Interface Orientation Research Articles

Bulklike band-offset mystery solved through energy minimization: Lessons from perovskite oxide heterojunctions

The formation of band offset (BO) at isovalent semiconductor heterojunctions has been branded ``bulklike'' because of an insensitivity of the BO to the interface orientation and atomic structure and a transitivity in BOs. Even though tunability and nontransitivity of BO are frequently found for heterovalent interfaces, empirical theories with built-in bulklike characteristics have thus far dominated the explanation of experimental BOs. Presently, the distribution of charge density and the formation of BO at a large number of interfaces between lattice-matched perovskite oxides are studied in detail using density functional theory. Ionic screening is found to dominate the formation of the BO, as a sharp dependence of the (apparently tunable) BO on atomic structure for unrelaxed interfaces is essentially washed out upon lattice relaxation. Numerical experimentation with deliberate embedding of dipolar layers in perovskite oxides and their interfaces corroborates the effectiveness of ionic screening. The relaxed, converged (bulklike) BOs are found to be in good agreement with the prediction of the neutral polyhedra theory (NPT). The success of the NPT, presently for ionic interfaces and previously for covalent zinc-blende interfaces, unmasks a possible connection between the partition into neutral symmetric cells and the energy-minimization requirement on the interface charge distribution. The independence of the BO on interface specifics, i.e., bulklike behavior, is shown to stem directly from such a property of charge distribution with minimized electrostatic energy. As energy minimization governs the formation of charge distribution in general, NPT is expected to describe the band offset of a wide variety of material interfaces.

A predicted-Newton's method for solving the interface positioning equation in the MoF method on general polyhedrons

Solving the interface positioning equation is the key procedure of the PLIC-VoF methods. Most of previous research only focused on the planar constant calculation and paid less attention to the relationship between the planar constant and the approximate interface orientation. The latter issue is important especially for the second order iteration based PLIC-VoF method, such as the MoF and LVIRA method. In these methods, the most accurate interface orientation is calculated through an iterative procedure, so the interface positioning equation has to be solved multiple times for the given volume fraction with different interface orientations. In this situation, if the incremental relation between the planar constant and the interface orientation is known, a predicted planar constant can be estimated. In this paper, we deduce the analytical partial derivatives of the planar constant with respect to the interface orientation and use them to predict the planar constant. A predicted-Newton's method is proposed to solve the interface positioning equation which takes the predicted planar constant as the initial guess. A great deal of numerical tests are also presented in this paper to verify the robustness of the new scheme. The efficiency of the proposed predicted-Newton's method is compared with the commonly used secant/bisection method by Ahn and Shashkov, and the numerical results indicate that the new method can reduce the iteration steps by 60%∼66% in solving the interface positioning equation and reduce the CPU time by 32%∼39% when implemented in the MoF method.

Representative volume element (RVE) based crystal plasticity study of void growth on phase boundary in titanium alloys

Crystal plasticity based finite element method (CPFEM) studies have been successfully used to model different material behaviour and phenomenon, including but not limited to; fatigue, creep and texture evolution. This capability can be extended to include the ductile damage and failure in the model. Ductile failure in metals is governed by void nucleation, growth, and coalescence. High strength titanium alloys can be formed from sheets and components and are prone to ductile failure. α – β Titanium alloys are in widespread use, ranging from aerospace, automotive, energy to oil and gas. They have multiple phases present in the microstructure but α and β phases are dominant and are present in various morphologies. This study focuses on the 3D representative volume element (RVE) simulations of spherical void of known initial porosity at the interface of α and β phase single crystals. The effect of initial porosity, applied triaxiality and orientation of RVE with respect to the loading direction is investigated. Slip based crystal plasticity formulation implemented as a user subroutine in commercially available software was used to simulate the void growth and the results of the same are presented. Lastly, a generalised correlation among loading type, loading direction, crystal orientation, phase interface orientation, and void growth is presented.

Predicting crack patterns at bi-material junctions: A coupled stress and energy approach

Failure of bi-material joints is affected significantly by the elastic material properties of both adherends as well as by geometrical parameters such as material interface orientation. Addressing these effects, scarf joint specimens made of a polymer foam and either PMMA or aluminum are examined employing a four-point bending test setup with varying scarf angle. The joints’ effective strengths are predicted using a coupled stress and energy criterion within the concept of finite fracture mechanics defining failure as spontaneous formation of finite sized cracks. Since numerous competing crack configurations are admissible, efficient modelling is crucial. Crack initiation triggered by the singular stress field at the bi-material junction is modelled using a semi-analytical approach based on the method of matched asymptotic expansions. In order to accurately capture stresses in the vicinity of the notch and energy dissipation due to crack onset, higher order terms associated to complex deformation modes are considered in the asymptotic expansion. Results based on the asymptotic approach are compared to numerical reference data. Theoretical predictions of the effective joint strength as well as the orientation of the associated initiating cracks are validated against experimental findings from literature.

Microstructural Evolution of Superelasticity in Shape Memory Alloys

Superelasticity in shape memory alloys (SMAs) is an important feature which caused by martensitic transformation and microstructural evolution. The composition of crystal variants and microstructures during the superelasticity process are simulated by molecular dynamics in the current work. Then, a computational post-processing scheme, which can identify variants type and interface orientation are proposed. This method reveals that the crystal adopts a multi-phase mixture during the superelsticity process, including many crystal systems, such as orthorhombic (O), R-phase (R), monoclinic (M) and body-centered-orthorhombic (BCO). In addition, the interface normal vectors between two different variants are examined by the compatibility equation, and the result has good agreement. The stress–strain curve, volume fraction for present variants and total energy diagram are illustrated. The microstructural evolution shows that R phase can serve as the transitional region between pairs of BCO variants. The variants O, BCO, M phase coherent between each other with specific pattern to form twinned arrangement. Furthermore, different random seeds for the initial velocity of the atoms, are used in the simulation to obtain equivalent microstructural evolution paths. The corresponding macroscopic properties are analyzed. The microstructural evolution path can have different energy barrier of initialization, which also leads to different present variant in the crystal. The results discovered in the current work are expected to provides design guidelines for the applications in SMAs.

Simulation of free surface flows with non-hydrostatic pressure distribution

In this work, a free surface flow simulator is developed in which Navier–Stokes equations using Marker and Cell (MAC) method in the framework of finite difference methodology have been solved. The free surface is tracked by the volume of fluid (VOF) method. The numerical code without free-surface is validated against the flow past a square cylinder. Three different free surface flows, i.e., dam break flow, two-dimensional cavity filling and undular bore, are studied to demonstrate the efficacy of the developed numerical model to simulate free surface flow. The numerical model used in the present work involves tracking of free surface of a single fluid in a two-fluid system. The parameters which can affect the interface orientation of the fluid is given as boundary condition at the interface. The inherent advantage of such numerical models is its ability to track free surface for high density and viscosity ratio fluids like air–water. The numerical model used in the present work is capable of solving such high density ratio two-fluid systems for which the effects due to surface tension are negligible. Results from all the problems are compared with earlier results available in literature.

Effects of alloying on the interface energy of the δ- and η-phase in nickelbase superalloys

Wrought 718-type nickel-base superalloys are forged at temperatures where the strengthening phases ( or ) are dissolved, but additional phases (δ-phase of Ni3Nb type or the η-phase of Ni3Ti-type) are present to ensure a fine-grained microstructure. The precipitation of these phases is influenced by the interfacial energy. We use density functional theory calculations at 0 K to study the interfacial energy of the δ- and η-phase with a coherent interface orientation. By transferring alloying elements from the center of a phase to the boundary, we calculate the influence of these elements on the interface energy. A simple model that uses the change in volume caused by the alloying element and its chemical binding energy in the different phases is presented and shown to be able to reasonably predict the interface transfer energy of the elements in most cases.

Discontinuous Phase Transitions in Nonlocal Schloegl Models for Autocatalysis: Loss and Reemergence of a Nonequilibrium Gibbs Phase Rule.

We consider Schloegl models (or contact processes) where particles on a square grid annihilate at a rate p and are created at a rate of k_{n}=n(n-1)/[N(N-1)] at empty sites with n particles in a neighborhood Ω_{N} of size N. Simulation reveals a discontinuous transition between populated and vacuum states, but equistable p=p_{eq} determined by the stationarity of planar interfaces between these states depends on the interface orientation and on Ω_{N}. The behavior for large Ω_{N} follows from continuum equations. These also depend on the interface orientation and on Ω_{N} shape, but a unique p_{eq}=0.211 376 320 4 emerges imposing a Gibbs phase rule.

Effect of interface fiber angle on the mode I delamination growth of plain woven glass fiber-reinforced composites

In the present study, the effect of the fiber angle orientation of the delamination interface on the initiation and the propagation fracture toughness of plain woven composites with stacking sequences of [012//012], [011/30//0/011], [011/45//0/011] and [011/30//−30/011], where “//” stands for the location of starter delamination in DCB specimens, under mode I loading was investigated. These stacking sequences were chosen so as to eliminate the effects of the remote ply orientation on the delamination behavior of the double cantilever beam (DCB) specimens. The samples were manufactured using the wet hand lay-up method and the fracture tests were conducted on specimens using the universal testing machine according to ASTM standards. The experimental results showed that the interface ply orientation had a negligible effect on the magnitude of initiation and propagation fracture toughness of plain woven composites due to the delamination propagation in the resin-fiber interface of delamination. Experimental investigations of fracture surface demonstrated the impact of different mechanisms on the delamination propagation, where the crack propagation in the resin-fiber interface proved to be one of the primary mechanisms for increasing the fracture toughness of such specimens. In addition, the experimental observations revealed that the fiber bridging was not an influential mechanism for increasing the fracture toughness during the delamination propagation, unlike the unidirectional DCB specimens. In order to predict the experimental load-displacement curve, the traction-separation curve was obtained using the J-integral approach and experimental data. Then, using the obtained curve, the crack initiation and propagation of DCB specimens was simulated by using the Abaqus finite element package. The numerical results indicated that having outputs of the 0//0 interface is enough to predict the load-displacement curve of other interfaces.

Unprecedented Bistability in Spin-Crossover Solids Based on the Retroaction of the High Spin Low-Spin Interface with the Crystal Bending.

There has been in recent years a continuous increase in spatiotemporal investigations of the dynamics of the first-order transitions in spin-crossover (SCO) solids. In single crystals, this phenomenon proceeds via a single domain nucleation and propagation, characterized in some systems with the presence of two equivalent and symmetric interface orientations, between the high-spin (HS) and low-spin (LS) phases, due to the anisotropic structural change of the unit cell at the transition. The present investigations bring an experimental evidence of the reversible driving of the translational and rotational degrees of freedom of the HS-LS interface. In addition to its rectilinear displacement, the interface rotates between two stable angles, 60o and 120o. It is demonstrated that while the translation motion is accompanied by a crystal's length change, the interface rotation is controlled by the crystal's bending. These results are well-explained in the frame of an elastic theoretical description in which the effect of the crystal bending, on the stability of the interface's orientation, is simulated by applying a moment of forces on the crystal. It is found that the interface orientation becomes unstable beyond a threshold load value, announcing the emergence of a bistability in SCO solids, taking place at constant HS fraction and volume. This work underlines the sensitive character of the interface orientation to any macroscopic crystal bending, an idea that can be used to develop a new generation of robust stress sensors, working at constant volume, thus avoiding deterioration problems due to crystal fatigue.

Cohesive laws for shearing of iron/precipitate interfaces

The behaviour of 11 differently oriented iron-precipitate interfaces under a shear load is studied with molecular dynamics simulations. We find that the behaviour depends not only on the interface orientation but also on the shear direction. Furthermore, for many interfaces the presence of a dislocation at the interface triggers a structure change in the interface, and with that completely modifies the shear behaviour. Several interface characteristics are inspected for their possibly decisive influence on the observed loading curves. However, none of these characteristics is found to correlate conclusively with the shear behaviour of the different interfaces. This indicates that actual shear behaviour is rooted in a deeper level of complexity than just depending on the properties of the initial interface. Clearly the time evolution of the interface during shearing is crucial. From the observations a comprehensive cohesive law is derived that represents the shear behaviour for every interface and for both shear directions. This cohesive law can be used in numerical methods at a larger length sale, such as discrete dislocation plasticity.

In pursuit of barrierless transition metal dichalcogenides lateral heterojunctions

There is an increasing need to understand interfaces between two-dimensional materials to realize an energy efficient boundary with low contact resistance and small heat dissipation. In this respect, we investigated the impact of charge and substitutional atom doping on the electronic transport properties of the hybrid metallic-semiconducting lateral junctions, formed between metallic (1T and 1Td) and semiconducting (1H) phases of MoS2 by means of first-principles and non-equilibrium Green function formalism based calculations. Our results clearly revealed the strong influence of the type of interface and crystallographic orientation of the metallic phase on the transport properties of these systems. The Schottky barrier height, which is the dominant mechanism for contact resistance, was found to be as large as 0.63 eV and 1.19 eV for holes and electrons, respectively. We found that armchair interfaces are more conductive as compared to zigzag termination due to the presence of the metallic Mo zigzag chains that are directed along the transport direction. In order to manipulate these barrier heights we investigated the influence of electron doping of the metallic part (i.e. 1Td-MoS2). We observed that the Fermi level of the hybrid system moves towards the conduction band of semiconducting 1H-MoS2 due to filling of 4d-orbital of metallic MoS2, and thus the Schottky barrier for electrons decreases considerably. Besides electron doping, we also investigated the effect of substitutional doping of metallic MoS2 by replacing Mo atoms with either Re or Ta. Due to its valency, Re (Ta) behaves as a donor (acceptor) and reduces the Schottky barrier for electrons (holes). Since Re and Ta based transition metal dichalcogenides crystallize in either the 1Td or 1T phase, substitutional doping with these atom favors the stabilization of the 1Td phase of MoS2. Co-doping of hybrid structure results in an electronic structure, which facilities easy dissociation of excitons created in the 1H part.

Effect of erosion angle and Fe2B orientation on cavitation erosion and interfaces of Fe-B alloy in high-velocity flowing zinc

The effects of erosion angle and Fe2B orientation on cavitation erosion and interface structures of a directionally solidified (DS) Fe-B alloy in flowing liquid zinc were investigated to clarify erosion mechanism of liquid metal. The results indicate that the cavitation erosion rate of DS Fe-B alloy with Fe2B [001] orientation vertical to interface exhibits better erosion cavitation resistance. Erosion angle can strongly affect erosion performance that depends not only on interfacial orientation structure but also on local microturbulence. Erosion morphologies manifest that an obvious cavitation erosion occurs in strong flowing zinc disturbed and agitated zone, which results in severe cavitation pits and slip deformation of products, thus stimulating subsequent localized corrosion and pit-aggregation cracks. Cavitation erosion craters coupled with performance variations confirm that microturbulence-assisted fluid eddies and backflow are responsible for the cavitation pits. The combined effects of flow pattern and interface orientation significantly govern the epitaxial grown ζ removal/accumulation behaviors and multiphase pinning film adhesion. A model of hydraulic liquid-hammer action by cavitation-induced microjet and interface orientation interaction is proposed and discussed to account for cavitation pits and slip cracking, which reveals underlying erosion mechanism and material design of DS Fe-B alloy in flowing liquid zinc.

Effects of printing-induced interfaces on localized strain within 3D printed hydrogel structures

Additive manufacturing, or 3D printing, is a promising approach for the fabrication of biological structures for regenerative medicine applications using tissue-like materials such as hydrogels. Herein, inkjet printing is implemented as a model droplet-based 3D printing technology for which interfaces have been shown to form between printed lines within printed layers of hydrogel structures. Experimental samples with interfaces in two orientations are fabricated by inkjet printing and control samples with and without interfaces are fabricated by extrusion printing and casting, respectively. The formation of partial and full interfaces is modeled in terms of printing conditions and gelation parameters, and an approach to predicting the ratio of interfacial area to the total contact area between two adjacent lines is presented. Digital image correlation is used to determine strain distributions and identify regions of increased localized deformation for samples under uniaxial tension. Despite the presence of interfaces in inkjet-printed samples, strain distributions are found to be homogeneous regardless of interface orientation, which may be attributed to the multi-layer nature of samples. Conversely, single-layer extrusion-printed samples exhibit localized regions of increased deformation between printed lines, indicating delamination along interfaces. The effective stiffness, failure strength, and failure strain of inkjet-printed samples are found to be dependent on the orientation of interfaces within layers. Specifically, inkjet-printed samples in which tensile forces pull apart interfaces exhibit significantly decreased mechanical properties compared to cast samples.

Assessing the mechanisms of common Pb incorporation into titanite

Common Pb, the portion of non-radiogenic Pb within a U bearing mineral, needs to be accurately accounted for in order to subtract its effect on U-Pb isotopic ratios so that meaningful ages can be calculated. The propensity to accommodate common Pb during crystallization, or later, is different across the range of U bearing minerals used for geochronology. Titanite frequently accommodates significant amounts of common Pb. However, the most appropriate method to correct for this requires knowledge on the mechanism and timing of common Pb incorporation; information that is commonly difficult to extract. In this study, the spatial and compositional distribution of trace elements (including Pb) in metamorphic titanites from a Greenland amphibolite is investigated on the grain- to nano-scale. Titanites have an isotopically similar signature for both common and radiogenic-Pb in all grains but significantly different quantities of the non-radiogenic component. Microstructural and compositional examination of these grains reveals undeformed, but high common Pb (F207%) titanites have homogeneous element distributions on the atomic scale suggesting common Pb is incorporated into titanite during its growth and not during later processes. In contrast, deformed titanite comprising low-angle boundaries, formed by subgrain rotation recrystallization, comprise networks of dislocations that are enriched in Mg, Al, K and Fe. Smaller cations may migrate due to elastic strain in the vicinity of the dislocation network, yet the larger K cations more likely reflect the mobility of externally-derived K along the orientation interface. The absence of Pb enrichment along the boundary indicates that either Pb was too large to fit into migrating lattice dislocations or static low-angle boundaries and/or that there was no external Pb available to diffuse along the grain boundary. As the common Pb composition is distinctly different to regional Pb models, the metamorphic titanite grew in a homogeneous Pb reservoir dominated by the break-down of precursor U-bearing phases. The different quantity of common Pb in the titanite grains indicates a mineral-driven element partitioning in an isotopically homogeneous metamorphic reservoir, consistent with low U, low total REE and flat LREE signatures in high F207% analyses. These results have implications for the selection of appropriate common Pb corrections in titanite and other accessory phases.

Deformation response of cube-on-cube and non-coherent twin interfaces in AgCu eutectic under dynamic plastic compression

Split-Hopkinson pressure bar dynamic compression experiments were conducted to determine the defect/interface interaction dependence on interface type, bilayer thickness and interface orientation with respect to the loading direction in the Ag-Cu eutectic system. Specifically, the deformation microstructure in alloys with either a cube-on-cube orientation relationship with {111}Ag||{111}Cu interface habit planes or a twin orientation relationship with {3̅13}Ag||{1̅12}Cu interface habit planes and with bilayer thicknesses of 500nm, 1.1µm and 2.2µm were probed using TEM. The deformation was carried by dislocation slip and in certain conditions, deformation twinning. The twinning response was dependent on loading orientation with respect to the interface plane, bilayer thickness, and interface type. Twinning was only observed when loading at orientations away from the growth direction and decreased in prevalence with decreasing bilayer thickness. Twinning in Cu was dependent on twinning partial dislocations being transmitted from Ag, which only occurred for cube-on-cube interfaces. Dislocation slip and deformation twin transfer across the interfaces is discussed in terms of the slip transfer conditions developed for grain boundaries in FCC alloys.

Determining device position through minimal user input

In many co-located, collaborative systems there is a need for the constituent devices used to be aware of the physical positions of their networked counterparts. This paper addresses this challenge by presenting a novel method of utilising users’ judgement of direction to obtain the location and orientation of a touch interface. The technique requires a user to draw several arrows on an interface which point towards physical landmarks in an environment. This allows for the setup of interface locations in a way which is (i) quick; (ii) inexpensive; (iii) not encumbering; and (iv) capable of being performed despite obstructions in the environment. A user study is presented which investigates what influence a user’s accuracy has on the technique’s resulting calculated location of an interface. The study reveals that the magnitude of a user’s inaccuracies is proportional to the size of the error in the result and that there is no improvement in user accuracy with practice. Finally, we make observations on the future extension and application of this technique.

Microstructural and Mechanical Implications of Microscaled Assembly in Droplet-based Multi-Material Additive Manufacturing.

To reveal the potential and limits of multi-material three-dimensional (3D) printed parts in droplet-based additive manufacturing, a study combining tensile experiments and 3D imaging technique is proposed. A polymeric composite structure made of acrylonitrile butadiene styrene and thermoplastic polyurethane is manufactured using a two extrusion head printer. The quality of the interface between the two thermoplastics is quantified by adjusting the number of intertwining droplets at the interface. Tensile experiments assisted with digital image correlation are performed with two-interface orientation to discriminate shearing and traction at the interface. The 3D imaging results, which are based on X-ray micro-tomography, show the distinct features of droplet-based additive manufacturing in terms of porosity content and connectivity. Interface properties are found to control, in an incomparable way, the mechanical response. It is found that the interface quality is determinant for enhancing the ultimate performance whereas the interface orientation is found to be the perfect leverage for varying the slope of the linear part.

A molecular dynamics study of the nucleus interface structure and orientation relationships during the austenite-to-ferrite transformation in pure Fe

ABSTRACTMolecular dynamics simulations using an embedded-atom method potential for pure iron (Fe) were performed to investigate the solid-state transformation of a body-centred-cubic phase from a polycrystalline face-centred-cubic matrix. A Kurdjumov–Sachs orientation relationship accompanied with a step-ledge disconnection structure was detected in high mobility phase boundaries of a full 3D classical and barrier-free nucleation specimen. The results imply that interface coherency is an important factor for both nucleation and growth processes, which may significantly affect the transition rate of the austenite–ferrite transformation.

Geometrical aspects of turbulent/non-turbulent interfaces with and without mean shear

The geometry of turbulent/non-turbulent interfaces (TNTIs) arising from flows with and without mean shear is investigated using direct numerical simulations of turbulent planar jets (PJET) and shear free turbulence (SFT), respectively, with Taylor Reynolds number of about Reλ≈100. In both flows, the TNTI is preferentially aligned with the tangent to the TNTI displaying convex, where the turbulent fluid nearby tends to have a stronger enstrophy, more frequently than concave shapes. The different flow configurations are reflected in different orientations of the TNTI with respect to the flow direction (and its normal). While the interface orientation with respect to the mean flow direction in PJET has an influence on the velocity field near the TNTI and the enstrophy production in the turbulent sublayer, there is no particular discernible dependence on the interface orientation in SFT. Finally, the intense vorticity structures or “worms,” which are possibly associated with “nibbling” entrainment mechanism, “feel” the local geometry of the TNTI, and it is shown that in PJET, a smaller local radius of these structures arises in regions near the TNTI where the local TNTI faces the mean flow direction.