Incorporation of the carbon in the a-SiH film not only increased the bandgap, but also led to an increase in corrosion resistance in an electrolyte. Adding H2 during the fabrication process led to a decrease in the density of states (DOS) in the films. Immersing the a-SiC:H(p)/a-SiC:H(i) structure in a pH 2 electrolyte showed excellent durability up to 100 h (so far tested); the photocurrent increased and the photocurrent onset shifted anodically after the 100-h durability test, which is encouraging. It was also found that a SiO x layer is formed on the surface of a-SiC:H when exposed to air, which led to a decrease in the photocurrent, and the photocurrent onset shifted cathod...

Influence of field evaporation on Radial Distribution Functions in Atom Probe Tomography

Abstract: This paper examines the extraction of structural information in the form of Radial Distribution Functions (RDFs) using Atom Probe Tomography (APT) data. These functions are generated in a highly efficient manner, thus allowing for the analysis of large data sets typical of APT. Experimental RDF calculations were performed for crystalline aluminium and a Mg65Cu25Y10 bulk metallic glass. For the pure aluminium sample, significant pair distance information was extracted, the quality of which was found to vary throughout the data set. Through a novel analysis procedure, the measured total RDF was used to map the local pair distance quality about each reconstructed atom. Surprisingly, the RDF quality maps indicated improved pair distance quality around poles and zone lines. In the case of the metallic glass, however, significant pair correlations were not discernible within the data set, despite short-range ordering being observed using TEM diffraction. The lack of correlations is thought to be associated with...

Nanostructured silicon and its application to solar cells, position sensors and thin film transistors

Abstract: This paper reports the performance of small area solar cells, 128 linear integrated position sensitive detector arrays and thin film transistors based on nanostructured silicon thin films produced by plasma-enhanced chemical vapour deposition technique, close to the onset of dusty plasma conditions, within the transition region from amorphous to microcrystalline. The small area solar cells, produced in a modified single chamber reactor, exhibited very good electrical characteristics with a conversion efficiency exceeding 9%. The 128 integrated position sensitive detector arrays, based on a similar pin structure, allow real-time 3D object imaging with a resolution higher than 90 l p/mm. The thin film transistors produced exhibited field effect mobility of 2.47 cm2/V/s, threshold voltage of 2 V, on/off ratio larger than 107 and sub-threshold slopes of 0.32 V/decade, which are amongst the best results reported for this type of device.

Elemental partitioning between α and β phases in the Ti–5Al–5Mo–5V–3Cr–0.5Fe (Ti-5553) alloy

Abstract: Using atom probe tomography, the partitioning of alloying elements between α and β in the alloy Ti metal-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe) has been investigated as a function of heat-treatment. It has...

Adhesion of small spheres

Abstract: Bradley 1 calculated the adhesive force between rigid spheres to be 2πRΔγ, where Δγ is the surface energy of the spheres. Johnson et al. (JKR) [2] calculated the adhesive force between elastic spheres to be (3/2) πRΔγ and independent of the elastic modulus. Derjaguin et al. [3] published an alternative theory for elastic spheres (DMT theory), and concluded that Bradley's value for the pull-off force was the correct one. Tabor [4] explained the discrepancy in terms of the range of action of the surface forces, z 0, and introduced a parameter μ≡(RΔγ²/E²z 0³)1/3, determining which result is applicable. Subsequently, detailed calculations by Derjaguin and his colleagues [5] and others, assuming a surface force law based on the Lennard-Jones 6–12 potential law, covered the full range of the Tabor parameter. Greenwood and Johnson [6] presented a map delineating the regions of applicability of the different theories. Yao et al. [7] repeated the numerical calculations but using an exact sphere shape instead of th...

Physics of the Lindemann melting rule

Abstract: We investigate the thermodynamics of melting for 74 distinct chemical elements including several actinides and rare earths. We find that the observed melting points are consistent with a linear relationship between the correlation entropy of the liquid and the Gruneisen constant of the solid, and that the Lindemann rule is well obeyed for the elements with simple structures and less well obeyed for the less symmetric more open structures. No special assumptions are required to explain the melting points of the rare earths or light actinides.

Structural properties and magnetic interactions in martensitic Ni-Mn-Sb alloys

Abstract: We study the structural and magnetic properties of Heusler based Ni50Mn50−x Sb x in the composition range 5.0 ≤ x ≤ 25.0. The cubic phase is preserved in the range 17.0 ≤ x ≤ 25.0, and the presence of martensitic transformations are found in alloys with x ≤ 15.0. In a critical composition range 12.5 ≤ x ≤ 10.0, the magnetic coupling in both austenitic and martensitic phases is paramagnetic, however, the presence of antiferromagnetic short-range correlations is observed in both phases. We discuss the consequences of these correlations on the behavior of the magnetic moment with respect to electronic properties.

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

Abstract: By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration....

The gauge theory of dislocations: Static solutions of screw and edge dislocations

Abstract: We investigate the T(3)-gauge theory of static dislocations in continuous solids. We use the most general linear constitutive relations in terms of the elastic distortion tensor and dislocation density tensor for the force and pseudomoment stresses of an isotropic solid. The constitutive relations contain six material parameters. In this theory, both the force and pseudomoment stresses are asymmetric. The theory possesses four characteristic lengths l1, l2, l3 and l4, which are given explicitly. We first derive the three-dimensional Green tensor of the master equation for the force stresses in the translational gauge theory of dislocations. We then investigate the situation of generalized plane strain (anti-plane strain and plane strain). Using the stress function method, we find modified stress functions for screw and edge dislocations. The solution of the screw dislocation is given in terms of one independent length l1 = l4. For the problem of an edge dislocation, only two characteristic lengths l2 and ...

Elasto-hydrodynamics of quasicrystals

Abstract: This work suggests the equations on elasto-hydrodynamics of quasicrystals and as an application gives some solutions of dislocation dynamics for three-dimensional icosahadral, cubic and one-dimensional hexagonal quasicrystals in terms of perturbation and variation methods. Some comparisions on the results to those given by other models have been discussed.

Hardness of materials at high temperature and high pressure

Abstract: The intrinsic character of the correlation between hardness and thermodynamic properties of solids has been established. The proposed thermodynamic model of hardness allows one to easily estimate hardness and bulk moduli of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements or on the enthalpy at the melting point. The correctness of this approach is illustrated by an example of the recently synthesized superhard diamond-like BC5 and orthorhombic modification of boron, γ-B28. The pressure and/or temperature dependences of hardness were calculated for a number of hard and superhard phases, i.e. diamond, cBN, B6O, B4C, SiC, Al2O3, β-B2O3 and β-rh boron. Excellent agreement between experimental and calculated values is observed for temperature dependences of Vickers and Knoop hardness. In addition, the model predicts that some materials can become harder than diamond at pressures in the megabar range.