A numerical model for the dissolution of spherical particles in binary alloys under mixed mode control

A general numerical model is described for the dissolution kinetics of spherical particles in binary systems for any combination of first order reactions at the particle-matrix interface and long distance diffusion within the matrix. The model is applicable to both finite and infinite media and handles both complete and partial particle dissolution. It is shown that interfacial reactions can have a strong effect on the dissolution kinetics, the solute concentration at the particle-matrix interface and the solute concentration profile in the matrix.     

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.     

工艺条件对镁合金微弧氧化的影响

在含有Na2SiO3、NaF、甘油及KOH的电解液中以恒电流方式对AZ31B镁合金进行微弧氧化处理,研究了电解液组分、浓度、电流密度及氧化处理时间等对微弧氧化过程及膜层性能的影响．研究表明：随着电解质浓度的增加,起火时间、起火电压基本呈下降趋势,氧化膜厚度呈增长趋势;过量的NaF会抑制放电;甘油的存在可稳定电解液,抑制尖端放电,使膜层的厚度降低;电流密度的增加可以降低起火时间,增加氧化膜的厚度,对放电电压没有明显影响;随着氧化处理时间的延长,氧化膜的厚度不断增长．     

纳米晶软磁合金环形试样有效磁各向异性的研究

采用趋近饱和定律首次测定了纳米晶合金环形试样有效磁各向异性常数〈K〉.为了对比测量的准确度,同时测试了传统的晶态坡莫合金环形试样的磁晶各向异性.结果表明,用环形试样可以完成对低矫顽力的软磁材料进行磁各向异性的测定.     

高导电的铜碲合金抗氧化性能的研究

将碲加入工业纯铜中。改善了纯铜500℃高温抗氧化性能。这是由于高温氧化时在铜的亚表层生成一种新相，阻碍或减缓空气中的氧在铜基体中的扩散所至；氧化温度低于400℃下的抗氧化能力随碲含量的增加略有降低，这与晶格缺陷增多有关。     

溶胶成分对镁合金阳极氧化膜层的影响研究

研究了AZ91D镁舍金材料在普通阳极氧化条件下，通过往碱性阳极氧化溶液中加入硅-铝溶胶成分(含量为0％～5％)，在60～70V的电压条件下进行交流阳极氧化处理，获得的膜层经过厚度测量和表面、断面微观形貌观察表明；溶胶成分在镁合金氧化成膜过程中，可以有效地提高镁合金表面的阳极氧化膜层厚度和膜层的致密程度。同时由于溶液中硅-铝溶胶成分的作用，使得阳极氧化成膜速度出现阶段性快速增长和缓慢增长。而溶胶成分的加入对阳极氧化膜层的X射线衍射相结构的影响不大。     

Wetting behavior of liquid Fe–C–Ti alloys on sapphire

Wetting behavior in the (Fe–C–Ti)/sapphire system was studied at 1823 K. The wetting angle between sapphire and Fe–C alloys is higher than 90° (93° and 105° for the alloys with 1.4 and 3.6 at.% C, respectively). The presence of Ti improves the wetting of the iron–carbon alloys, especially for the alloys with carbon content of 3.6 at.%. The addition of 5 at.% Ti to Fe–3.6 at.% C provides a contact angle of about 30°, while the same addition to Fe–1.4 at.% C decreases the wetting angle to 70° only. It was established that the wetting in the systems is controlled by the formation of a titanium oxicarbide layer at the interface, which composition and thickness depend on C and Ti contents in the melt. The experimental observations are well accounted for by a thermodynamic analysis of the Fe–Ti–Al–O–C system.     

Concentration dependence of the spectral emissivity of liquid binary metallic alloys

A review is given on the literature data on the concentration dependence or the emissivity of liquid binary metallic alloys (Ni-Fe, Ce-Cu, Ni-Cr, Ti-Al). Our measurements at the liquidus temperatures are presented for the systems Ni-V, Fe-V, and Fe-Nb and the pure components at=547 nm and=650 nm. All available results are interpolated in comparison to the phase diagrams of the systems. This comparison indicates that the nonvariant liquidus temperatures have the highest deviation from a linear interpolation between the emissivity of the pure components. The corresponding concentration ranges are characterized by stronger atomic interactions in the melt. Therefore errors in noncontact temperature measurements occur if the concentration dependence is neglected or estimated from the pure components.     

The mechanism of early oxidation stages of Fe20Cr5Al-type alloys at 1123 K

Abstract

Fe20Cr5Al-type alumina forming alloys are known as α alumina-forming materials at relatively high temperatures (≥1273 K). This paper reports on the oxidation mechanism of commercial and synthetic Fe20Cr5Al alloys, at a considerably lower temperature, 1123 K. Some of the alloys were implanted with oxygen or yttrium ions prior to the oxidation exposure.

Two-stage-oxidation, secondary ion mass spectrometry, secondary electron microscopy and lowangle X-ray diffraction were used as experimental tools. Results showed that even prolonged exposures are not sufficient to ensure that the scale consists essentially of α-Al₂O₃. Mechanisms involved from the observed scale morphology, microstructure and phase composition are discussed     

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.     

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs.

The kinetics of formation of Al₂O₃ in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol⁻¹. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.     

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 comparison of breakaway oxidation in wedge-shaped and parallel sided coupons of FeCrAl alloys

Abstract

At elevated temperatures, FeCrAl based alloys go into non-protective breakaway oxidation when the aluminium content drops below a critical level and α-alumina ceases to form. For 2-mm thick coupons, even at 1,300°C, this may take several thousand hours of isothermal oxidation and it can be quite difficult to obtain samples in the critical period immediately before breakaway commences. An alternative approach has been developed, which involves the oxidation of wedge-shaped samples with a 7° taper. The surface to volume ratio varies along the wedge, hence the aluminium reservoir is depleted first at the thin end of the sample, where breakaway commences, and then moves progressively up to the wedge. There is always a region on each sample which can be studied next to the breakaway area. Although the test can be used to rank alloys by the ease with which breakaway progresses, there is clearly a difference between a parallel sided coupon just prior to breakaway and a wedge where one adjacent region has already gone into breakaway. Electron microscopy and analysis have been used to explore these differences and preliminary results are reported.     

Microstructure stability and oxidation behaviour of (FeCoNiMo)90(Al/Cr)10 high-entropy alloys

Simple ordered L1₂ phase and complex ordered µ phase were found in the (FeCoNiMo)₉₀Al₁₀ alloy, while disordered FCC and µ phases were detected in the (FeCoNiMo)₉₀Cr₁₀ alloy. After annealing at 900°C, nano-size precipitates were observed in the L1₂ phase, and splitting and spheroidisation were occurred in some regions of the eutectic structures in the (FeCoNiMo)₉₀Cr₁₀ alloy. After holding for 100?h at 900°C, (FeCoNiMo)₉₀Al₁₀ alloy showed an excellent hardness resistance. The oxidation kinetics of both (FeCoNiMo)₉₀Al₁₀ and (FeCoNiMo)₉₀Cr₁₀ alloys followed a parabolic rate law at 900°C. An external layer of Al₂O₃ and an underlying MoO₂ subscale were found in the (FeCoNiMo)₉₀Al₁₀ alloy. A semi-continuous oxide layer of MoO₂ and Cr₂O₃ was observed in the (FeCoNiMo)₉₀Cr₁₀ alloy.     

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     

Development of an effective interatomic potential for transition metals and alloys: Modified Wills-Harrison model

This paper deals with the development of an effective interionic interaction for transition metals and alloys in the spirit of a hybridized pseudopotential tight-binding formalism, developed among others by Wills and Harrison. Stated briefly, the recipe consists in writing the total interionic interaction as a sum of contributions arising from nearly free s-electrons and that of tightly bound d-electrons. The s-d hybridization is also accounted for by allowing for the relative change in the occupancy of s and d electron counts. This potential, although not taking into account the magnetic contribution, contains otherwise all the essentials of a physically meaningful interatomic interaction. In this paper, a modified version of the Wills-Harrison model is used to calculate the structural and cohesive properties of transition metals at 0 K. The calculated values are in reasonable agreement with the experimental ones. This method is also used to derive an effective interatomic interaction for γ-TiAl. The use of this potential in obtaining an estimate of point defect energetics in TiAl is discussed.     

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.     

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.     

One parameter model for strength properties of hardenable aluminium alloys

Models for strength properties are proposed for commercially aluminium alloys. The alloy group investigated are the hardenable alloys from the 2000 (Al–Cu and Al–Cu–Mg), 6000 (Al–Mg–Si) and 7000 (Al–Zn–Mg) series. The same model for solid solution hardening that has successfully been applied to non-hardenable alloys has been used. For precipitation hardening, particle cutting and the Orowan mechanism have been considered. The same basic model is used for all strength properties. It is demonstrated that with one fitting parameter for each property, a representation with reasonable accuracy can be obtained that is applicable to a wide range of alloys. Such models are useful in materials optimisation and selection.     

The likelihood of achieving quantified road safety targets: A binary logistic regression model for possible factors

In past several decades, many countries have set quantified road safety targets to motivate transport authorities to develop systematic road safety strategies and measures and facilitate the achievement of continuous road safety improvement. Studies have been conducted to evaluate the association between the setting of quantified road safety targets and road fatality reduction, in both the short and long run, by comparing road fatalities before and after the implementation of a quantified road safety target. However, not much work has been done to evaluate whether the quantified road safety targets are actually achieved. In this study, we used a binary logistic regression model to examine the factors – including vehicle ownership, fatality rate, and national income, in addition to level of ambition and duration of target – that contribute to a target’s success. We analyzed 55 quantified road safety targets set by 29 countries from 1981 to 2009, and the results indicate that targets that are in progress and with lower level of ambitions had a higher likelihood of eventually being achieved. Moreover, possible interaction effects on the association between level of ambition and the likelihood of success are also revealed.