A three-dimensional model for particle dissolution in binary alloys

A three-dimensional model for particle dissolution in binary alloys is proposed and its numerical solution procedure is described. The model describes dissolution of a stoichiometric particle as a Stefan problem. The numerical method is based on a level set method, which has a wide applicability in modeling moving boundary problems. The present model relies on local thermodynamic equilibrium on the interface between the dissolving particle and the diffusive phase. The level set method is shown to handle complex topological changes in particle break-up very well. The potential of the technique is demonstrated in describing the globularization of planar, perturbed and cracked cementite plates in a pearlitic microstructure.     

Validation of an effective oxidation pressure model for liquid binary alloys

Dynamic surface-tension measurements using the sessile drop method and acquisition times of a few seconds make it possible to study the evolution of the surface of molten metals and alloys and so reliably validate the predictive models of the interactions between pure liquid metals and an oxidizing atmosphere, in both an inert gas carrier and in a vacuum. The presence of active oxidation contributes to maintaining surface cleanness and then strongly affects the shape of the boundary separating oxidation and de-oxidation regimes. Recently the general physical–mathematical analysis we developed for pure liquid metals has been extended to liquid binary alloys and their oxides. In this work we present the experimental results of tests on some binary alloys chosen as test systems to try to obtain a preliminary validation of the extended model. The theoretical results obtained, indicating that the behaviour of the alloy towards oxidation tends to be similar to that of the less oxidizable component, have thus been confirmed experimentally.     

A first principles technique for the analysis of alloy phase stability in random binary alloys

In this communication we introduce the augmented-space recursion method coupled with the orbital peeling technique, as a powerful tool for the calculation of effective cluster interactions, useful for the study of alloy phase stability. An application to the well studied PdV system has been carried out. This work was done in collaboration with Tanusri Saha and Indra Dasgupta, S N Bose National Centre for Basic Sciences, Calcutta     

Evolution and calibration of a numerical model for modelling of hybrid-fibre ECC panels under high-velocity impact

A material model for hybrid-fibre engineered cementitious composites (ECC) under impact loading is developed and calibrated in this paper, and size effect, appropriate erosion criteria and strain rate effect are investigated and accounted for in the model. Employing the new material model, a numerical model and modelling technique are developed to model the impact behaviour and impact process of hybrid-fibre ECC panels using LS-DYNA commercial software. The material model and the numerical model developed in this paper are validated against the experimental results.     

Thermodynamic–kinetic model for the simulation of solidification in binary copper alloys and calculation of thermophysical properties

An earlier developed thermodynamic–kinetic solidification model for binary copper alloys is extended to take into account the formation of the bcc phase via the peritectic transformation and the formation of binary compounds from the fcc phase. Also the eutectic and eutectoid transformations are simulated but only approximately, by modeling the movement of the fcc/eutectic and fcc/eutectoid interfaces due to the diffusion kinetics of the fcc phase only. The new model can handle binary copper alloys containing solutes Ag, Al, Cr, Fe, Mg, Mn, Ni, P, Si, Sn, Te, Ti, Zn and Zr. Depending on the alloy composition, cooling rate and dendrite arm spacing, the model determines the fractions and compositions of the phases (liquid, fcc, bcc, compounds) and calculates thermophysical material properties (enthalpy, specific heat, thermal conductivity, density and liquid viscosity), needed in heat transfer models, from the liquid state down to room temperature. The model is applied to Cu–Sn and Cu–Zn alloys but also to some other binary alloys to show the effect of cooling on the phases formed. Depending on the alloy system, the solidification structures obtained after real cooling processes are shown to be quite different from those estimated from phase diagrams.     

A micromechanical model for the prediction of the temperature fracture behaviour dependence in metallic alloys

In the present paper, a micro-mechanical model based on energetic considerations is developed to simulate the effect of environmental temperature on the fracture toughness of metallic alloys. By considering a reference elementary volume (REV) with the same composition of the real material, the stress-strain field inside such a volume and the corresponding strain energy due to a temperature variation is determined. The energy balance to determine the material fracture toughness is generalised in order to take into account the temperature effects. The proposed micro-mechanical model is governed by few parameters which can be simply estimated, and allows us to determine the fracture toughness for any temperature below the room temperature. Such a model is applied to three metallic alloys which show a ductile-brittle transition temperature: ASTM A471, Carbon Steel D6ac, Steel S275 J2. From the comparison of theoretical results with experimental data, it can be concluded that the model seems to be able to correctly predict the fracture toughness at low temperatures.     

A numerical wave tank model for cavitating hydrofoils

In this paper, an iterative boundary element method (IBEM) for both 2-D and 3-D cavitating hydrofoils moving steadily inside a numerical wave tank (NWT) is presented and some extensive numerical results are given. The cavitating hydrofoil part, the free surface part and the wall parts of NWT are solved separately, with the effects of one on the others being accounted for in an iterative manner. The cavitating hydrofoil surface, the free surface, the bottom surface and the side walls are modelled by a low-order potential based panel method using constant strength dipole and source panels. Second-order correction on the free surface in 2-D are included into the calculations by the method of small perturbation expansion both for potential and for wave elevation. The source strengths on the free surface are expressed in terms of perturbation potential by applying first-order (linearized) and second-order free surface conditions. The IBEM is applied to a 2-D (NACA16006 and NACA0012) and a 3-D rectangular cavitating hydrofoil and the effect of number of iterations, the effect of the depth of the hydrofoil from finite bottom and the effect of the walls of NWT, on the results are discussed.     

Effect of eutectic particles on the grain size control and the superplasticity of aluminium alloys

Aluminium alloys containing eutectic particles of the Al-Ni, Al-Mg-Si, Al-Ni-Ce and Al-Cu-Ce systems are investigated. The particles which control grain growth and stimulate grain nucleation are studied. The Zener-Smith law about dependence between grain size and particle parameters is confirmed and experimental coefficients are found. Experimental coefficients of the Zener-Smith equation obtained in this study depend on the particle size and differ from theoretical coefficients proposed by Zener and Smith. Some alloys with grain size about 3 μm demonstrate very good superplasticity indicators, namely: the strain rate sensitivity index m = 0.5-0.6 and the elongation over 400% at constant strain rate 5 × 10⁻³ s⁻¹.     

Solute-homogenization model and its experimental verification in Mg-Gd-based alloys

Composition homogenization in solid solution is important for industrial alloys. In the present work, a solute homogenization model is proposed based on the chemical short-range-order tendency in Mg-Gd-based alloys. After a calculation using the cluster-plus-glue-atom model, the stable Mg-Gd structural unit is derived, [Gd-Mg₁₂]Mg₆, where one solute Gd is nearest-neighbored with twelve Mg atoms to form the characteristic hcp cluster [Gd-Mg₁₂] and this cluster is matched with six Mg glue atoms. Such a local unit is then mixed with [Mg-Mg₁₂]Mg₃, the stable unit for pure Mg. Assuming that the Gd-containing units are arranged in fcc- or bcc-like lattice points and the Mg units in their octahedral interstices, three proportions between the two units are obtained, 1:1, 2:3, and 1:3, which constitute three solute homogenization modes. The prevailing Mg-Gd-based alloys are consequently classified into three groups, respectively exemplified by GW103 K (Mg-10Gd-3Y-0.4Zr, wt%), GW83 K (Mg-8Gd-3Y-0.4Zr), and GW63 K (Mg-6Gd-3Y-0.4Zr). Mg-Gd-Y-Zr alloys were designed following the model (where Y and Zr were also added in substitution for Gd) and prepared by permanent-mould casting. According to their mechanical properties, the 1:3 alloy (Mg-5.9Gd-1.6Y-0.4Zr) shows the best comprehensive properties (ultimate tensile strength 305 MPa, yield strength 186 MPa, elongation 9.0%) in solution plus ageing state.     

Magnetic properties of binary Cu50–(Ni, Fe and Co)50 alloys prepared by ball milling and isothermal annealing

The magnetic properties of Cu–Ni, Cu–Fe and Cu–Co binary alloys prepared by ball milling and subsequent isothermal annealing have been investigated systematically. A detailed microstructural characterization by X-ray diffraction (XRD) and high-resolution transmission electron microscopic (HRTEM) analysis shows that single phase Cu–Ni solid solution formed by isothermal annealing of ball-milled Cu–Ni powder blend deteriorates the magnetic properties. In contrast, isothermal annealing of Cu–Fe and Cu–Co powder blends resulted into significant improvement of magnetic properties due to precipitation of Fe and Co from the respective supersaturated solid solution. Dispersion of Co nanoparticle in Cu matrix yielded the most attractive magnetic properties.     

A life prediction model for the chemical failure of FeCrAlRE alloys: preliminary assessment of model extension to lower temperatures

Abstract

FeCrAl alloys are being deployed increasingly for industrial applications at elevated temperatures, primarily because their environment protection derives from the formation of an alumina scale. However, such protection is limited ultimately by chemical failure of the scale resulting in catastrophic, non-protective corrosion rates. The accurate prediction of component failure, therefore, is essential for economic and safety reasons.

A model for the chemical failure of alumina scales has been developed, which predicts the lifetimes of commercial FeCrAlRE alloys in oxidising environments. This model is based on the consumption of aluminium in the alloy through scale growth and also takes into account the effect of mechanical scale failure/spallation accelerating the rate of aluminium depletion – over the temperature range (1100–1400°C) this model has been shown to be applicable, when the scale formed was α-Al₂O₃. Its growth and failure processes are essentially similar over the range, although, of course, rates are temperature dependent.

In other potential technological applications, for example automotive catalytic converters, service temperatures can be much lower, in some instances as low as 750°C, while component sections are considerably thinner than for the high temperature (≥1100°C) structural applications.

At these lower temperatures scale growth mechanisms are more complex, involving the formation initially of transitional aluminas, which in some instances transform with time into the more stable α alumina. Additionally, the role of the reactive element (RE) and also of the mechanical interaction between substrate and scale can vary over the lower temperature range.

Component lifetime prediction at these temperatures is vital also. So, to initiate this action, this paper will present a preliminary assessment of the ability of the existing high temperature model to predict the lifetime of FeCrAlRE alloys in oxidising environments at temperatures in the range 750–1050°C.     

Effect of phase on the formation of excited particles under ion bombardment of Fe-Co alloys

Investigations of the main parameters of ion-photon emission during Ar⁺ ion bombardment of Fe and Co metals and Fe-Co alloys have been made. It was shown that the dependence of the quantum yields of the emissions of ejected excited Fe and Co atoms on the concentration of corresponding elements in the alloys generally decreased but showed local maxima. These maxima are related to concentrations of Fe and Co at which intermetallic compounds (FeCo, Fe₃Co, FeCo₃) are present. It is suggested that this local increase in excited metal atom signals is related to the breaking of intermetallic bonds.     

Fracture of U-notched specimens under mixed mode: Experimental results and numerical predictions

The first part of the paper gives an account of 153 fracture tests on blunted notched specimens (with notches of root radius ranging from 0.3 to 4.0 mm), loaded under mixed mode (ranging from almost pure mode I to mode II, and beyond). Maximum loads and initial crack angles were measured as a function of notch root radius and loading mixity. Such results can help in evaluating numerical models of the fracture of notched components. The second part of the paper deals with the suitability of the cohesive crack concept for predicting fracture loads under mixed mode. Use of local mode I was considered for numerical computations. Comparison of experimental results with numerical predictions was significantly accurate. Diagrams of fracture loci for notched components loaded under mixed mode are discussed.     

Calculations of diffusion in FCC binary alloys using on-the-fly kinetic Monte Carlo

We present a systematic, microscopic approach to diffusion for intermetallic alloys using accelerated molecular dynamics. On-the-fly kinetic Monte Carlo is combined with an efficient saddlepoint search to find the saddlepoints exiting a valley, based on energetics from the embedded atom method. With this technique, we compute the tracer diffusivities, migration energies, short-range order and long-range order as a function of composition and temperature for examples of moderately ordered (Cu₃Au), weakly ordered (Au–Ag) and weakly clustered (Cu–Ni) alloys. We find that away from any critical temperature, the calculations produce reliable results, but when critical behavior dominates the approach is overcome by critical fluctuations.     

Improved version of the eikonal model for absorbing spherical particles

Abstract

We present a new expression for the scattering amplitude, valid for spherical absorbing objects, which leads to an improved version of the eikonal method outside the diffraction region. Limitations of this method are discussed and numerical results are presented and compared successfully with the Mie theory.     

A generalised notch stress intensity factor for U-notched components loaded under mixed mode

A novel notch stress intensity factor (NSIF) for U-notched specimens loaded under mixed mode is examined in this article. The concept is based on the averaged strain energy density criterion, or alternatively on the cohesive zone model, as well as the equivalent local mode approach. To a certain extent, it is a generalisation of Glinka’s NSIF for mode I, where σ_tip is replaced by σ_max.The applicability of a fracture criterion based on this new NSIF is checked against 171 fracture tests with PMMA (at −60 °C) performed on U-notched specimens, with different notch root radii and loaded under mixed mode. The asymptotic behaviour of the new NSIF as the notch becomes a crack (when the notch root radius tends to zero) or when the notch disappears (when the notch root radius tends to infinity) is also discussed.     

A constitutive model for cyclic actuation of high-temperature shape memory alloys

In this work, a three dimensional constitutive model for High Temperature Shape Memory Alloys (HTSMAs) is presented. To describe the evolution of the cyclic actuation behavior of such alloys, viscoplastic mechanisms and transformation induced plasticity are introduced in addition to the classical transformation behavior of shape memory alloys. Based on continuum thermodynamics, the evolution of phase transformation, plasticity induced transformation, retained martensite and viscoplasticity are described. Deformation mechanisms that occur over the operational range of such HTSMAs have been identified from the thermomechanical behavior of a NiTiPd alloy. The proposed model has therefore been calibrated and validated based on the thermomechanical response of the studied NiTiPd HTSMA alloy during thermal cycles under compression. Careful attention is devoted to the calibration procedure to identify the contribution of the different mechanisms independently. Finite Element Analysis (FEA) is performed to demonstrate the capabilities of the model to describe the cyclic behavior of HTSMA devices.     

A cellular automaton model of steady-state columnar-dendritic growth in binary alloys

A two-dimensional cellular automaton model has been developed to examine the evolution and coarsening behaviour of solid-solution dendrites during steady-state columnar freezing. Using an empirical rule to account for interface growth, realistic dendrite geometries were obtained for different assumed compositions and process conditions. Coarsening occurred by a coalescence mechanism associated with bridging of adjacent dendrite arms.