Phase Transformations in Cu-Al and Cu-Zn-Al alloys

Phase transformations in Cu-12.4% Al and Cu-14.4% Zn-8.4% Al alloys were examined by DTA. The influence of the rate of temperature change on the sequence of phase transformations was studied. It was found that the rates of heating and cooling were the major factors determining the transformations which take place in these alloys.

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Zusammenfassung Phasenübergänge in den Legierungen Cu - 12,4% Al und Cu - 14,4% Zn - 8.4% Al wurden thermoanalytisch untersucht. Der Einfluß der Geschwindigkeit der Temperaturänderung auf die Reihenfolge der Phasenübergänge wurde untersucht. Aufheiz- und Abkühlgeschwindigkeit sind die wichtigsten Faktoren, von denen die Phasen übergänge in diesen Legierungen abhängen.

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- 12,4% - 14,4% - 8,4% . . , , .

     

Features of structure and phase transformations in shape memory TiNi-based alloys after severe plastic deformation

Amorphous and nanostructured TiNi-based alloys (Ti₅₀Ni₅₀, Ti_49.5 Ni_50.5, Ti₅₀Ni₄₉Fe₁, in at.%) were first produced using two techniques of severe plastic deformation (SPD), namely high pressure torsion (HPT) and equal channel angular pressing (ECAP). X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to examine the structural states and phase compositions of initial and SPD specimens, their thermostability on annealing and cooling, including in situ experiments. The nanocrystallization temperatures of the amorphous alloys and critical points of martensitic transformations in the crystalline alloys were also determined by means of measurements of the temperature dependence of electrical resistance. It was shown that martensitic transformations in the sequence B2↔R↔B19′ occur in nanostructured TiNi-based alloys or, on the contrary, only a single B2↔R transition can occur in the amorphous-nanocrystalline alloys. In nanostructured SPD-alloys cooled to below the M_s′ or M_s temperatures, nucleation and growth of R- and B19′-martensites occur by a “B2-austenite single nanocrystal — martensite single crystal” mechanism without microtwinning. Only in submicrocrystalline SPD-alloys with coarser B2-grains (larger than 100 – 200 nm) the R and B19′ martensites had a twinned packet morphology.     

Kinetics of thermal transformations in nanosized chromium films

The transformations in nanosized chromium layers at different layer thicknesses (d = 14–154 nm) and thermal treatment temperatures (T = 673–873 K) were studied by optical spectroscopy, microscopy, and gravimetry. The kinetic curves of conversion at different chromium film thicknesses and treatment temperatures are well approximated by the linear, inverse logarithmic, cubic, and logarithmic functions. The contact potential difference for Cr and Cr₂O₃ films and photo-emf for Cr-Cr₂O₃ systems were measured. An energy band diagram of Cr-Cr₂O₃ systems was constructed. A model of thermal transformation was constructed for Cr films that included the stages of oxygen adsorption, charge carrier redistribution in the contact field of Cr-Cr₂O₃, and chromium(III) oxide formation.     

Energetics of phase transformations in polyethylene

The molecular mechanisms involved in the orthorhombic-to-monoclinic phase transformation in polyethylene were investigated by the computer simulation of a structure–energy map based on empirically justified intermolecular potential functions. Stable packing structures for the orthorhombic and monoclinic form were isolated as relative minima, cohesive energies were determined from the energy minima, specific chain motions involved in the transformation were identified by the minimum energy path connecting the packing minima, and the activation energy for the transformation was determined from the energy barrier along the minimum energy transformation path. The packing structure parameters predicted from the energy map were in excellent agreement with unit cell dimensions observed near 0°K. The activation energy predicted for the transformation is relatively low (～0.5 kcal/mole of ethylene at 0°K and 0.25 kcal/mole of ethylene near the melting point, 411°K). Monoclinic packing was predicted to be slightly more stable than orthorhombic. Since this result is inconsistent with a large body of observations, we propose that the intramolecular energy of chain folds plays a dominant role in establishing chain-packing geometry. The inclusion of fold-transition energetics could give rise to transformation mechanisms which differ in details from those proposed in this work.     

Kinetics of photostimulated transformations in nanosized bismuth films

Optical spectroscopy, gravimetric analysis, and microscopy studies show that irradiating bismuth films (d = 3–55 nm) with light (λ = 360 nm and I = 1.8 × 10¹⁵–7.0 × 10¹⁵ quantum cm^?2 s^?1) leads to major changes in their absorption and reflectance spectra and in film mass. The kinetic curves of the degree of photochemical transformation of bismuth films are shown to obey linear, inverse logarithmic, cubic, and logarithmic laws. The contact potential difference (CPD) of Bi and Bi₂O₃ films and the photo-electromotive force (emf) of Bi-Bi₂O₃ systems are measured. An energy band diagram of Bi-Bi₂O₃ systems is constructed. A model that includes the stages of generation, recombination, and redistribution of nonequilibrium charge carriers in the contact field of a Bi-Bi₂O₃ system, oxygen adsorption, diffusion of cation vacancies, and Bi₂O₃ formation is proposed.     

Pressure-induced phase transformations in LiAlH4

The pressure-induced phase transformations in pure LiAlH4 have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic alpha-LiAlH4 phase (P2(1)/c) to a high pressure phase (beta-LiAlH4, reported recently to be monoclinic with space group I4(1)/b) having a distorted [AlH4]- tetrahedron. The Al-H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure beta-LiAlH4 phase was pressure quenchable and can be recovered at lower pressures ( approximately 1.2 GPa). The Al-H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al-H bond.     

Kinetics of Low-Temperature Conversion of Carbon Monoxide with Water Vapor on Cu-Zn-Al Oxide Catalysts

Theoretical and Experimental Chemistry - According to the analysis of literature data and our experimental results, a kinetic equation for the low-temperature conversion of carbon monoxide with...     

High-pressure structures and phase transformations in elemental metals

At ambient conditions the great majority of the metallic elements have simple crystal structures, such as face-centred or body-centred cubic, or hexagonal close-packed. However, when subjected to very high pressures, many of the same elements undergo phase transitions to low-symmetry and surprisingly complex structures, an increasing number of which are being found to be incommensurate. The present critical review describes the high-pressure behaviour of each of the group 1 to 16 metallic elements in detail, summarising previous work and giving the best present understanding of the structures and transitions at ambient temperature. The principal results and emerging systematics are then summarised and discussed.     

Kinetics and mechanism of precipitation processes in Al-Ag alloys

During the heating of a supersaturated solid solution, four peaks were observed in the DTA curve. Two exotherms were due to the precipitation of Guinier-Preston zones and an equilibrium phase. Endotherms accompanied dissolution of the zones and a stable precipitate.The temperatures of the peaks, the thermal effects and the activation energy of precipitation appear to depend on the chemical compositions of the alloys.

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Zusammenfassung Während des Aufheizens einer übersättigten festen Lösung wurden in der DTA-Kurve vier Peaks beobachtet. Zwei exotherme Effekte wurden der Ausscheidung von Guinier-Preston-Zonen und einer Gleichgewichtsphase zugeschrieben. Die Auflösung der Zonen und des stabilen Präzipitats gehen mit endothermen Effekten einher. Die Peaktemperaturen, die thermischen Effekte und die Aktivierungsenergie der Ausscheidung scheint von der chemischen Zusammensetzung der Legierungen abzuhängen.

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Structural transformations of metal alloys under electrocatalytic conditions

Nanostructured alloys are efficient catalysts for mediating renewable energy storage and recovery reactions. The morphology and composition of an alloy can change during catalysis; particularly for potentials more oxidizing than the reversible hydrogen electrode. The formation of noble-metal shells covering an alloy core, or bi-continuous nanoporosity, is a common way an alloy can evolve by a corrosion mediated process. Recently, it was found that alloys can reconstruct within the bulk and form an ordered intermetallic material with different crystal structures and compositions than the starting material during corrosion. This review will discuss the different pathways alloys can be altered by electrochemistry. We will discuss the mechanisms which cover known structural changes and the more recently discovered process involving electrochemically driven incongruent phase transformations. Insights into the transformation of alloy materials are important for understanding how to prepare catalysts with improved electrochemical stability, and for synthesizing materials.     

Phase transformations in Al-Zn alloys solidifying at various rates

The influence of the alloying elements magnesium, copper and silicon on phase transformations in Al-60 wt% Zn alloy solidified at rates from 0.4 up to 65 deg/s has been investigated by means of DTA method.The dendritic segregation of zinc in the solid solution manifested itself by changes of the shape of the respective parts of the cooling or heating curves between the solidus and liquidus temperatures. The appearance of non-equilibrium transformations has been recorded as peaks in the last stage of crystallization.The shift of liquidus and eutectic temperatures in the direction of lower temperatures was observed in dependence on the cooling rate.Thermal analysis carried out at various cooling rates revealed nonequilibrium transformations at higher velocities. The occurrence of such effects was confirmed using X-ray microanalysis.

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Zusammenfassung Mittels DTA wurde der Einfluß der Legierungselemente Magnesium, Kupfer und Silizium auf Phasenumwandlungen in Al-60m% Zn-Legierungen mit Erstarrungsgeschwindigkeiten zwischen 0,4 und 65 K/s untersucht.Die dentritische Segregation von Zink in Mischkristallen macht sich durch Unterschiede des Verlaufes der jeweiligen Abschnitte der Abkühloder Aufheizkurven zwischen Solidus- und Liquidustemperaturen deutlich. Das Auftreten von Nicht-Gleichgewichts-Umwandlungen wurde als Peak im letzten Abschnitt der Kristallisierung aufgezeichnet.Die Verschiebung von Liquidustemperaturen und Eutektika in Richtung niedriger Temperaturen wurde als Funktion der Kühlgeschwindigkeit beobachtet.Die bei verschiedenen Kühlgeschwindigkeiten durchgeführte Thermoanalyse deckte das Auftreten von Nicht-Gleichgewichts-Umwandlungen bei höheren Geschwindigkeiten auf. Das Auftreten solcher Effekte wurde durch Anwendung von Röntgen-Mikroanalyse bekräftigt.

     

Peculiarities of phase transformations in aqueous isopropanol solutions

Peculiarities of phase transformations in aqueous isopropanol solutions are studied in the range from room temperature to −15°C. It is found that solution cooling results in the formation of macroaggregates with regular geometrical shapes, which are dispersed in the liquid phase and directedly move in the field of a temperature gradient. In aqueous isopropanol solutions with concentrations of 10–30 vol %, the aggregates are formed at temperatures of −10 to −15°C. The average aggregate size is several millimeters. Under the action of a temperature gradient, the aggregates move to the region of higher temperatures. At temperature gradients of 1–2 K/cm, the aggregate velocity is 1–2 mm/s. The characters of the motion and interaction of aggregates are very sensitive to the temperature distribution in a solution. After the aggregate motion ceases, crystallization of the liquid phase that initially is outside of the aggregates is observed.     

Kinetics of heat-induced and photochemical transformations of quinonimines and their derivatives

Heat-induced transformations of N-[2,6-diisopropylphenyl]-3,5-di(tert-butyl)-, N-[2,6-diethylphenyl]-3,5-di(tert-butyl)-, and N-[2-methyl-6-ethylphenyl]-3,5-di(tert-butyl)-o-benzoquinonimines in nonane follow the first-order rate equation, whereas that of N-[2,5-di(tert-butyl)phenyl]-3,5-di(tert-butyl-o-benzoquinonimine obey the second-order rate equation. Kinetic parameters of these reactions have been determined. 4aH-Phenoxazine derivatives of quinonimines are intermediates of the heat-induced transformations following the first-order kinetics; under the irradiation with 405 nm light they undergo the ring opening to give the starting compounds with quantum yield close to unity.     

Size- and shape-dependent phase transformations in wurtzite ZnS nanostructures

This paper describes the equilibrium morphologies of zinc sulfide nanoparticles in the wurtzite phase as a function of size, determined using ab initio Density Functional Theory (DFT) simulations and a shape-dependent thermodynamic model predicting the Gibbs free energy of a nanoparticle. We investigate the relative stabilities of a variety of nanoparticle shapes based on the wurtzite structure and show how the aspect ratio of wurtzite nanorods moderates the size-dependent phase transformation to the zinc blende phase. We find that while wurtzite nanoparticles are thermodynamically unstable with respect to the low energy rhombic dodecahedron morphology in the zinc blende phase at all sizes, shape- and size-dependent phase transformations occur when other zinc blende morphologies are present. Despite popular synthesis of zinc sulphide nanoparticles in the wurtzite phase, an in-depth thermodynamic study relating to the relative stability of wurtzite shapes and comparison with the zinc blende phase does not exist. Therefore this is the first thermodynamic study describing how shape can determine the solid phase of zinc sulfide nanostructures, which will be of critical importance to experimental applications of nanostructured zinc sulfide, where phase and shape determines properties.     

Phase and chemical transformations of lanthanum and iron alloys in a nitrogen-hydrogen medium

The behavior of La_xFe_1–x alloys in a nitrogen-hydrogen medium at 293–723 K and a pressure of up to 1.5 × 10⁷ Pa was studied. The alloys decompose to give LaH_2.9 and -Fe at 293 K, LaN, LaH_2.4, and -Fe at 623 K, and LaN and -Fe at 723 K. Ammonia was detected in the gas phase at 623–723 K.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 8, 2004, pp. 1241–1243.Original Russian Text Copyright © 2004 by Shilkin.This revised version was published online in April 2005 with a corrected cover date.     

Chemical transformations and phase transitions in polymer rheology and technology

Fundamental rheokinetic effects observed in processes of linear polymerization and three-dimensional oligomer curing are discussed. It was shown that changes in rheological properties during linear polymerization can be described by relationships based on treating a reactive mass as a polymer solution, if a newly formed polymer is soluble in reactive medium. Phase separation limits this approach and leads to some new rheological consequences such as change in the rate of viscosity growth (in some cases viscosity can even decrease in the course of polymerization). Shearing influences temperature of a phase transition and kinetics of chemical reaction if it proceeds in heterogeneous conditions. The same phenomena were also observed for oligomer curing which can occur in heterogeneous manner with microgelation and formation of colloid particles of a new phase before the gelation of a system as a whole.     

Kinetics and mechanism of di-tert-butyl peroxide-initiated liquid-phase transformations of 2-alkoxytetrahydropyrans

The di-tert-butyl peroxide-initiated (by thermal decomposition) liquid-phase transformations of 2-alkoxytetrahydropyrans to the corresponding esters of n-valeric acid and -valerolactone were studied by kinetic methods. Their ratio is determined by the structure of the alkoxy group. The reaction proceeds via a mechanism involving an unbranched chain reaction with quadratic breaking of the chains in the rearranged radicals.Translated from Khimiya Geterotsiklicheskikh. Soedinenii, No. 4, pp. 456–459, April, 1977.