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.     

Effect of partial oxygen pressure on the initial stages of high-temperature oxidation of γ-NiCrAl alloys

Abstract

The effect of the partial oxygen pressure (pO₂) on the initial stages of oxide-layer growth of a γ-Ni-27Cr-9Al (at.%) alloy at 1373 K was studied using X-ray photoelectron spectroscopy, X-ray diffractrometry, scanning electron microscopy and electron probe X-ray microanalysis. The alloy was oxidised at a pO₂ of 0.1 and 0.2 × 105 Pa in an UHV processing chamber and a furnace, respectively. For both pressures, a double-layered oxide structure evolved upon oxidation, which consisted of a NiCr₂O₄/Cr₂O₃ layer on top of an α-Al₂O₃ layer. Two different oxidation stages were identified: an initial, very fast oxidation stage followed by a second stage of slow, parabolic growth. The initial oxidation stage was associated with the preferential oxidation of Al, followed by the internal oxidation of Al and simultaneous formation of Cr₂O₃ (and some NiO). The onset of the second stage ran parallel with the coalescence of internal α-Al₂O₃ crystallites into a continuous closed layer. Then, the internal oxidation of Al and growth of Cr₂O₃ ceased. A lower pO₂ reduced the activity of oxygen at the oxide/alloy interface during the initial, fast oxidation stage, thereby enhancing the rate of formation of a continuous closed α-Al₂O₃ layer and suppressing the formation of Cr₂O₃ (and NiO).     

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     

Anomalous formation of micrometer-thick amorphous oxide surficial layers during high-temperature oxidation of ZrAl2

The thermal oxidation of ZrAl₂ in the temperature range of 550–750 °C in pure oxygen has been investigated by a combinational experimental approach using X-ray diffraction, scanning electron microscopy/energy dispersive spectrometer, Auger electron spectroscopy and cross-sectional transmission electron microscopy. The thermal oxidation leads to the growth of anomalously thick (up to 4.5 μm) amorphous (Zr_0.33Al_0.67)O_1.66 surficial layers at temperatures as high as 750 °C. The oxidation kinetics obeys a parabolic law with an activation energy of 143 kJ/mol. The underlying mechanism for the formation of such micrometer-thick amorphous oxide surficial layers has been discussed on the basis of interface thermodynamics and the occurrence of high interface stability associated with a synchronous oxidation of Al and Zr elements.     

Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Abstract

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.

Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.     

微弧氧化处理对钛铌合金力学及摩擦磨损性能的影响

采用微弧氧化的方法于磷酸盐电解液中在二元β型TiNbx(x=5,10,15,20,25)合金表面制备了微弧氧化涂层.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)表征各合金表面微弧氧化涂层的物相组成和微观形貌.采用纳米压痕仪、球-盘摩擦磨损实验仪分析了微弧氧化处理对钛铌合金力学性能、耐磨性的影响.结果表明,通过微...     

电解液组成对AZ91D镁合金微弧氧化的影响

在含有NaAlO2、KF的电解质溶液中,采用恒电流方式对AZ91D镁合金进行微弧氧化获得陶瓷膜.研究了电解液组分及浓度对陶瓷氧化膜厚度及表面形貌的影响,同时,采用动电位极化曲线及电化学交流阻抗评价了陶瓷氧化膜的耐蚀性.研究发现:NaAlO2单独存在时即可产生火花放电现象,但得到的氧化膜较薄;氟化钾的加入可以显著增加氧化膜厚度,膜厚的增长速度与氟化钾的加入量呈线性关系.SEM表面形貌分析表明:电解质浓度较低时产生的氧化膜宏观上较粗糙、微观上颗粒结合紧密;高浓度时得到的氧化膜宏观上细致光滑,微观上存在明显的孔洞和放电隧道,呈熔融状态结合在一起.动电位极化曲线及电化学交流阻抗的测试一致表明,经微弧氧化处理后的镁合金耐蚀性显著提高.     

Microstructural studies of NiAl-based model alloys and commercial coatings after isothermal oxidation

Abstract

Four commercial aluminide diffusion coatings (one Pt-free) and four polycrystalline NiAl model materials, isothermally oxidised in laboratory air at 1050°C for 1 hour, have been investigated. The coatings were deposited on a single crystal Ni-based superalloy, CMSX-4. Two of the model materials are binary alloys containing different Al amounts. The two other have the same Ni/Al ratio as the binary alloys but were alloyed with 4 at% Pt. In order to control the surface roughness, all materials were polished prior to oxidation. Microstructural studies have been conducted using X-ray diffraction and scanning electron microscopy. To get a deeper insight on the platinum influence on the oxide structure, two commercial coatings (one Pt free and the other modified with Pt) were also investigated using transmission electron microscopy. The results obtained are discussed according to the influence of Al and Pt contents in the materials.     

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.     

High-temperature oxidation of FeCrAl(Y,Pt) alloys in oxygen–water vapour

Abstract

High-temperature oxidation behaviour of FeCrAl(Y, Pt) alloys was studied in oxygen and oxygen – water vapour (47 vol%) for 18 ks at 1473, 1573 and 1673 K, by mass gain measurements, the amount of spalled oxide, X-ray diffraction, scanning electron microscopy and field emission-transmission electron microscopy – energy dispersive spectroscopy. After oxidation at 1473K for 18 ks in oxygen, the mass gain of the FeCrAl alloy with appropriate addition of both yttrium and platinum was the smallest value, and alumina – alloy interface of the alloy showed good coherency by TEM. After oxidation at 1673K in oxygen – water vapour (47 vol%) for 18 ks, the amount of spalled oxide of FeCrAl0.5Pt alloy was zero. This result is thought to relate to the formation of concentrated platinum layer at the oxide – alloy interface. Spalling of oxide scale on FeCrAl and FeCrAl0.1Y alloys was observed, however, oxide scale on FeCrAl0.5Y alloy showed good adherence during five cycles (one cycle=18 ks) in oxygen – water vapour (47 vol%) at 1573 K.     

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.     

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.