Orwik

Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel.

A precipitation hardened maraging TRIP steel was analyzed using a pulsed laser atom probe. The laser pulse energy was varied from 0.3 to 1.9nJ to study its effect on the measured chemical compositions and spatial resolution. Compositional analyses us... expand abstracting proximity histograms did not show any significant variations in the average matrix and precipitate compositions. The only remarkable change in the atom probe data was a decrease in the ++/+ charge state ratios of the elements. The values of the evaporation field used for the reconstructions exhibit a linear dependence on the laser pulse energy. The adjustment of the evaporation fields used in the reconstructions for different laser pulse energies was based on the correlation of the obtained cluster shapes to the TEM observations. No influence of laser pulse energy on chemical composition of the precipitates and on the chemical sharpness of their interfaces was detected. collapse abstract

Atom probe tomography characterization of heavily cold drawn pearlitic steel wire.

Atom Probe Tomography (APT) was used to analyze the carbon distribution in a heavily cold drawn pearlitic steel wire with a true strain of 6.02. The carbon concentrations in cementite and ferrite were separately measured by a sub-volume method and co... expand abstractmpared with the literature data. It is found that the carbon concentration in ferrite saturates with strain. The carbon concentration in cementite decreases with the lamellar thickness, while the carbon atoms segregate at dislocations or cell/grain boundaries in ferrite. The mechanism of cementite decomposition is discussed in terms of the evolution of dislocation structure during severe plastic deformation. collapse abstract

Thermal stability of TiAlNCrN multilayer coatings studied by atom probe tomography.

This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission el... expand abstractectron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600°C. At 700°C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000°C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. collapse abstract

Efficient and accurate simulations of deformable particles immersed in a fluid using a combined immersed boundary lattice Boltzmann finite element method

The deformation of an initially spherical capsule, freely suspended in simple shear flow, can be computed analytically in the limit of small deformations [D. Barthes-Biesel, J. M. Rallison, The Time-Dependent Deformation of a Capsule Freely Suspend... expand abstracted in a Linear Shear Flow, J. Fluid Mech. 113 (1981) 251-267]. Those analytic approximations are used to study the influence of the mesh tessellation method, the spatial resolution, and the discrete delta function of the immersed boundary method on the numerical results obtained by a coupled immersed boundary lattice Boltzmann finite element method. For the description of the capsule membrane, a finite element method and the Skalak constitutive model [R. Skalak et al., Strain Energy Function of Red Blood Cell Membranes, Biophys. J. 13 (1973) 245-264] have been employed. Our primary goal is the investigation of the presented model for small resolutions to provide a sound basis for efficient but accurate simulations of multiple deformable particles immersed in a fluid. We come to the conclusion that details of the membrane mesh, as tessellation method and resolution, play only a minor role. The hydrodynamic resolution, i.e., the width of the discrete delta function, can significantly influence the accuracy of the simulations. The discretization of the delta function introduces an artificial length scale, which effectively changes the radius and the deformability of the capsule. We discuss possibilities of reducing the computing time of simulations of deformable objects immersed in a fluid while maintaining high accuracy. collapse abstract

Second-order convergence of the deviatoric stress tensor in the standard Bhatnagar-Gross-Krook lattice Boltzmann method

It is shown numerically that the deviatoric stress tensor is second-order accurate in the bulk Bhatnagar-Gross-Krook lattice Boltzmann (LB) method. In an earlier work [T. Krüger et al., Phys. Rev. E 79, 46704 (2009)], we have already predicted the se... expand abstractcond-order convergence. However, numerical simulations using a duct flow were not fully in line with this prediction. In particular, the convergence rate of the stress tensor was observed to depend on the LB boundary condition. In the present paper, we examine a pure bulk system, the decaying Taylor-Green vortex flow. Our prediction on the second-order accuracy of the stress tensor is unambiguously evidenced via these studies. collapse abstract

Lattice Boltzmann study of pattern formation in reaction-diffusion systems

Pattern formation in reaction-diffusion systems is of great importance in surface micro-patterning [Grzybowski et al. Soft Matter. 1, 114 (2005)], self-organization of cellular micro-organisms [Schulz et al. Annu. Rev. Microbiol. 55, 105 (2001)] an... expand abstractd in developmental biology [Barkai et al. FEBS Journal 276, 1196 (2009)]. In this work, we apply the Lattice Boltzmann method (LBM) to study pattern formation in reaction-diffusion systems. As a first methodological step, we consider the case of a single species undergoing transformation reaction and diffusion. In this case, we perform a third-order Chapman-Enskog multiscale expansion and study the dependence of the Lattice Boltzmann truncation error on the diffusion coefficient and the reaction rate. These findings are in good agreement with numerical simulations. Furthermore, taking the Gray-Scott model as a prominent example, we provide evidence for the maturity of the LBM in studying pattern formation in non-linear reaction-diffusion systems. For this purpose, we perform linear stability analysis of the Gray-Scott model and determine the relevant parameter range for pattern formation. Lattice Boltzmann simulations allow not only to test the validity of the linear stability phase diagram including Turing and Hopf instabilities, but also permit going beyond the linear stability regime, where large perturbations give rise to interesting dynamical behavior such as the so called self replicating spots. We also show that the length scale of the patterns may be tuned by rescaling all relevant diffusion coefficients in the system with the same factor while letting all the reaction constants unchanged. collapse abstract

Ab Initio Guided Design of bcc Ternary Mg–Li–X (X  Ca, Al, Si, Zn, Cu) Alloys for UltraLightweight Applications

Ab initio calculations are becoming increasingly important for designing new alloys as these calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis. In this paper, fund... expand abstractamental physical properties (like formation energies and elastic constants) of a set of bcc Mg–Li and Mg–Li-based compounds are calculated using density functional theory (DFT). These DFT-determined properties are in turn used to calculate engineering parameters such as (i) specific Young's modulus (Y/ρ) or (ii) shear over bulk modulus ratio (G/B) differentiating between brittle and ductile behavior. These parameters are then used to identify those alloys that have optimal mechanical properties for lightweight structural applications. First, in case of the binary Mg–Li system, an Ashby map containing Y/ρ versus G/B shows that it is not possible to increase Y/ρ without simultaneously increasing G/B (i.e., brittleness) by changing only the composition of a binary alloy. In an attempt to bypass such a fundamental materials-design limitation, a set of Mg–Li–X ternaries (X = Ca, Al, Si, Cu, Zn) based on stoichiometric Mg–Li with CsCl structure was studied. It is shown that none of the studied ternary solutes is able to simultaneously improve both specific Young's modulus and ductility. collapse abstract

Small droplets on superhydrophobic substrates

We investigate the wetting behavior of liquid droplets on rough hydrophobic substrates for the case of droplets that are of comparable size to the surface asperities. Using a simple three-dimensional analytical free-energy model, we have shown in a r... expand abstractecent letter [M. Gross, F. Varnik, and D. Raabe, EPL 88, 26002 (2009)] that, in addition to the well-known Cassie-Baxter and Wenzel states, there exists a further metastable wetting state where the droplet is immersed into the texture to a finite depth, yet not touching the bottom of the substrate. Due to this new state, a quasistatically evaporating droplet can be saved from going over to the Wenzel state and instead remains close to the top of the surface. In the present paper, we give an in-depth account of the droplet behavior based on the results of extensive computer simulations and an improved theoretical model. In particular, we show that releasing the assumption that the droplet is pinned at the outer edges of the pillars improves the analytical results for larger droplets. Interestingly, all qualitative aspects, such as the existence of an intermediate minimum and the “reentrant transition,” remain unchanged. We also give a detailed description of the evaporation process for droplets of varying sizes. Our results point out the role of droplet size for superhydrophobicity and give hints for achieving the desired wetting properties of technically produced materials. collapse abstract

Anodic oxides on a beta type Nb–Ti alloy and their characterization by electrochemical impedance spectroscopy

Anodic oxides were grown on the surface of an electropolished (Ti–30 at% Nb) beta-titanium (β-Ti) alloy by cyclic voltammetry. The scan rate was 100 mV s−1 between 0 and 8 V in increments of l V in ... expand abstractan acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was carried out right after each anodic oxide growth increment to study the electronic properties of the oxide/electrolyte interface in a wide frequency range from 100 kHz to 10 MHz with an AC perturbation voltage of 10 mV. A film formation factor of 2.4 nm V−1 was found and a relative permittivity number (dielectric constant) of 42.4 was determined for the oxide film formed. Mott–Schottky analysis on a potentiostatically formed 7 nm thick oxide film was performed to assess the semiconducting properties of the mixed anodic oxide grown on the alloy. A flat band potential of −0.47 V (standard hydrogen electrode, SHE) was determined, connected to a donor density of 8.2 × 1017 cm−3. β-Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer α-Ti compound. Its application, however, requires a passivation behaviour comparable or better than α-Ti which in fact is found. collapse abstract

Inertia effects and stress accumulation in a constricted duct: A combined experimental and lattice Boltzmann study

We experimentally and numerically investigate the flow of a Newtonian fluid through a constricted geometry for Reynolds numbers in the range $0.1 - 100$. The major aim is to study non-linear inertia effects at larger Reynolds numbers (>10) on the s... expand abstracthear stress evolution in the fluid. This is of particular importance for blood flow as some biophysical processes in blood are sensitive to shear stresses, e.g., the initialization of blood clotting. We employ the lattice Boltzmann method for the simulations. The conclusion of the predictions is that the peak value of shear stress in the constriction grows disproportionally fast with the Reynolds number which leads to a non-linear shear stress accumulation. As a consequence, the combination of constricted blood vessel geometries and large Reynolds numbers may increase the risk of undesired blood clotting. collapse abstract

Revealing the design principles of high-performance biological composites using ab initio and multiscale simulations: the example of lobster cuticle.

A review of crystallographic textures in chemical vapor-deposited diamond films

Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the... expand abstract texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond. Keywords  CVD diamond films - deposition parameters - texture - EBSD - van der Drift growth selection mechanism collapse abstract

Microtexture and Grain Boundaries in Freestanding CVD Diamond Films: Growth and Twinning Mechanisms

Three groups of free-standing chemical vapor deposition (CVD) diamond films formed with variations in substrate temperature, methane concentration, and film thickness are analyzed using high-resolution electron back-scattering diffraction. Primarily ... expand abstract{001}, {110}, and {111} fiber textures are observed. In addition, corresponding primary and higher order twinning components are found. As interfaces, high angle, low angle, primary twin, and secondary twin boundaries are observed. A growth and a twinning model are proposed based on the sp3 hybridization of the bond in the CH4 molecule that is used as the deposition medium. collapse abstract

Texture prediction from a novel grain cluster-based homogenization scheme

We propose a method, termed relaxed grain cluster (RGC), which homogenizes the response of polycrys- tals, subjected to mechanical loads, from the monocrystal constitutive law in order to predict the evolution of deformation resistance and microstruc... expand abstractture properties, e.g. texture. Generalizing existing grain interaction models, we consider a clus-ter of homogenous grains at each continuum material point. Allowing for additional displacements of the grain interfaces introduces relaxations with respect to the classical full-constraints (FC) Taylor model. This decouples the local grain deformation gradients, but may induce interfacial mismatch between grains. The relaxations are determined as minimizers of the cluster′s total mechanical work density being biased by a (penalty) energy density associated with the interfacial mismatch. In this work the bias is neglected, thus the minimum energy criterium is equivalent to stress equi- librium at each interface. As an example, the evolution of texture for plane-strain compression (simplification for cold rolling) of a commercial aluminum alloy is compared for different configurations of interfacial relaxations. We discuss the resulting variation in texture intensity in light of the different relaxation modes allowed and point out the fully-relaxed RGC scheme to be closest to and in decent agreement with experimental reference. Keywords  Grain interaction - Homogenization method - Texture evolution collapse abstract

Wetting gradient induced separation of emulsions: A combined experimental and lattice Boltzmann computer simulation study

Guided motion of emulsions is studied via combined experimental and theoretical investigations. The focus of the work is on basic issues related to driving forces generated via a step-wise (abrupt) change in wetting properties of the substrate alon... expand abstractg a given spatial direction. Experiments on binary emulsions unambiguously show that selective wettability of the one of the fluid components (water in our experiments) with respect to the two different parts of the substrate is sufficient in order to drive the separation process. These studies are accompanied by approximate analytic arguments as well as lattice Boltzmann computer simulations, focusing on effects of a wetting gradient on internal droplet dynamics as well as its relative strength compared to volumetric forces driving the fluid flow. These theoretical investigations show qualitatively different dependence of wetting gradient induced forces on contact angle and liquid volume in the case of an open substrate as opposed to a planar channel. In particular, for the parameter range of our experiments, slit geometry is found to give rise to considerably higher separation forces as compared to open substrate. collapse abstract

Fall and rise of small droplets on rough hydrophobic substrates

Liquid droplets on patterned hydrophobic substrates are typically observed either in the Wenzel or the Cassie state. Here we show that for droplets of comparable size to the roughness scale an additional local equilibrium state exists, where the dr... expand abstractoplet is immersed in the texture, but not yet contacts the bottom grooves. Upon evaporation, a droplet in this state enters the Cassie state, leading to a qualitatively new self-cleaning mechanism. The effect is of generic character and is expected to occur in any hydrophobic capillary wetting situation where a spherical liquid reservoir is involved. collapse abstract

Effect of aspect ratio on transverse diffusive broadening: a lattice Boltzmann study.

We study scaling laws characterizing the interdiffusive zone between two miscible fluids flowing side by side in a Y-shape laminar micromixer using the lattice Boltzmann method. The lattice Boltzmann method solves the coupled three-dimensional (3D) h... expand abstractydrodynamics and mass transfer equations and incorporates intrinsic features of 3D flows related to this problem. We observe the different power-law regimes occurring at the center of the channel and close to the top/bottom wall. The extent of the interdiffusive zone scales as the square root of the axial distance at the center of the channel. At the top/bottom wall, we find an exponent 1/3 at early stages of mixing as observed in the experiments of Ismagilov [Appl. Phys. Lett. 76, 2376 (2000)]. At a larger distance from the entrance, the scaling exponent close to the walls changes to 1/2 [J.-B. Salmon and A. Adjari, J. Appl. Phys. 101, 074902 (2007)]. Here, we focus on the effect of the finite aspect ratio on diffusive broadening. Interestingly, we find the same scaling laws regardless of the channel's aspect ratio. However, the point at which the exponent 1/3 characterizing the broadening at the top/bottom wall reverts to the normal diffusive behavior downstream strongly depends on the aspect ratio. We propose an interpretation of this observation in terms of the shear rate at the side walls. A criterion for the range of aspect ratios with non-negligible effect on diffusive broadening is also provided. collapse abstract

Designing Ultrahigh Strength Steels with Good Ductility by Combining Transformation Induced Plasticity and Martensite Aging

We present results on precipitation hardened ductile high strength maraging-TRIP steels with up to 1.5 GPa strength and good ductility. The alloys have low carbon content (0.01 wt% C), 9–15 wt% Mn, and additions of Ni, Ti, and Mo (1–2 wt%... expand abstract) for precipitation. Martensite aging leads to a surprising simultaneous increase in strength and total elongation for samples with 9 wt% Mn and 12 wt% Mn. collapse abstract

Shear stress in lattice Boltzmann simulations.

A thorough study of shear stress within the lattice Boltzmann method is provided. Via standard multiscale Chapman-Enskog expansion we investigate the dependence of the error in shear stress on grid resolution showing that the shear stress obtained by... expand abstract the lattice Boltzmann method is second-order accurate. This convergence, however, is usually spoiled by the boundary conditions. It is also investigated which value of the relaxation parameter minimizes the error. Furthermore, for simulations using velocity boundary conditions, an artificial mass increase is often observed. This is a consequence of the compressibility of the lattice Boltzmann fluid. We investigate this issue and derive an analytic expression for the time dependence of the fluid density in terms of the Reynolds number, Mach number, and a geometric factor for the case of a Poiseuille flow through a rectangular channel in three dimensions. Comparison of the analytic expression with results of lattice Boltzmann simulations shows excellent agreement. collapse abstract

On strain gradients and size-dependent hardening descriptions in crystal plasticity frameworks

Strain gradient crystal plasticity models have been quite successful in describing size-dependent deformation. Various formulations have been suggested in the literature involving approaches that vary considerably in principle. In this respect it is ... expand abstractimportant to associate certain classes of material models to specific deformation mechanisms and to investigate possible inherent restrictions. To this end, the scaling relation between dislocation substructures and the order (first order, second order) of strain gradients in the corresponding continuum formulation are discussed. Conclusions for isotropic and kinematic hardening on slip system level and their related orders in the strain gradients are drawn. Keywords  strain gradient plasticity - size effects - crystal plasticity - dislocation density collapse abstract

Influence of Structural Principles on the Mechanics of a Biological FiberBased Composite Material with Hierarchical Organization: The Exoskeleton of the Lobster Homarus americanus

The cuticle of the lobster Homarus americanus is a nanocomposite, such as most structural biological materials. It consists of a matrix of chitin-protein fibers associated with various amounts of crystalline and amorphous calcium carbonate in the rig... expand abstractid parts of the body, and is organized hierarchically at all length scales. One prominent design principle found in the hierarchical structure of such biological fibrous composite materials is the twisted plywood structure. In the lobster cuticle, it is formed by superimposing and gradually rotating planes of parallel aligned chitin-protein fibers. To adjust the mechanical properties to the requirements on the macroscopic level, the spatial arrangement and the grade of mineralization of the fibers can be modified. A second design principle of lobster cuticle is its honeycomb-like structure, generated by the well-developed pore canal system, whose twisted ribbon-shaped canals penetrate the cuticle perpendicular to its surface. Due to the hierarchical structure, the mechanical properties of the lobster cuticle have to be investigated at different length scales, which is essential for the understanding of the structure–mechanical function relations of mineralized tissues (e.g., potentially also bone and teeth). In order to investigate the influence of the structural principles on the mechanical properties on the macroscopic scale miniaturized tensile, compression, and shear tests were carried out to obtain integral mechanical data. Characterization of the microstructure included scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) measurements. collapse abstract

Growth rate and EBSD texture analysis of nitrogen doped diamond films

Chemical-Vapor-Deposition (CVD) diamond films were prepared using a variation of nitrogen addition into the gas source admixture by a direct current CVD method. The influence of nitrogen addition on the crystallographic texture and grain shape evol... expand abstractution in heteroepitaxial polycrystalline diamond films was investigated using high-resolution electron backscattering diffraction (HR-EBSD) and X-ray diffraction (XRD). The analysis reveals that an addition of 1.5% N2 to the CH4 gas flow leads to a strong enhancement of a {110} fiber texture. The phenomenon is discussed in terms of a competitive growth selection mechanism. collapse abstract

Abstract Accepted for publication in Computational Materials Science A Hybrid Model for Mesoscopic Simulation of Recrystallization

A brief summary of simulation techniques for recrystallization is given. The limitations of the Potts model and the Cellular Automaton model as used in their standard forms for grain growth and recrystallization are noted. A new approach based on a h... expand abstractybrid of the Potts model (MC) and the Cellular Automaton (CA) model is proposed in order to obtain the desired limiting behavior for both curvature-driven and stored energy-driven grain boundary migration. collapse abstract

A comparison of polycrystalline elastic properties computed by analytic homogenization schemes and FEM

Body-center-cubic (BCC) magnesium–lithium alloys are a promising light-weight structural material. As a first step in a theoretically guided materials design strategy single crystal elastic coefficients for BCC magnesium-lithium alloys with dif... expand abstractferent compositions were computed using ab initio methods. These single crystal elastic coefficients were then used to predict the corresponding polycrystalline elastic properties using various analytic homogenization techniques (Voigt, Reuss, and a self-consistent approach) as well as the finite element method. As expected, the Voigt and Reuss bounds form the upper and lower bounds on the polycrystalline elastic properties, which the predicted values of the self-consistent approach and finite element approaches fall in between. Additionally, the difference between the polycrystalline elastic properties derived from the self-consistent approach and the finite element method is small illustrating the power and value of the self-consistent approach for non-textured materials. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) collapse abstract

Error propagation in multiscale approaches to the elasticity of polycrystals

The error propagation properties of the Voigt, Reuss, Voigt–Reuss–Hill–Gilvarry, and Hershey schemes for the determination of the integral elastic response of texture free polycrystalline aggregates with cubic structure were studied... expand abstract. The sensitivity of the homogenized polycrystalline shear modulus was tested (i) analytically on the partial derivatives of the shear modulus with respect to the individual elastic constants within extremal Voigt and Reuss schemes, and (ii) numerically for all four methods. The sensitivity of the Hershey shear modulus on the input parameters, the single crystalline elastic constants B, C ′, C44, is shown to be within the limiting values found for the Voigt and Reuss schemes. This conclusion is illustrated numerically on a set of five cubic materials with very different physical properties. The influence of the bulk modulus was found to be approximately two orders of magnitude smaller than that of C ′ and C44. The Hershey modulus was also found to be non-linear, asymmetric, and strongly dependent on the level of the elastic anisotropy of the studied system. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) collapse abstract