Enthalpies of formation of solid Ni---Cr and Ni---V alloys by direct reaction calorimetry

A SETARAM HT1500 calorimeter has been used successfully to measure the enthalpies of formation of solid Ni---Cr alloys at 1583 K by a direct reaction method. The results are in good agreement with those given in the literature. Having proved that the apparatus was suitable to measure such quantities, measurements of the enthalpies of formation of solid Ni---V alloys were made at 1473 K. Experiments were made at 16 compositions; data are reported for Ni-rich f.c.c. and V-rich b.c.c. solid solutions and for the σ phase.     

Thermodynamic investigation of the Lu---Pb system

The Lu---Pb alloys were studied by different techniques. The molar heat capacities of the solid compounds Lu₅Pb₃ and Lu₆Pb₅ were determined in the range 525–823 K using differential scanning calorimetry. The enthalpy of formation of LuPb₂ was obtained by both emf and calorimetric methods. Potentiometric measurements were performed in the range 610–730 K and a value of −35 ± 2 kJ (mol at.)⁻¹ was obtained for the enthalpy of formation of LuPb₂ in the solid state at 298 K. By using a direct isoperibolic aneroid differential calorimeter the value Δ_formH = −34 ± 2 kJ (mol at.)⁻¹ was determined. The data obtained in this study are compared with those of other similar rare earth-lead compounds and discussed briefly.     

Chromate conversion coatings on aluminium-copper alloys

The study compares the formation of chromate/fluoride conversion coatings, composed mainly of amorphous hydrated chromia, on model, solid-solution, binary Al-Cu alloys, of a range of compositions, and on 2014-T6 aluminium alloy. The model alloys, produced by magnetron sputtering, reveal the importance of copper in limiting the thickness of the coatings by promoting loss of coating material. This occurs at an earlier time in the treatment with increasing copper content of the alloy. The coating loss follows closely upon the achievement of the required level of copper enrichment for incorporation of copper into the coating, with a thin alumina film beneath the hydrated chromia sustaining the enrichment process. The coating on the 2014-T6 alloy is of non-uniform thickness, with much thinner coating developing at copper-rich second phases, consistent with the results of model alloys.     

Optimal modifying and refining processes for A356 aluminium alloys

M illions of alum inium alloy com ponents are produced using A356 alum inium alloys, which provide high fluidity, good 'castability'and m echanical properties. A356 alloys are widely used to fabricate structural castings for autom otive and other industrial applications. For α (Al) refining,A356 alum inium alloys usually apply Al-Tim aster alloys in the range 0.08-0.20% Ti, resulting in high consum ption of Al-Ti m aster alloys and higher production cost.M odification ofthe Al-Sieutectic …     

Influence of pre-treatments in cerium conversion treatment of AA2024-T3 and 7075-T6 alloys

The role of pre-treatment in the formation of a cerium conversion coating is investigated for the protection of AA2024-T3 and 7075-T6 alloys. The alloys were alkaline-etched and de-smutted in nitric acid, prior to cerium treatment in Ce(NO₃)₃ at 85 °C with H₂O₂ accelerator. Potentiodynamic polarization studies in 3.5% NaCl solution revealed a large shift of  300 mV of the corrosion potential below the pitting potential for the 7075-T6 alloy, which correlated with the development of a finely-textured, uniform coating. However, the formation of a uniform coating and protection was dependent upon the time of de-smutting, with non-uniform coatings resulting from extended times of de-smutting. In contrast, non-uniform coatings developed on the 2024-T3 alloy, with pitting potential at the corrosion potential, irrespective of the time of de-smutting. Findings for the 2024-T3 alloy indicate that extended de-smutting affects the enrichment of alloying elements.     

Cerium-based conversion coatings on aluminium alloys: a process review

Abstract

Rare earths are among the most promising options for replacing chromate conversion coatings on aluminium. In nearly three decades of research, several hundred papers have been published in the area, the bulk of which has never been reviewed. This paper reviews the literature on rare earth coating processes, with particular emphasis on those based on cerium. It is concluded that several process areas are poorly understood, some of which are critical to further progress in the field. These include the development of industrially suitable pretreatments, technologies for coating non-aerospace alloys and seals to enhance corrosion performance and paint adhesion.     

Influence of oxidation rate and alloy composition on alloy enrichments of anodized Al–W alloys

The effect of oxidation rate on enrichment of tungsten in metastable, solid-solution Al–W alloys during anodizing has been investigated by Rutherford backscattering spectroscopy and medium energy ion scattering. The oxidation rate has negligible influence upon enrichment for current densities in the range 0.01–100 mA cm⁻², with levels between 1.3 × 10¹⁵ and 1.5 × 10¹⁵ W atoms cm⁻² for an Al–0.7at.%W alloy. Further, enrichment factors, expressing the ratio of the enrichment of tungsten in the alloy, in units of 10¹⁵ atoms cm⁻², to the bulk composition of the alloy, in units of at.%, for alloys of compositions in the range 0.1–30 at.%W, decrease progressively in the approximate range 2.5–0.2.     

Electrochemical formation of holmium-cobalt alloys

The electrochemical formation processes of holmium-coball alloys on cobalt cathode in molten HoCl3-KCl were investigated by cyclic voltammetry and open current potential-time curve after potentiostatic electrolysis. The structure of Ho-Co alloys' films deposited on cobalt electrode by potentiostatic electrolysis was characterized by X-ray diffraction. The standard Gibbs free energies of formation for the intermetallic compounds of Ho and Co were determined. The diffusion coefficient and diffusion activation energy of Ho atom in the alloy phase were calculated to be 10-10-10-11 cm2/s and 96.0 kJ/mol, respectively, from the current-time curve at potential step.     

铝合金精炼理论要点和r艺原则

叙述了铝合金熔体精炼理论和工艺方面某些有说服力的研究成果,涉及“临界流速、“污染－净化动态平稳”概念,引出旋转喷吹净化工艺的优越性,提出了净化工艺应遵循的原则,指明了铝熔体中Ａｌ２Ｏ３和Ｈ２之间的相互依存关系。     

A corrosion study of the main constituent phases of AZ91 magnesium alloys

The different constituents of an AZ91 alloy (α,β, and MnAl phases) were synthesized and their corrosion resistance was studied by electrochemistry in ASTM D1384 water, pH 8.3. The pure phases were characterised through the corrosion potential, the polarisation resistance, and polarisation curves, then systematically coupled to assess the galvanic corrosion occurring in the AZ91 alloy. The aluminium content of the oxide film was obtained by X-ray photoelectron spectroscopy measurements. The corrosion rate of the α solid solution alloys depends closely on their Al content. Aluminium enhances the corrosion resistance of the α-phase through the formation of an Al enriched superficial layer. The β-phase is 150 mV nobler than the α-phase, but their corrosion rates are similar. The galvanic currents are low (below 20 μA cm⁻²) whatever the implemented couples and close to the corrosion current previously measured for the AZ91 alloys.     

Factors influencing the deposition of Ce-based conversion coatings, part I: The role of Al ions

The mechanism of deposition of cerium-based conversion coatings and the influence of Al³⁺ ions was modeled using titrations of cerium-based conversion coating solutions with and without added Al³⁺ ions. Precipitates resulting from these titrations were characterised by Raman spectroscopy and thermogravimetric analysis. Cerium peroxo species and precipitated aluminium compounds were detected in the precipitates. The titrations indicated that cerium and aluminium compounds precipitate independently. The coating deposited onto AA2024-T3 was studied using Raman spectroscopy and optical and scanning electron microscopy. Similar species were observed to those in the precipitates. Changes to the properties of the coating during ageing were monitored using contact angle measurements and Raman spectroscopy. A model is presented that proposes a mechanism for the deposition of the coating over the matrix of AA2024-T3.     

铝合金Na—Mg变质规律研究

热分析法可以记录试样在冷运过程中的时间－温度曲线。在本文中，根据热分析曲线和大量的试验数据，得到了铝合金Ｎａ－Ｍｇ变质的综合规律，同时也给出了σｂ与ΔＴ之间的数学计算公式。最后把公式计算数据和实验结果做了对比。实践表明：利用本文结论来指导现场生产是一种行之有效的方法。     

Material flow and void defect formation in friction stir welding of aluminium alloys

An ingenious experimental programme by combining artificially thickened oxide layer as marker material and ‘stop-action’ welding were used to study the material flow and defect formation in friction stir welding of aluminium alloys. The results showed that material flow around the pin on the advancing side (AS) was severer than that on the retreating side (RS) and the fastest velocity of material flow in the middle stir zone (SZ) was 43.9?mm?s^?1. Moreover, the material under the RS shoulder included extruded metal only and the material under the AS shoulder included extruded and rotated metal. Lastly, instantaneous void occurrence and insufficient inflow material were reasons for the preferential formation of void defects in the top SZ on the AS.     

Rare earth contribution in solid state electrolytes, especially in the chemical sensor field

In solid state electrolytes, rare earths are commonly used, and a number of their properties are exploited in these applications. Firstly, the rare earth can be utilized as a dopant to produce the mobile ion vacancies in an electrolyte and to enhance the ionic conducting characteristics. Secondly, they can be used as the principal constituent of the electrolytes. In both these case, the characteristics that the rare earths hold a stable trivalent state even though the main ionic mobility is via mono- or trivalent ions. Thirdly, the rare earth trivalent ion alone can migrate in some solids. This unexpected feature has been recently clarified by selecting the appropriate structure for the trivalent ion conduction in order to reduce the electrostatic interaction between the mobile ions and the framework of the solid. In solid electrolytes, only one ionic species can migrate. This feature is very advantageous for chemical sensor applications, especially from the viewpoint of selective and quantitative detection of specific gas species. The rare earth contribution to solid state electrolytes has been rapidly increasing, and has been expanding further the realms of application not only in the industrial field but also in the whole science field. Here, the rare earths contribution to the solid state electrolyte field is briefly reviewed.     

Modelling intermetallic phase formation in dissimilar metal ultrasonic welding of aluminium and magnesium alloys

Abstract

Intermetallic compound (IMC) formation at the joint line usually has strongly detrimental effect on the performance of dissimilar metal welds. To understand the formation of IMC interlayers, and explore strategies to control their growth, a model has been developed and applied to the case of dissimilar joining of aluminium and magnesium alloys using ultrasonic welding. The model accounts for microbond formation during welding, diffusion across the joint line, as well as nucleation, spreading and thickening of the first IMC layer (Mg₁₇Al₁₂ phase) and the formation and simultaneous thickening of the second (Al₃Mg₂) layer. The model predictions match measurements reasonably well and the model has been used to predict the sensitivity of IMC layer thickness to weld temperature and time.     

Bulge Testing under Constant and Variable Strain Rates of Superplastic Aluminium Alloys

The present paper deals with superplastic forming of aluminium alloy AA5083 sheet metals tested at specific strain rates, temperatures and counter pressures by means of bulge testing using circular and elliptical dies and by the cone-cup testing method. Further, differences from batch to batch can lead to a different strain rates at the maximum m value. It is shown by experimental investigations that pulsating strain rates can lead to higher m values and to increased thickness strains.     

Enthalpies of formation of binary Laves phases

Enthalpies of formation of binary Laves phases have been critically surveyed and reviewed in this paper. The enthalpy-of-formation data indicate that both geometric and electronic factors are important in stabilizing Laves phases. Analysis of enthalpy data reveals that there are metallic, covalent, and ionic bonds, or a mixed metallic-covalent-ionic bond, in Laves phases. The enthalpies of formation for binary transition-metal lanthanide Laves phases including ReCo₂, ReNi₂, ReFe₂, ReRu₂, ReRh₂, ReOs₂, ReIr₂, and RePt₂ (Re—lanthanide element), as calculated by the semiempirical Miedema model, are found to be in good agreement with the available experimental data. This indicates that Miedema's theory is capable of predicting the enthalpy of formation of transition-metal lanthanide Laves-phase systems.     

Al-Si基铸铝合金阳极氧化

Al-Si合金由于其良好的性能已在工业中得到了广泛的应用.阳极氧化成为铝合金铸件表面处理的一种重要方式,而阳极氧化膜色差直接影响到铸件的表面质量、使用寿命及外观要求.探讨了影响铸铝阳极氧化的因素,研究了铸铝合金的表面预处理和阳极氧化工艺,讨论了铸铝合金的表面预处理和工艺条件对阳极氧化膜性能的影响,据此优化工艺,获得了合适的表面预处理方法和阳极氧化最佳工艺条件.该工艺操作简便,节约能源,生产效率高,易于推广应用.     

稀土元素变质A356铝合金的研究进展

总结了目前国内外采用稀土对A356铝合金变质的研究进展,归纳了稀土元素对A356铝合金的变质效果。综合目前的研究结果,结合商业化A356铝合金应用的实际情况,认为采用DOE成分设计方法对相对价廉的二元、甚至多元稀土或稀土与其他元素的复合变质体系进行成分设计是有前景且符合实际的。     

Enthalpies of formation of compounds in Al-Ni-Y system

The enthalpies of formation of the ternary compounds Al4NiY, Al2NiY, Al2Ni6Y3, Al16 Ni3Y, AlNiY, Al3Ni2Y, AlNi8Y3, Al7Ni3Y2, and of the binary comp ounds Al2Y containing nickel and Ni5Y containing aluminum have been determined by high temperature reaction calorimetry. The enthalpy values measured are compared to previously published results where available as well as extended Miedema model predictions. The melting points of the compounds were determined by DTA and X-ray diffraction was used to confirm the crystal structures of the compounds. The enthalpi es of formation of the ternary compounds show a maximum along the 50%Al (mole fr action) section. The ternary compounds appear along lines of constant yttrium content consistent with binary compound solubility extensions.