Pressure effect on the phase transformations and structure of an Al-12 at % Si alloy

Phase transformations of an Al-12 at % Si alloy have been studied by differential barothermal analysis at temperatures of up to 700°C in argon compressed to 100 MPa. Heating at a rate of 8°C/min under a pressure of 100 MPa has been shown to increase the temperature of the L → (Al) + Si eutectic transformation in the alloy by 4°C. After crystallization of the binary eutectic, the differential thermal analysis cooling curve showed an additional exothermic peak, corresponding to the decomposition of the aluminum-based solid solution (Al) at 547°C and precipitation of silicon particles in the nanometer range. A barothermal scanning cycle reduced numerical porosity characteristics of the alloy, except for the pore number density. The lattice parameter of silicon microparticles in the alloy approaches values in the literature, whereas that of nanoparticles is slightly greater. The lattice parameter of the aluminum remains unchanged during crystallization and cooling in compressed argon. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.     

High-pressure phase transformations, microstructure, and magnetic properties of the hypereutectic alloy 10Ni-90Al

Phase transformations of the hypereutectic alloy 10Ni-90Al have been studied by differential barothermal analysis at temperatures of up to 900°C and pressures of up to ?100 MPa. We have determined the pressure coefficients of the liquidus and solidus temperatures and the temperatures of solid-state dissolution and precipitation of the intermetallic phase Al₃Ni. At high pressures, the solid-state decomposition of the supersaturated solid solution of nickel in aluminum occurs at 622°C and yields an (Al) + Al₃Ni mixture. We assume that, during cooling of the alloy at high pressure, intermetallic particles form in three steps. Barothermal analysis data have been compared to the canonical t-x phase diagram of the Al-Ni system. As a result of melting and crystallization at high pressure, the microcrystalline structure of the as-prepared alloy transforms to a macrocrystalline, dendritic structure and the micropore concentration increases. X-ray diffraction data indicate an increase in the unit-cell volumes of the Al and Al₃Ni in the alloy crystallized at 100MPa. We have studied the magnetic properties of the alloy after barothermal analysis.     

Extraction Selectivities of Crown Ethers for Alkali Metal Cations: Differences between Single-Species and Competitive Solvent Extractions

Separation factor values for pairs of alkali metal cations determined in competitive solvent extractions of alkali metal picrates from aqueous solutions into chloroform by a variety of benzo- and cyclohexano-group-containing crown ethers vary significantly from extrapolations based upon the results of single-species extraction experiments. For almost all of the crown ether-alkali metal cation combinations examined, the separation factor values are greater for competitive solvent extraction. In view of the unexpected results for sodium picrate extraction by dibenzo-24-crown-8, the solid-state structure of the isolated complex was determined.     

Alkylation of Aromatic Compounds in the Presence of Catalysts Based on Mesoporous Phenol–Formaldehyde Polymers

A catalyst based on a mesoporous phenol–formaldehyde polymer (MPF) as an organic support modified with the IMHSO₄ ionic liquid has been synthesized. The catalytic activity of the sample has been studied in the alkylation of aromatic compounds with octene-1. It has been found that the use of the catalyst in phenol alkylation leads to the formation of both alkylphenols (C-alkylates) and alkyl phenyl ethers (O-alkylates) with a total yield of up to 60%. In the case of alkylation of benzene and benzene derivatives, significant conversion values (45–50%) are achieved only for toluene and anisole.     

Optical determination of thallium(I) and cesium(I) with a fluorogenic calix[4]arenebis(crown-6 ether) containing one pendent dansyl group

A fluorogenic derivative of 1,3-alternate calix[4]arenebis(crown-6) (1) containing a dansyl group in the proton-ionizable side arm has been employed in selective sensing of Tl+ and Cs+ at low concentration levels in MeCN-H2O (1:1) mixed solvent. Optical recognition of these two metal cations by 1 occurs in contrasting modes. On the basis of the results of fluorescence, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), and 1H NMR studies, Tl+ and Cs+ react with 1 via formation of 1:1 complexes that differ in coordination arrangement around the metal ion.     

Noble metal ion sorption by pyridyl and bipyridyl group‐containing chelating polymers

A series of eighteen 4‐vinylpyridine and 4‐methyl‐4′‐vinylbipyridine copolymers with different crosslinkers was examined as sorbents for the noble metal ions of Ag(I), Au(III), Pd(II), and Pt(II) from aqueous solutions. The chelating polymers possess appreciable sorption selectivity for Au(III) over Ag(I) and for Pd(II) over Pt(II). Binding abilities of the copolymers toward the noble metal ions vary as the identity of the chelating heterocyclic amine moiety and the structure of the crosslinkers are altered. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 207–213, 2001     

Effect of Barothermal Processing on the Solid-State Formation of the Structure and Properties of 16 at % Si–Al Hypereutectic Alloy

We describe barothermal processing (hot isostatic pressing) of a 16 at % Si–Al binary alloy for 3 h at a temperature of 560°C and pressure of 100 MPa for 3 h, in combination with measurements of heat effects during cooling. The results demonstrate that this processing leads to the fragmentation of the silicon structural constituent and ensures a high degree of homogenization of the as-prepared alloy. Heat treatment of the 16 at % Si–Al alloy at 560°C and a pressure of 100 MPa leads to a thermodynamically driven enhanced silicon dissolution, up to ~10 at %, in the aluminum matrix, resulting in the formation of a supersaturated solid solution, which subsequently decomposes during cooling. We analyze the complete porosity elimination process, which makes it possible to obtain a material with 100% relative density. According to differential barothermal analysis, microstructural analysis, and scanning and transmission electron microscopy data, barothermal processing of the 16 at % Si–Al alloy produces a bimodal size distribution of the silicon phase constituent: microparticles 3.6 μm in average size and nanoparticles down to ~1 nm in diameter. The Al matrix has been shown to contain a high density of edge dislocations. Barothermal processing reduces the thermal expansion coefficient and microhardness of the hypereutectic alloy. We conclude that solid-state barothermal processing is an effective tool for completely eliminating microporosity from the 16 at % Si–Al alloy, reaching a high degree of homogenization, and controlling the microstructure of the alloy, in particular by producing high dislocation density in the aluminum matrix.     

High-pressure phase transitions and structure of Al–20 at % Si hypereutectic alloy

Phase transformations of an Al–20 at % Si high-silicon hypereutectic alloy have been studied by differential barothermal analysis at temperatures of up to 800°C in argon compressed to 100 MPa. High pressure has been shown to raise the melting point of the alloy by 5°C during heating and to lower the eutectic solidification temperature by 5°C during cooling in comparison with the canonical phase diagram of the Al–Si system. At a temperature of 553°C, heating and cooling lead to silicon dissolution and decomposition of the aluminum-based solid solution, respectively. After high-pressure solidification, the silicon particles in the alloy have a bimodal size distribution. Quantitative porosity characteristics in the alloy after a barothermal scanning cycle are similar to those in the as-prepared alloy. The lattice parameters of the silicon and aluminum remain unchanged. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.     

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.     

Phase transformations at high pressures and temperatures and the structure of a hypoeutectic 1Ni-99Al alloy

The phase transformations in a hypoeutectic 1Ni-99Al alloy are studied by differential barothermal analysis in the temperature range up to 750°C at a compressed argon pressure up to ～100 MPa. The Al matrix of the initial alloy is found to be saturated by micropores at a concentration of 3.7 × 10¹⁰ cm^?3. After melting and solidification in a compressed argon atmosphere, the micropore concentration increases to 3.2 × 10¹¹ cm^?3. As a result of melting and solidification at a high pressure, the initial fine-grained structure of the alloy with an average grain size of 16 μm transforms into a coarse-grained structure during dendritic solidification. The processing of electron-microscopic images is used to determine the volume content of intermetallic compound Al₃Ni in the Al matrix. The liquidus temperature of the alloy at 100 MPa increases by 10°C, and the solidus temperature is 5°C higher than the eutectic transformation temperature in aluminum-rich Al-Ni alloys. The solid-phase decomposition of the supersaturated solid solution of nickel in aluminum occurs at 630°C. At 100 MPa, the field of solid solutions of nickel in aluminum extends to 1.2 at % Ni as compared to the Al-Ni system at atmospheric pressure. The lattice parameters of Al and Al₃Ni are found to increase in the alloy solidified at 100 MPa. The microhardness of the Al matrix in the alloy is measured after a barothermography cycle. A portion of the Al-Ni phase diagram is proposed for a pressure of 100MPa in the nickel content range 0–4.3 at %.