Behavior Of Bulk Material Research Articles

Effect of Segregation and Surface Condition on Corrosion of Rheo-HPDC Al–Si Alloys

Corrosion properties of two Al–Si alloys processed by Rheo-high pressure die cast (HPDC) method were examined using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The effects of the silicon content, transverse and longitudinal macrosegregation on the corrosion resistance of the alloys were determined. Microstructural studies revealed that samples from different positions contain different fractions of solid and liquid parts of the initial slurry. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and surface quality.

Modification of implicit algorithm for solving a problem on the elastic plasticity of bulk materials

A mathematical statement is given of the elastic-plastic behavior of isotropic bulk material using a classic model of Drucker-Prager. We have improved the return-mapping algorithm for solving numerically a problem on the mechanical state of bulk material. In order to solve a system of nonlinear equations by the Newton method, it is proposed, at each step of iterations, instead of finding the inverse matrix, to solve a system of algebraic equations, linearized by Newton, by applying the Gauss exclusion method. This makes it possible to reduce the number of arithmetic operations by about 3n2 (n is the dimensionality of SLAE) at each iteration step for each plastic finite element. We have tested the programming code developed on the high-level programming language Fortran on the example of a model material, characterized by the associative law of current, at different values of the angle of natural repose. Comparison of the obtained results of numerical experiment with the data received by applying the proprietary software revealed a deviation within 0.25–5.3 % depending on the desired magnitude

Microstructure and Room Temperature Compressive Deformation Behavior of Cold-Sprayed High-Strength Cu Bulk Material

This study investigated the room temperature compressive deformation behavior of Cu bulk material manufactured by cold spray process. Initial microstructural observation identified a unique microstructure with grain size of hundreds of nm in the particle interface area and relatively coarse grains in all other areas. Room temperature compressive results confirmed cold-sprayed Cu to have a yield strength of 340 MPa, which is similar to materials manufactured by severe plastic deformation process such as equal channel angular press. In addition, strain softening phenomenon, which is rarely found in room temperature compressive deformation, was observed. According to such unique characteristics, continuous microstructure evolution and surface fractures according to the strain (e t = 0.3/0.6/0.9) of the material were observed, and considerations were made for deformation and fracture behavior. Microstructural observation after compressive deformation confirmed that average grain size decreased as the strain increased, and the fraction of the low-angle boundary, which has an indirect relationship with dislocation density, showed a tendency to decrease in e t = 0.3-0.6 region where the strain softening phenomenon occurs. Based on the results described above, this study was able to identify the possibility of manufacturing cold-sprayed Cu bulk material for structural material and its room temperature deformation behavior.

Coupling of FEM and DEM simulations to consider dynamic deformations under particle load

Coupled FEM–DEM simulations enable the direct analysis of the load, the deformation and the stresses inside machine parts which interact with bulk materials. The analysis of large deformations of elastic parts is interesting as the deformation will significantly influence the bulk material behaviour. In this paper a bidirectional coupling method for the FEM software $$\hbox {ANSYS}^\circledR $$ Classic and the DEM software $$\hbox {LIGGGHTS}^\circledR $$ is presented. The coupling algorithm was verified and validated using a modified draw down test rig. The results from the experimental investigations and the FEM–DEM simulations are compared. A very good correlation between experiments and simulations could be found.

Optimization of Surface Properties for Nanoindentation Studies on Nuclear Structural Materials

Nanoindentation using static loading of indenter tip is one of versatile methods used for evaluation of materials in smaller volume. It includes investigation of structural phases and thin layers on substrates etc. In the field of nuclear core and structural materials, nanoindentation has become an important tool to assess mechanical properties and correlate to the level of radiation damage at elevated and room temperatures. Nanoindentation, ideally in combination with Transmission Electron Microscopy, can describe the extent of damage and behaviour from the nanoand micro scale. Due to the high sensitivity of nanoindentation system, that typically uses loads in the range of tens of nN up to several tens of mN, the precise sample preparation is challenging and to be performed especially to understand behaviour of bulk materials. In the current study, samples from Austenitic Stainless Steel (ASS) 321, which is the representative structural material used for reactor internals, were prepared by standard methods - fine polishing, chemical etching, electrolytic etching, electrolytic polishing, electrolytic polishing & etching and ion polishing. Firstly, non-irradiated samples were used for optimization of the sample preparation methodology and then it will be applied on irradiated samples to obtain local mechanical properties. After each preparation step, nanoindentation was performed and load was optimized leading to the minimum standard deviation, also taking into account an indent size and pile-up mechanism. Scanning Probe Microscopy (SPM imaging) and nanoindentation results showed the multi-grained austenitic structure with minimal roughness. Local mechanical properties can be measured according to the knowledge of radiation damage type and location, with focus on grain boundaries to be evaluated. This study shows that advanced methods such as ion polishing are not suitable for ASS preparation, but standard methods based on chemical reaction with structure, especially electrolytic polishing and etching, are highly recommended.

Far infrared and terahertz spectroscopy of ferroelectric soft modes in thin films: A review

ABSTRACTFar-infrared and terahertz spectroscopy of ferroelectric soft and central modes in thin films on substrates is reviewed. In addition to classical displacive proper ferroelectrics, also incipient and relaxor ferroelectrics and multiferroics are discussed. Special attention is paid to differences between the soft-mode behavior in thin films and bulk materials (ceramics and single crystals) and their influence on the low-frequency permittivity. Particularly the effects of the thin film strains and depolarizing electric fields of the probing waves on the grain boundaries are emphasized. The soft-mode spectroscopy is shown to be a very sensitive tool to reveal the thin film quality.

The mechanical behaviour of SentryGlas $$^{\circledR }$$ ® ionomer and TSSA silicon bulk materials at different temperatures and strain rates under uniaxial tensile stress state

An innovative type of connections for glass components, called laminated connections, has been developed in the last years. Two materials have been used for laminated connections: the transparent ionomer SentryGlas® (SG) from Kuraray (former Dupont) and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. In this paper, the mechanical behaviour of SG and TSSA bulk materials is studied under uniaxial tensile stress condition. The effects of strain rate and temperature variations are investigated. Particular attention is paid (i) to the study of these polymers in cured condition and (ii) to the computation of true stress and strain field during the tests. Firstly, it is observed that the mechanical behaviour of both SG and TSSA are temperature and strain rate dependent. These effects are quantitatively determined in the paper. Secondly, two additional phenomena are observed. For TSSA, it is observed that the material goes from fully transparent to white colour, exhibiting the so-called whitening phenomenon. For SG, instead, it is observed that the strain field distribution is dependent on the temperature. More specifically, the material exhibits a non-uniform strain field distribution due to the occurring of the necking phenomenon. Measurements along the specimens, using Digital Image Correlation techniques, showed that the localized strain propagates over the full specimen length, resulting in a cold-drawing phenomenon. Finally, it is also shown that engineering and true stress–strain definition exhibits large deviation indicating that the finite deformation theory should be used for the computation of the stress–strain curves to be implemented in numerical modelling.

Determination of the Self-ignition Behaviour of Bulk Materials from Heat Storage Tests below Atmospheric Pressure

Is a displacement of the self-ignition temperature up to higher temperatures possible by a pressure reduction? Therefore, the influence of an air pressure reduction on the self-ignition temperature of dust bulks is investigated. First, major effects which might influence the self-ignition temperature and the combustion behaviour of dust bulks under reduced pressure are described. Based on these theoretical considerations, basic experiments are carried out to determine the self-ignition temperature under reduced pressure. Thereby, the determination of the self-ignition temperature is carried out by isoperibolic heat storage tests according to DIN EN 15188:2007. The results of the investigations show that self-ignition temperatures of different bulk materials shift to higher temperatures within a pressure reduction.

Towards predictive simulations of machining

Machining simulations are challenging with respect to both numerical issues and physical phenomena occurring during machining. The latter are mainly related to the description of the bulk material behaviour (plasticity) and surface properties (friction and wear). The aim of this paper is to present what is required for predictive models, depending on their scopes, as well as the needed developments for the future. The paper includes a short review of selected works that are relevant for this purpose as well as conclusions based on our own experience.

Tailoring the texture of IN738LC processed by selective laser melting (SLM) by specific scanning strategies

Selective laser melting (SLM) is an emerging technology of additive manufacturing, which is used to directly produce metallic parts from thin powder layers. This study aims at correlating laser scanning strategies with the resulting textures and corresponding anisotropy of the elastic behavior of bulk materials. Tensile test specimens made of the γ’-containing Ni-base superalloy IN738LC were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy-specimens). Their bulk mechanical properties were determined at room temperature and at 850°C. Specimens were investigated in the ‘as-built’ condition and after recrystallization heat treatment. SEM-based electron backscatter diffraction (EBSD) was applied to measure their crystallographic preferred orientations (texture) and to correlate the anisotropy of Young's modulus with the texture of the material. It is shown that the applied laser scanning strategies allow to tailor the crystallographic texture locally. The possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. A recrystallization heat treatment reduces the degree of crystallographic texture and thus the elastic anisotropy by abundant annealing twinning. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.

Mid-IR spectroscopy of Fe:ZnSe quantum dots.

We report spectroscopic characterization of Fe:ZnSe quantum dots (for 2% of Zn/Fe molar ratio) fabricated by microemulsion hydrothermal synthesis. Mid-IR photoluminescence of the 5E↔5T2 transition of Fe2+ ions over 3.5-4.5 μm spectral range was observed in Fe:ZnSe quantum dot samples and kinetics of luminescence have been characterized at temperatures of 30-300 K under direct (2.788 μm) mid-IR excitation and indirect (0.355 μm) photoionization excitation. The radiative lifetime (τrad) was estimated from these measurements to be 48 µs while lifetime at room temperature was measured to be 440 ns. This agrees closely with the behavior of bulk material.

Phase field modeling and simulation of coupled fracture and twinning in single crystals and polycrystals

A phase field theory incorporating both fracture and deformation twinning behaviors in crystalline solids is described and implemented in finite element calculations. A variational approach is used to derive governing equations for quasi-static loading. The constitutive theory accounts for possible anisotropy of surface energy of fracture, enabling preferential cleavage on intrinsically weak crystallographic plane(s). Both linear elastic and nonlinear elastic models for bulk material behavior are addressed, the latter via compressible neo-Hookean elasticity. Numerical implementation is undertaken via the finite element method, wherein nodal degrees of freedom are displacement components and order parameters associated with twinning shear and local elastic stiffness reduction from fracture. Three dimensional simulations are reported, with solutions obtained via incremental energy minimization subjected to appropriate boundary and irreversibility constraints. Two sets of calculations are considered: a single crystal with a geometric notch, from which a crack and/or twin may extend upon mode I or mode II loading, and simple tension of a polycrystal consisting of grains with various lattice orientations. Results from the first set of calculations demonstrate a tendency for fracture before twinning when surface energies of the two mechanisms are equal, and a tendency for twinning to delay fracture when the fracture energy substantially exceeds the twin boundary energy. Results from the second set demonstrate effects of relative orientations of cleavage planes to habit planes (parallel or perpendicular), effects of initial orientation distributions, and effects of secondary grain boundary phases differing in strength and stiffness from surrounding crystals.

Thermal and Optical Characterization of a Sn-Se-Pb-Ge Glassy Alloy

Thermal behavior of bulk material analyzed through differential scanning calorimetry and optical properties of thin films have been studied. The material exhibits a high glass transition temperature (Tg) and wide thermal stability region (ΔT), which make the glass of potential use in temperature sensitive technological applications. We see the enhanced cross-linking and stronger bonding network in the glassy matrix by substituting Pb with Ge, which plays a significant role in tuning the properties of the material. Addition of heavier Pb in base glass matrix causes a red shift in the absorption edge and reduces optical losses in chalcogenide glass. The material possesses a tunable band gap around 1.75 eV and high refractive index around 2.8. Effect of compositional variation and thickness on optical constants has also taken into account.

Modeling of Experimental Atherosclerotic Plaque Delamination.

A cohesive zone model (CZM) approach is applied to simulate atherosclerotic plaque delamination experiments in mouse abdominal aorta specimens. A three-dimensional finite element model is developed for the experiments. The aortic wall is treated as a fiber-reinforced, highly deformable, incompressible material, and the Holzapfel-Gasser-Ogden (HGO) model is adopted for the aortic bulk material behavior. Cohesive elements are placed along the plaque-media interface along which delamination occurs. The 3D specimen geometry is created based on images from the experiments and certain simplifying approximations. A set of HGO and CZM parameter values is determined based on values suggested in the literature and through matching simulation predictions of the load vs. load-point displacement curve with experimental measurements for one loading-delamination-unloading cycle. Using this set of parameter values, simulation predictions for four other loading-delamination-unloading cycles are obtained, which show good agreement with experimental measurements. The findings of the current study demonstrate the applicability of the CZM approach in arterial tissue failure simulations.

Deformation and relaxation of grafted polymer layers at elevated temperatures

The mechanical deformation and relaxation behaviour of thin-grafted polymer layers is investigated by a scanning force microscope (SFM) in contact mode. The layers were considerably plastically deformed at different temperatures. By scratching across the surface, a ripple structure is created. The temperature- and time-dependent relaxation of the ripple structure is followed directly by SFM and discussed in comparison to classical viscoelastic relaxation behaviour and time-temperature superposition of bulk material. In contrast to classical viscoelastic behaviour, here, the strong plastic deformation of the grafted layers accelerates the relaxation behaviour.

Radiation induced effects on mechanical properties of nanoporous gold foams

It has recently been shown that due to a high surface-to-volume ratio, nanoporous materials display radiation tolerance. The abundance of surfaces, which are perfect sinks for defects, and the relation between ligament size, defect diffusion, and time combine to define a window of radiation resistance [Fu et al., Appl. Phys. Lett. 101, 191607 (2012)]. Outside this window, the dominant defect created by irradiation in Au nanofoams are stacking fault tetrahedra (SFT). Molecular dynamics computer simulations of nanopillars, taken as the elemental constituent of foams, predict that SFTs act as dislocation sources inducing softening, in contrast to the usual behavior in bulk materials, where defects are obstacles to dislocation motion, producing hardening. In this work we test that prediction and answer the question whether irradiation actually hardens or softens a nanofam. Ne ion irradiations of gold nanofoams were performed at room temperature for a total dose up to 4 dpa, and their mechanical behavior was measured by nanoindentation. We find that hardness increases after irradiation, a result that we analyze in terms of the role of SFTs on the deformation mode of foams.

Probing Porous Structure of Single Manganese Oxide Mesorods with Ionic Current

Characterization of materials in confined spaces, rather than attempting to extrapolate from bulk material behavior, requires the development of new measurement techniques. In particular, measurements of individual meso- or nanoscale objects can provide information about their structure which is unavailable by other means. In this report, we perform measurements of ion currents through a few hundred nanometer long MnO2 rods deposited in single polymer pores. The recorded current confirms an existence of a meshlike character of the MnO2 structure and probes the effective size of the mesh voids and the polarity of surface charges. The recorded ion current through deposited MnO2 structure also suggests that the signal is mostly due to metal cations and not to protons. This is the first time that ionic current measurements have been used to characterize mesoporous structure of this important electrode material.

MEMS packaging reliability assessment: Residual Gas Analysis of gaseous species trapped inside MEMS cavities

The reliability of MEMS sensors or actuators such as accelerometers, gyroscopes, resonators, RF switches, microbolometers… is critically dependent on the vacuum level and on the nature of the gaseous species present inside the MEMS cavity (∼1mm3 or less, usually in the 10−2–10−3mbar vacuum range after sealing by Wafer Level Packaging process). Although a lot of methods have been explored, the Residual Gas Analysis (RGA) method – however destructive-reveals to be an interesting technique to determine the possible sources of gaseous species present in the MEMS cavity or outgassed by materials, in order to assess to which extent a given packaging technology is capable of maintaining the required environmental conditions all along the device lifetime.This paper introduces results recently achieved with two specifics RGA test benches developed at CEA–LETI (i) to analyze very small amounts of gases trapped inside MEMS cavities, and (ii) to assess the outgassing or pumping behavior of bulk or getter materials. These Ultra High Vacuum (UHV) RGA equipments operate with a residual background pressure level in the 10−10mbar range and use Quadripolar Mass Spectrometers (QMS) for gas analysis.Experimental results confirm that the RGA technique is successfully used to identify the nature and proportions of gases outgassed by materials or trapped in cavities as small as 1mm3 under a residual pressure lower than 10−2mbar.

DEM speedup: Stiffness effects on behavior of bulk material

A number of techniques exist for minimizing the computational cost of discrete element simulations (DEMs). One such method is a reduction of particle stiffness, which allows for bigger time steps and therefore fewer iterations in a simulation. However, the limits and drawbacks of this approach are still unclear, and may lead to invalid results. This paper investigates the effect of a stiffness reduction on bulk behavior by examining three case studies. Two cases demonstrate that particle stiffness can be reduced without affecting the bulk material behavior, whereas the third test shows that a stiffness reduction influences the bulk behavior.

Modelling of microstructure effects on the mechanical behavior of aluminium tubes drawn with different reduction areas

The elastoplastic self-consistent model is one of the effective models to study the initial microstructure effect on the mechanical behavior of bulk materials. However, the use of this micromechanics-based model which only takes into account the Hall–Petch relationship at grain level for predicting the grain size effects on the strength of the deformed materials is not accurate yet. This is because of the fact that the dislocation density also contributes to the strengthening of deformed materials. In this study, a modification made to the Hill-Hutchinson elastoplastic self-consistent model was proposed for investigating the microstructure dependence of the mechanical behavior of deformed materials. Meanwhile, the application of the proposed model for the prediction of mechanical behavior of cold-drawn 6063 aluminium tubes with variable wall thickness was studied. Because of the novel modification, an optimization procedure with two objectives was required to identify the parameters of this micromechanical model. An acceptable agreement between experimental and theoretical stress-strain curves was achieved.