Cathodic behavior of scandium(III) on reactive copper electrodes in LiF–CaF2 eutectic molten salt

The electrochemical formation of Sc–Cu alloy was investigated in LiF–CaF₂ eutectic molten salt employing various electrochemical methods. Cyclic voltammetry and square wave voltammetry indicate that the reduction of scandium(III) on the tungsten electrode is a one-step reduction process with three electrons transfer. And underpotential deposition of scandium on the copper electrode occurs owing to the depolarization effect, i.e., forming Sc–Cu intermetallic compounds. The thermodynamic properties of the Sc–Cu intermetallic compounds ScCu₄, ScCu₂, and ScCu in the temperature range of 1153–1223 K were estimated by open-circuit chronopotentiometry. Moreover, the Sc–Cu alloys were prepared by potentiostatic electrolysis and characterized by optical microscope, X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The results reveal that only Sc–Cu alloy composed of Cu-ScCu₄ can be synthesized at low cathodic current density and above eutectic temperature.     

Continuous preparation of composition-controllable Y–Ni alloy in LiF–YF3–Y2O3 molten salt

The Y–Ni alloy is a primary precursor for the preparation of high-performance La–Y–Ni-based hydrogen storage materials. However, it cannot be produced continuously at low cost, which limits the wide popularization and application of La–Y–Ni-based materials. In this paper, this problem was solved perfectly using electrochemical reduction of Y₂O₃ in the LiF-YF₃ system. It is found that the reversible reduction from Y³⁺ to Y on the W electrode takes only one step, namely a significant soluble–soluble reaction controlled by Y³⁺ diffusion throughout the melt. Four typical signals of square wave voltammetry (SWV) corresponding to different kinds of Y–Ni intermetallic compounds are observed in LiF−YF₃ and LiF–YF₃–Y₂O₃ melts, and reduction potential can become positive with the addition of Y₂O₃, probably because of the formation of more complexes in the melts. Homogeneous Y–Ni alloy samples were produced continuously and prepared via galvanostatic electrolysis by using bargain-price raw material (Y₂O₃) and setting the current density at 10 A/cm² on the nickel electrode, before they were collected into a bottom receiver. A series of analyses including scanning electron microscopy-energy idspersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS), demonstrate that concentration of yttrium in Y–Ni alloy is adjustable within the wide range of 44 wt% to 72 wt% by fine-tuning the electrolysis temperature (875–1060 °C) in the LiF-YF₃ system to ensure the optimal hydrogen storage performance and economic efficiency of La–Y–Ni-based hydrogen-storage materials.     

Electrochemical co-reduction of Y(III) and Al(III) in a fluoride molten salt system and electrolytic preparation of Y–Al intermediate alloys

In this study, a molten salt co-reduction method was proposed for preparing Y–Al intermediate alloys and the electrochemical co-reduction behaviors of Y(III) and Al(III) and the reaction mechanism of intermetallic compound formation were investigated by transient electrochemical techniques. The results show that the reduction of Y(III) at the Mo electrode is a reversible electrochemical process with a single-step transfer of three electrons, which is controlled by the mass transfer rate. The diffusion coefficient of Y(III) in the fluoride salt at a temperature of 1323 K is 5.0238 × 10^?3 cm²/s. Moreover, the thermodynamic properties associated with the formation of Y–Al intermetallic compounds were estimated using a steady-state electrochemical method. Y–Al intermediate alloy containing 92 wt% yttrium was prepared by constant current electrolysis at 1323 K in the LiF–YF₃–AlF₃–Y₂O₃ (6 wt%)–Al₂O₃ (1 wt%) system at a cathodic current density of 8 A/cm² for 2 h. The Y–Al intermediate alloy is mainly composed of α-Y₂Al and Y phases. The development and application of this innovative technology have solved major technical problems, such as a long production process, high energy consumption, and serious segregation of alloy elements at this stage.     

Effect of the Al–Zr–Y master alloy composition on the modifying effect in the Al–4% Cu alloy

The results of modifying the Al–4% Cu alloy with test Al–1.04% Zr–0.70% Y and Al–1.23% Zr–0.39% Y master alloys are reported; the Zr-to-Y atomic-percentage ratio in the alloys is 1.41 and 3.08, respectively. The effect of small amounts of Zr and Y (from 0.1 to 0.3%) added in the form of test ternary master alloys of different compositions and binary Al–Zr and Al–Y master alloys on the grain refinement in the Al–4% Cu alloy has been studied. The structure of the initial alloy is characterized by pronounced directional solidification of the α phase. As the Zr + Y content increases, the columnar-crystal zone decreases and the equiaxed-crystal zone increases; at a (Zr + Y) content of 0.326%, only equiaxed crystals ~200 μm in size are present in an ingot. When Zr and Y are added with binary master alloys, the macrostructure of the modified Al–4% Cu alloys indicates that columnar crystals grow until their contact at the center of the ingot, and their growth is independent of the amount of added Zr and Y.     

Estimation of porosity and shrinkage in a cast eutectic Al–Si alloy

Estimation of volume deficit of US 413 cast aluminium alloy has been discussed in the present study. Decrease in specific volume leads to volume deficits in castings and it can be envisaged as a casting defect. Hence, information about volume deficit and its distribution is essential in minimising casting defects. The volume deficit of a given casting is the combination of macro cavities, internal porosity, surface sinks and volumetric contraction. These defects are measured using mathematical formulae. Estimation of internal closed porosity has been addressed through X-ray computer tomography. Influence of pouring temperature on the volume deficit characteristics has been studied.

Dans cette étude, on discute de l’estimation du déficit volumique de l’alliage coulé d’aluminium US 413. La réduction du volume spécifique mène à un déficit volumique dans les moulages que l’on considère comme un défaut de coulée. En conséquence, l’information concernant le déficit volumique et sa distribution est essentielle dans la restreinte des défauts de coulée. Le déficit volumique d’un moulage donné consiste en une combinaison de macro cavités, de porosité interne, de puits de surface et de contraction volumétrique. Ces défauts sont mesurés en utilisant des formules mathématiques. On a abordé l’estimation de la porosité interne fermée au moyen de la tomographie par ordinateur avec rayons x. On a étudié l’influence de la température de coulée sur les caractéristiques du déficit volumique.     

Microstructural and Tribological Characteristics of Al–10Cu–Fe alloy Produced by Vertical Centrifugal Casting process

In this study, an attempt has been made to produce Al–10Cu–Fe alloy by vertical centrifugal casting at speeds ranging from 800 to 2850 rpm. The microstructural features, mechanical and wear properties have been investigated. The microstructure of Al–10Cu–Fe alloy consists of equiaxed grain morphology of the primary α-phase with eutectic phases in the interdendritic regions. It has been observed that there is a variation in the grain size from the inner surface of the casting to its outer surface. The speed also has a strong influence on the grain size and subsequent mechanical properties of the alloy. The wear properties of the alloy have been evaluated at a constant sliding velocity of 1 m/s for a range of applied load and sliding distance. The variations in the wear behavior are attributed to the size and solidification morphology of the castings.     

Heterogeneous nucleation of Al2Cu in AlCu eutectic liquid droplets embedded in an al matrix

The heterogeneous nucleation of Al₂Cu in AlAl₂ Cu eutectic liquids droplets embedded in an Al matrix has been studied by a combination of optical microscopy, transmission electron microscopy and differential scanning calorimetry. Nucleation of Al₂Cu is stimulated catalytically by the surrounding matrix at a temperature approx. 25°C below the eutectic temperature. With increasing cooling rate, the solidification onset, peak and end temperatures decrease and the peak height and width of the solidification exotherm increase. the contact angle at the AlAl₂ Cu liquid triple point is calculated to be 24.6° from the variation of exothermic peak width with cooling rate, but the corresponding calculated value of the number of potential catalytic nucleation sites is physically unrealistic.     

Solidification of the eutectic Ga–Sn alloy

Cyclic thermal analysis is used to study the effect of overheating of the eutectic Ga–8.5 mol % Sn melt on the presolidification supercooling. It is found that, when the liquid eutectic is overheated above the eutectic temperature (T_e = 293.5 K) and is subsequently cooled, the dependence of the presolidification supercooling on the overheating temperature exhibits monotonic ascending behavior. The maximum supercooling after heating of the melt to 339 K is 26 K. The kinetic and thermodynamic parameters of eutectic solidification are calculated using the thermal analysis curves measured during melting.     

High-temperature phase equilibria of Cu–O–Al2O3 system in air

Phase equilibria of Cu–O–Al₂O₃ system were experimentally investigated in a temperature range of 1100–1400°C under 0.21?atm oxygen pressure. The experiments were conducted employing a high-temperature equilibration and quenching method. Microstructures of the quenched samples were observed with scanning electron microscope. The phase compositions in the samples were analysed with electron probe microanalysis technique. Measured solubility of Al₂O₃ into the molten oxide ranged from 0.0 to 1.8?wt-%. A small solubility of Cu₂O into Al₂O₃ was also observed ranging from 1.20 to 1.58?wt-%.     

Density of Na3AlF6–AlF3–LiF–MgF2–Al2O3–Sm2O3 molten salt melt for Al–Sm alloy

Samarium (Sm) has been widely used in making aluminum (Al)–Sm magnet alloy materials. The research team for this study developed a molten salt electrolyte system which directly produces Al–Sm alloy to replace the energy intensive conventional distillation technology. In this study, molten melt density was measured and operation conditions were optimized to separate Al–Sm alloy product from the fluoride molten melt electrolysis media based on density differences. Archimedes' principle was applied to measure density for the basic molten fluoride system (BMFS: Na₃AlF₆–AlF₃–LiF–MgF₂) electrolysis media in the temperature range from 905 to 1055 °C. The impact of temperature (t) and the Al₂O₃ and Sm₂O₃ addition ratio (w_(Al2O3), w_(Sm2O3)) in the basic fluoride system on molten melt density was examined. The fluoride molten melt density relationship was determined to be: ρ = 3.11701 ? 0.00802w_(Al2O3) + 0.027825w_(Sm2O3) ? 0.00117t. The test results showed that molten density decreases with increase in temperature and Al₂O₃ addition ratio, and increases with the addition of Sm₂O₃, and/or Al₂O₃ + Sm₂O_3. The separation of Al–Sm (density 2.3 g/cm³) product melt from the BMFS melt is achieved by controlling the BMFS density to less than 2.0 g/cm³. It is concluded that the optimal operation conditions to control the BMFS molten salt density to less than 2.0 g/cm³ are: maintain addition of Al₂O₃ + Sm₂O₃ (w_(Al2O3) + w_(Sm2O3)) < 9% of Na₃AlF₆, Al₂O₃/Sm₂O₃ ratio (w_(Al2O3):w_(Sm2O3)) > 7:3, and temperature between 965 and 995 °C.     

Synthesis and properties of Cu–Al–Ni shape memory alloy strips prepared via hot densification rolling of powder preforms

Abstract

Cu–Al–Ni shape memory alloy strips were successfully prepared by a powder metallurgy route consisting of preparing powder preforms from premixed Cu, Al and Ni powders by cold compaction, stepwise sintering in the range 873–1273 K, followed by unsheathed multipass hot rolling at 1273 K in protective atmosphere. The densification behaviour of the sintered powder preforms during hot rolling has been discussed. Homogenisation of the hot rolled strips was carried out at 1173 K for 4 h. It has been shown that the finished Cu–Al–Ni alloy strip consisted of self-accommodated plates ofβ' and γ' martensites together with a small amount of nanocrystalline Cu₉Al₄ phase. The finished hot rolled Cu–Al–Ni strips had fracture strength of 476 MPa, coupled with 2·5% elongation. The shape memory tests showed almost 100% recovery after 10 thermomechanical cycles in the hot rolled strips at 1 and 2% applied prestrain.     

Effect of different cooling rates on the microstructure of Cu–Al–Fe alloy

Abstract

The phases obtained in copper aluminium bronze alloy (Cu–10Al–2Fe) cast into a permanent die were investigated. The parameters examined were the preheating temperatures of the die and the graphite coating thickness. The phases α and γ₂ were detected as well as the metastable phases β′ and γ′. The intermetallics of the system Fe–Al were obtained in various stoichiometric compositions. The different cooling rates of the casting resulted in two mechanisms of transformation to α grains out of the unstable β phase, one being nucleation and growth producing needle shaped α grains and the other exhibiting a massive transformation to spherical α grains.

On a étudié les phases obtenues dans l’alliage de cuivre aluminium bronze (Cu–10Al–2Fe) coulé dans un moule permanent. Les paramètres examinés incluaient les températures de préchauffage du moule et l’épaisseur du revêtement de graphite. On a détecté les phases α et γ₂ ainsi que les phases métastables β′ et γ′. On a obtenu les composés intermétalliques du système Fe–Al sous des compositions stoechiométriques variées. Les différentes vitesses de refroidissement du moulage ont résulté en deux mécanismes de transformation en grains α à partir de la phase β instable, l’un étant la nucléation et la croissance produisant des grains α en forme d’aiguille, l’autre exhibant une transformation massive en grains α sphériques.     

Microstructure and properties of the eutectic 12Si–Al alloy subjected to barothermal treatment

A binary 12Si–Al alloy is subjected to barothermal treatment (hot isostatic pressing) at a temperature of 560°C and a pressure of 100 MPa for 3 h. This treatment is shown to result in a high degree of homogenization in the chemically and structurally heterogeneous initial alloy. As follows from the morphology of silicon microparticles, barothermal treatment of the 12Si–Al alloy leads to thermodynamically promoted silicon dissolution in the aluminum matrix up to ~10 at % with the formation of a metastable supersaturated solid solution, which decomposes upon cooling. The process of removal of porosity, which results in the formation of a high-density homogeneous material, is analyzed. After a cycle of barothermal treatment, a bimodal size distribution of the silicon phase constituent forms in the 12Si–Al alloy at an average microparticle size of 2.7 μm and an average nanoparticle size of 36 nm. The linear thermal expansion coefficient of the alloy decreases after barothermal treatment, and the microhardness of the eutectic alloy is determined after this treatment. Barothermal treatment of the 12Si–Al alloy is shown to be an effective tool for the removal of microporosity, achieving a high degree of homogenization, and forming a near-optimum bimodal size distribution of the silicon structural constituent, which is comparable with or even exceeds the results of conventional heat treatment of the material at atmospheric or lower pressure.     

Physical simulation study of the dynamic recrystallization kinetics of an Ni–Cu–Al alloy

The influence of temperature and deformation on the dynamic recrystallization kinetics of an Ni–30Cu–3Al heat-resistant alloy has been studied on the basis of experiments using a torsion plastometer. The process is simulated and mathematical model is proposed for estimating the dynamically recrystallized grain sizes. Good agreement between the calculated and experimental data is achieved.     

Characterization of metastable crystalline phases in the AlGe alloy system

Metastable crystalline phases have been produced in Al-Ge alloys by melt spinning liquid alloys and by rapidly heating thin films which are initially amorphous. The structures and compositions of these nonequilibrium phases have been characterized using convergent beam electron diffraction and energy dispersive X-ray spectroscopy. The results of these studies are compared directly with those of previous authors in order to resolve the controversy which has existed in the literature concerning the identity of the metastable phases which form in Al-Ge alloys when processed by novel techniques such as rapid solidification.     

Novel synthesis of single-crystalline TbCu7-type Sm–Fe powder by low-temperature reduction-diffusion process using molten salt

In this study, molten salt was used as a solvent for calcium (Ca) to let a reduction-diffusion (R-D) reaction occur below the melting point of Ca (1115 K), which is the lower limit temperature of the conventional R-D process. When the R-D reaction is conducted below 923 K with LiCl molten salt, submicron-sized TbCu₇-type Sm–Fe powder is formed. The c/a ratio of the powder estimated by a synchrotron X-ray diffraction pattern is 0.8456, which is consistent with the Sm_0.67Fe_5.667(SmFe_8.5) phase. An electron backscatter diffraction analysis reveals that single-crystalline TbCu₇-type SmFe_8.5 powder was synthesized for the first time.     

Mechanisms of eutectic solidification in Al–Si alloys modified with Ba,Ca, Y and Yb

The eutectic solidification mechanisms in an A356.0 (Al–7%Si–Mg) alloy modified by barium, calcium, yttrium and ytterbium have been determined. The crystallographic orientations of aluminium in the eutectic and the surrounding aluminium dendrites were measured by electron backscattering diffraction mapping, and samples were also quenched at different stages during the eutectic arrest and examined by optical microscopy. The combination of these two techniques shows that each of the elements added promote heterogeneous nucleation of eutectic grains in the interdendritic liquid, while the aluminium in the unmodified alloy grows epitaxially from the dendrites. Furthermore, calcium and yttrium result in a strong dependency of eutectic solidification on the thermal gradient, i.e. the eutectic evolves from the walls towards the centre of the sample on a macro-scale. These differences in eutectic solidification mode show a correlation with some thermal characteristics of the eutectic arrest.     

Microstructure evolution and densification behaviour of powder metallurgy Al–Cu–Mg–Si alloy

ABSTRACT

An Al–Cu–Mg–Si alloy was prepared by conventional press-sintering powder metallurgy using elemental Al powder. The phase transformation process of Al–Mg, Al–Si alloy and Cu during the sintering process was investigated in details. It was found that a series of phase transitions take place in the alloy to disrupt the oxide film of Al particle and enhance the densification process. The relative density of the sintered samples reached 98%. A new Al–Mg–Cu–O compound was found at the grain boundaries except the MgAl₂O₄ phase, it is speculated that the disruption of the oxide film was also associated with the other alloy compositions except for Mg. Furthermore, no detectable AlN compound was found at the grain boundary region although sintering with flowing nitrogen atmosphere, which is benefit from the high density of the green compact and the excellent wettability between the liquid phase and the aluminium.