Showing papers on "Solid solution published in 1992"

Mechanically driven alloying of immiscible elements

Abstract: We report on the formation by mechanical alloying of fcc solid solution of Fe and Cu, which are immiscible. Intense codeformation of Cu with Fe and other high-melting bcc metals generates small fragments with tip radii of the order 1 nm such that capillary pressures force the atoms on these fragments to dissolve. The critical radius for dissolution is found to increase with solute content inside the spinodal thus leading to full dissolution of one of the components. The fcc-FeCu solid solutions have Curie temperatures similar to those of vapor-deposited alloys and a positive heat of mixing that is in accord with theoretical results.

Thermodynamic Assessment and Calculation of the Ti-Al System

Abstract: A thermodynamic description of the Ti-Al system has been developed. Three different analytical descriptions were used to describe the three different types of phases occurring in the Ti-Al system: the stoichiometric compounds, the disordered solution phases, and the ordered inter-metallic compounds which have homogeneity ranges. A least-squares technique was used to optimize the thermodynamic quantities of the analytical description using experimental data available in the literature. The calculated phase diagram, as well as the thermodynamic func-tions, agree well with the critically evaluated experimental data from the literature.

The microstructure evolution of a Fe73.5Si13.5B9Nb3Cu1 nanocrystalline soft magnetic material

Abstract: The microstructure evolution in the course of crystallization of a splat-quenched Fe73,5Si13.5B9Nb3Cu1 amorphous alloy was investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HRTEM). All the alloying elements were found to be distributed homogeneously as an amorphous solid solution in the as-quenched state. At an initial stage of annealing, a concentration fluctuation of Cu was found to occur. Cu formed clusters of a few nanometer diameter and their composition was found to be approximately 30 at.% Cu at the beginning. In the later stage, a b.c.c. FeSi solid solution and the B and Nb enriched amorphous phase with the smaller Si content were found to coexist. In addition to these two phases, Cu enriched particles containing approximately 60 at.% Cu were found to be present in the intergranular regions, although we were not successful yet to determine whether this was a crystalline or amorphous phase. Based on these observations, we discuss the crystallization process of this alloy at 550°C which leads to the emergence of excellent soft magnetic properties.

Superconductivity at 8.4 K in calcium-doped C60

Abstract: IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.

Preparation and Thermoelectric Properties of Mg 2 Si 1− x Ge x ( x =0.0∼0.4) Solid Solution Semiconductors

Abstract: Mg 2 Si 1-x Ge x Solid Solution Semiconductors were prepared in the composition range 0.0≤x≤0.4. At a composition w=0.4, effects of impurities were investigated for dopants of Sb and Ag. Carrier concentrations were controlled up to 1.5×10 26 electrons/m 3 and 5.5×10 25 holes/m 3 by doping Sb and Ag, respectively. The thermal conductivity κ was measured at 300 K.

An experimental evaluation of the phase relationships and solubilities in the nbcr system

Abstract: The phase relationships and solubilities have been determined experimentally in a revised evaluation of the NbCr alloy system The results from a series of microstructural investigations on arc-cast and equilibrated samples, together with X-ray diffraction and compositional analyses, provide a basis for modifying the invariant reaction phase compositions and phase boundaries In addition, new evidence based on X-ray diffraction measurements further establishes the existence of a high temperature C14 Laves polytype as well as an intermediate C36 structure for NbCr2 Moreover, lattice parameters have been determined as a function of composition for the bcc and C15 Laves phases Within the equilibrium solubility limits of the Nb and Cr terminal bcc solid solutions, the lattice parameters exhibit a slight positive deviation from Vegard's rule For the NbCr2 C15 Laves phase the solubility behavior and the distinct change in the composition dependence of the lattice parameters suggest a different defect structure on either side of the stoichiometric composition in the intermetallic phase field

Experimentally produced oscillatory zoning in the (Ba, Sr)SO 4 solid solution

Abstract: WHEN crystals grow from a multicomponent fluid phase under conditions where ionic diffusion in the solid is negligible compared to that in the liquid, any compositional gradients in the crystal record the evolution of the solid/liquid interface composition during growth. For the particular case of oscillatory zoning1–3, a relatively common feature of natural crystal growth4, there has been considerable theoretical interest4–6, but the specific question of whether high or low supersaturations are required to explain the development of the zoning remains unanswered. Experimentally produced compositional oscillations have been observed7,8, but the role of supersaturation had to be inferred, rather than measured directly. Here we describe major-element oscillatory zoning in (Ba, Sr)SO4 solid solutions grown by the counter-diffusion of (Ba2+, Sr2+) and SO2−4 ions through a porous silica-gel transport medium. We demonstrate how the different solubilities of the two pure phases determine the threshold supersaturation for nucleation, and show how coupling between the compositional gradients in the solid and the liquid results in the observed oscilla-tory behaviour.

Temperature-induced diffusion of probe vanadium(IV) ions into the matrix of titanium dioxide as investigated by ESR techniques

Abstract: Heat treatments of multicomponent oxides are known to induce an interdiffusion of the cations. In the case of V 2 O 5 /TiO 2 (rutile) mixed compounds, vanadium(IV) ions can be included as dopants inside the TiO 2 matrix. This reaction leads formally to a V x Ti 1-x O 2 solid solution

Lattice Constants of Boron Carbides

Abstract: In this paper, the lattice constants of boron carbides are determined by powder X-ray diffraction for samples with compositions between about 7.7 and 20.5 at.% carbon. The boundaries of the single-phase region are at about 9 at.% carbon and near, but likely somewhat less than 20 at.% carbon. The composition dependence of the lattice constants thus established provides a method of assessing the carbon concentration of unknown materials. In particular, assignment of the approximate composition of single crystals used in previous studies allows for a systematic examination of changes in interatomic separation as a function of composition. These changes are discussed in terms of a structural model of the boron carbide solid solution.

High Mechanical Strength of Quasicrystalline Phase Surrounded by fcc-Al Phase in Rapidly Solidified Al–Mn–Ce Alloys

Abstract: A nonequilibrium structure consisting of fine quasicrystalline (icosahedral) grains surrounded by fcc-Al phase was found to form in rapidly solidified Al-Mn-Ce alloys. The formation of the nonequilibrium structure is limited to a composition range of 4 to 6 at% Mn and 2 to 3%Ce which is located around the phase boundary between fcc solid solution and icosahedral plus amorphous phases. The icosahedral phase consists of equiaxed grains with a size of 50 to 100 nm and the width of the fcc-Al phase along the grain boundary is as small as 5 to 15 nm

Neutron powder diffraction study of the åkermanite-gehlenite solid solution series

Abstract: The crystal structures of members of the akermanite-gehlenite solid solution series have been refined from high resolution neutron powder diffraction data. Synthetic samples of 0, 25, 50, 75 and 100% akermanite composition were used. The results are consistent with the ordering scheme for the tetrahedrally-coordinated cations represented by the structural formula Ca2[Mg x Al1-x]1[Si1+x Al1-x]2O7 where the superscripts denote inequivalent tetrahedral sites. No evidence was found for the existence of phase transitions in the samples with x ≤ 0.75, in contrast to the behaviour found in pure akermanite. The relationship between the cell parameters of meilite and the tetrahedral conformations and the resulting non-ideality of the solid-solution are discussed.

Kinetics of tweed and twin formation during an ordering transition in a substitutional solid solution

Abstract: A kinetic model based on the theory of strain-induced interactions is developed to describe the effect of elastic strain on structural transformation kinetics. An ordering transition resulting in a reduction in the crystal lattice point-group symmetry in a substitutional solid solution is considered. Our computer simulation of a two-dimensional model binary alloy revealed the detailed kinetics of formation of tweed morphologies and the tweed → twin coarsening process driven by the transformation-induced elastic strains.

Solid state amorphization in binary Ti-Ni, Ti-Cu and ternary Ti-Ni-Cu system by mechanical alloying

Abstract: An amorphous phase has been synthesized by mechanical alloying in a planetary mill over a nickel content range of 10–70 at.% in the TiNi system and a copper content range of 10–50 at.% in the TiCu system. In the case of ternary TiNiCu alloys the glass-forming composition range has been found to be given by x = 10–20 for Ti60Ni40 − xCux, x = 10 – 30 for Ti50Ni50 − xCux and x = 10 – 40 for Ti40Ni60 − xCux alloys. The difficulty in the amorphization of copper-rich compositions is explained in the light of enthalpy composition diagrams calculated for the ternary solid solution and the amorphous phase.

Formation of alkoxy-derived yttrium aluminium oxides

Abstract: Monoclinic Y4Al2O9 and hexagonal YAlO3 crystallize at low temperatures from amorphous materials prepared by the hydrolysis of yttrium and aluminium double alkoxides. Hexagonal YAlO3 transforms to the cubic phase with a garnet structure as an intermediate product at elevated temperatures. The formation process of YAlO3 is described. Solid solutions of hexagonal YAlO3 crystallize between 50 and 62.5 mol % Al2O3. Yttrium aluminium garnet Y3Al5O12(YAG) is formed by transformation of the solid solution.

Method for calculating alloy energetics.

Abstract: A semiempirical method for the computation of alloy energies is introduced. It is based on the equivalent-crystal theory of defect-formation energies in elemental solids. The method is both simple and accurate. Heats of formation as a function of composition are computed for some binary alloys of Cu, Ni, Al, Ag, Pd, Pt, and Au using the heats of solution in the dilute limit as experimental input. The separation of heats into strain and chemical components helps in understanding the energetics. In addition, lattice-parameter contractions seen in solid solutions of Ag and Au are accurately predicted. Good agreement with experiment is obtained in all cases.

Application of an Ion‐Packing Model Based on Defect Clusters to Zirconia Solid Solutions: I, Modeling and Local Structure of Solid Solutions

Abstract: Oxygen vacancies can be introduced into zirconia solid solution ZrO2–MOu (u= 1 and 1.5) to maintain electroneutrality. Recently, the local structures around Zr4+ and M2u+ ions in ZrO2–MOu solid solutions have been studied through EXAFS spectroscopy, diffuse scattering analysis, and single-crystal structure analysis. The present study constructs an ion-packing model for zirconia solid solutions based on some defect cluster models. The decrease of cell volume with the occurrence of vacancies is assumed to be expressed by decreasing the coordination number (CN) of cations around the vacancy. The distribution of CNs in a solid solution was calculated from a certain defect cluster model. The average interatomic distances, the average CN, and the short-range order parameters were calculated using this distribution of CNs. The local structures calculated from the model were compared with experimental data in the systems ZrO2–MO1.5 (M = Y, Gd, Yb, and Ca). In the ZrO2–YO1.5 system, the r(s–O) interatomic distance, where s represents Zr4+ or Y3+ and O represents O2−, decreased with Y content and therefore vacancy content. The probability of finding Y3+ around a vacancy increases with increasing yttria content from a comparison of the calculated results with the ones from recent EXAFS studies. The present model can qualitatively explain compositional and size dependences of the dopant on various local structures.

Characterization of C─B─N Solid Solutions Deposited from a Gaseous Phase between 900° and 1050°C

Abstract: The composition, the structure, and some physical properties of boron–carbon–nitrogen solid solutions prepared by chemical vapor deposition (CVD) have been investigated. Both crystalline films and amorphous granular materials resulting respectively from heterogeneous and homogeneous nucleation were characterized by X-ray diffraction, XPS, RBS, and TEM. The compositions of these single-phase materials are gathered in two main domains located in the B/N > 1 part of the C─B─N composition diagram. It is stated that the carbon-rich domain results from structural disorder of an ideal C5B2N composition. The thermal behavior of these films indicates that no mass loss can be detected after 1 h at 1700°C but a graphitization and a formation of small amounts of B13C2 are observed. Density and preliminary electrical conductivity measurements were also performed.

Compositional and Microstructural Characterization of RuO2 ‐ TiO2 Catalysts Synthesized by the Sol‐Gel Method

Abstract: In this study mixtures were prepared from solution using and three different precursors for, , and and tris (2, 4‐pentanedionate) Ru(III). Multi‐layer coatings were deposited by spin coating onto fused silica slides with and without an intermediate, 200 nm thick, vacuum deposited Ti film. Samples heat treated at 200, 300, and 400°C were studied by several techniques (Rutherford backscattering spectrometry (RBS), elastic recoil detection, (ERD), nuclear reaction analysis (NRA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and x‐ray diffraction (XRD) to characterize morphological and compositional changes and to test the reproducibility of the preparation method. In all cases a single crystalline phase, the solid solution of the two component oxides, was detected by XRD. The films appeared to have a low density, and hydrogen and carbon were still found in the coatings after each heat treatment; their content was reduced only at 400°C for samples B and C. The surface of the films displayed increasing percentages of .

Compositional range of the Bi2Sr2CaCu2OxHTc-superconductor and its surrounding phases

Abstract: The extension of the single-phase field of the two-layer superconductor Bi 2 Sr 2 CaCu 2 O 8 is clarified for 830°C in air. The single-phase field of “2212” is a trigonal bipyramid with a length of 13.7 mol% parallel to the SrOCaO edge, and an extension of 9 mol% perpendicular to it. The tolerance of the lattice concerning CuO variation is smaller than ± 1.2 mol%. The CaO-rich solid solution is deficient in CuO compared to the ideal 2212 solid solution, Bi 2 (Sr 1− x Ca x ) 3 Cu 2 O 8 , whereas the SrO-rich side has excess CuO. Multiphase equilibria around “2212” as well as the single phase field were derived by combining the XRD and EDX results of 91 samples of different compositions with the previously published results. The intersections of the multiphase equilibria with three different planar sections of the compositional tetrahedron are shown.

Phase diagram of the system Bi1.6Pb0.4Sr2CuO6-CaCuO2 between 825°C and 1100°C

Abstract: The phase diagram of the lead-substituted Bi 2 O 3 -SrO-CaO-CuO system has been investigated in the temperature range 825°C–1100°C, and in static air along the line Bi 1.6 Pb 0.4 Sr 2 Ca n −1 Cu n O 2 n +4+ x . It involves nine distinct solid phases and two liquid ones. The transformation and melting lines of the superconducting n = 1, 2, 3 phases are located below 900°C. Below the solidus (∼855°C), the sequence of stable { n } phases is {1}, {1}+{2}, {2}, {2}+{3}, and {3}. The {3} phase (110K) exists as the dominant phase between ∼835°C and ∼875°C for 3 n t >∼900°C) mainly contains the transformation and melting lines of CuO, (Ca, Sr) 2 CuO 3 , and (Ca, Sr) 14 Cu 24 O y . The liquids involves a eutectic point at (∼930°C, n =2) and a critical end point of miscibility of the two liquids L 1 (bismuth rich) and L 2 . The presence of Pb shifts the transformation lines of the superconducting phases downwards by 10–15°C. The practical implications of this diagram are discussed.

Melting and dissolving

Abstract: The diffusion-governed melting that occurs when a binary melt is placed in contact with a pure solid is described. It is shown that if the melt superheat is much greater than the solid supercooling, the melt composition at the interface equals that of the solid and so the solid will melt at a rate determined by the thermal diffusivity. However, as the liquid superheat decreases, chemical disequilibrium may lower the interface temperature and so the melt composition at the interface increases above that of the solid, according to the liquidus relation. In this case the solid will dissolve into the liquid at a rate determined by the solutal diffusivity. These diffusion-governed solutions are used to infer the different modes of convection which may rise when the interface between the solid and the melt is horizontal.The theory is generalized to investigate the diffusion-governed melting of a binary solid solution placed in contact with a binary melt. If the melt superheat is sufficient then the rate of phase change is again determined by the thermal diffusivity. In this case, owing to the very small solutal diffusivity in both the solid and the liquid, the melt composition at the interface is nearly equal to that of the solid. This corresponds to the melting regime. As the liquid superheat decreases, the rate of phase change decreases to values determined by the solutal diffusivity in the liquid, and the melt composition at the interface evolves towards that of the far-field liquid. This corresponds to the dissolving regime. As the melt superheat decreases further, with the solid still changing phase into liquid, then the melt composition at the interface remains approximately equal to that of the far-field melt. In each case, a compositional boundary layer develops in the solid, just ahead of the interface, in order to restore the solid at the interface to thermodynamic equilibrium. These different phase change regimes may arise if the composition of the solid is either higher or lower than that of the liquid.

Structural evolution of mechanically alloyed TiAl alloys

Abstract: Two titanium alloys with 24 and 50 at% Al respectively, were mechanically alloyed in a steel vial with hardened steel balls using a Spex 8000 mixer-mill The sequence of phases formed has been investigated by both X-ray diffraction and transmission electron microscopy With increasing milling time a solid solution of aluminum in titanium, intermetallic Ti 3 Al, an amorphous phase and an fcc crystalline phase formed sequentially The formation of the amorphous phase has been rationalized in terms of Miedema's thermodynamic analysis The results available on the fcc phase suggest that this phase may be a titanium nitride formed by pick-up of nitrogen during milling and that it has not formed by the crystallization of the amorphous phase per se

Optical-conductivity spectra of Sr1-xLaxTiO3: Filling-dependent effect of the electron correlation.

Abstract: Optical-conductivity spectra have been measured on the correlated electron system ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${\mathrm{TiO}}_{3}$ (0\ensuremath{\le}x\ensuremath{\le}1) in order to determine how the electronic structure depends on the filling of the 3d-electron conduction band. While metallic resistivity was observed in all samples with x\ensuremath{\ge}0.1, the optical-conductivity spectra have revealed a critical change in the electronic structure: The effective carrier mass, which has been deduced by analysis of the optical-conductivity spectra and the energy-loss function, is critically enhanced as the x=1 filling case (Ti 3${\mathit{d}}^{1}$ configuration) is approached, perhaps due to electron-correlation effects, in a manner reminiscent of the Mott-Hubbard transition.

Competition between stable and metastable phases during mechanical alloying and ball milling

Abstract: The TiAl and the TiSi system are chosen as model systems to study the origin of metastable phase formation during mechanical alloying of elemental powder blends with a negative heat of mixing The results demonstrate that kinetics play only a minor role in the phase selection, in particular for the final state Instead, the intermetallic phases are energetically destabilized with respect to the solid solutions and the amorphous phase due to the destruction of the chemical long-range order upon milling Thus the latter phases tend to form during mechanical alloying of elemental poweder belnds as well as during milling of powders of the stable intermetallic compounds The metastable phases can be predicted by the free energy curves which are calculated by the CALPHAD method In case of the TiAl and TiSi alloys, metastable structures and grain sizes in the nm range can be obtained which promise improved mechanical behaviour with respect to the processing of light-weight intermetallic compounds for high-temperature applications Die Systeme TiAl und TiSi wurden als Modellsysteme mit negativer Mungsenthalpie fur das mechanische Legieren elementarer Pulvermischungen ausgewahlt, um die Ursache fur die Bildung metastabiler Phasen zu untersuchen Die Ergebnisse zeigen, das die Kinetik bei der Phasenbildung keine wesentliche Rolle spielt, insbesondere fur den Endzustand Vielmehr werden aufgrund der Zerstorung der chemischen Fernordnung beim Mahlen die intermetallischen Phasen im Vergleich zu den Mischkristallen und der amorphen Phase energetisch destabilisiert Das begunstigt die Bildung dieser Phasen beim mechanischen Legieren elementarer Pulvermischungen sowie beim Mahlen von Pulvern intermetallischer Gleichgewichtsphasen Mit freien Enthalpiekurven, die nach der CALPHAD-Methode berechnet wurden, last sich die Struktur der metastabilen Phasen vorhersagen Im Falle von TiAl- und TiSi-Legierungen konnen metastabile Strukturen mit Korngrosen im nm-Bereich prapariert werden, die verbesserte mechanische Eigenschaften im Hinblick auf die Herstellung von intermetallischen Phasen mit geringer Dichte fur Hochtemperaturanwendungen erwarten lassen