Electron microscopic investigations on a melt-quenched Al-Rh alloy

An Al-2 wt% Rh alloy was subjected to rapid quenching from the molten state and transmission electron microscopy and diffraction techniques were extensively used to characterize the phases present. Melt quenching led to the formation of a homogeneous solid solution, indicating a substantial increase in solid solubility limit over the equilibrium value. Although stable up to about 640 K, the supersaturated solid solution decomposed at elevated temperatures to a mixture of aluminium solid solution and an intermediate phase, which has been identified to have a monoclinic structure witha=1.636 nm,b=0.805 nm,c=1.279 nm andΒ=107.77° with a stoichiometry corresponding to Al₁₃Rh₄. Although heterogeneous nucleation at grain boundaries was the predominant feature, the precipitate also formed as WidmanstÄtten platelets inside the grains. This phase has been shown to be an equilibrium phase. Very long annealing treatments at temperatures of 773 K resulted in the formation of an ordered phase. The long-range order was destroyed on heating to still higher temperatures. The equilibrium constitution of the alloy has been shown to be a mixture of Al₁₃Rh₄ and aluminium solid solution.     

Phase formation in rapidly quenched Cu-based alloys

In the present investigation, we report the formation of γ-brass type phase in the rapidly quenched Cu₅₀Ga₃₀Mg₅Ti₁₅ and Cu₅₀Al₃₀Mg₅Ti₁₅ alloys. Rapid solidification of Cu₅₀Ga₃₀Mg₅Ti₁₅ alloy shows the formation of simple cubic γ-brass type phase (a = 0.863 nm), which on annealing at 1,023 K for 60 h transforms to disordered type γ-brass phase (a = 0.879 nm). It has been observed that intensity modulation of electron diffraction spots corresponding to simple cubic γ-brass phase is similar to the intensity modulation observed in the mirror orientation of icosahedral quasicrystalline phase. Contrary to the crystalline phase formation in Cu–Ga–Mg–Ti alloy, rapid solidification of Cu₅₀Al₃₀Mg₅Ti₁₅ shows the formation of amorphous and nanocrystalline bcc γ-brass type phase (a = 0.870 nm), which on annealing transforms to ordered γ-brass type phase (a = 0.872 nm). The structural and microstructural characterization was done through X-ray diffraction and transmission electron microscopy.     

A structural investigation of stabilized oxygen evolution catalysts

A ternary mixed oxide containing SnO₂, RuO₂ and IrO₂ was shown to exhibit a more stable electrocatalytic behaviour than simpler catalysts consisting of noble metal oxides only. X-ray diffraction and transmission electron microscopy were used to characterize the structure of these oxide catalysts. It was found that precipitation of the fine catalyst powders resulted in the formation of a mixed (Sn, Ru, Ir)O₂ rutile phase. This phase was shown to be thermodynamically unstable, decomposing to SnO₂ and (Ru, Ir)O₂ on annealing at 800° C. X-ray photoelectron spectroscopy revealed that the surface composition of these mixed oxide catalysts varies considerably from the bulk composition. The formation of such metastable, mixed oxides is discussed.     

Crystallization of amorphous Te-20 at% Sn alloy

Amorphous Te-20 at% Sn alloy was obtained by rapid cooling from the liquid state. Phase transitions occurring during continuous heating of amorphous films were studied by differential thermal analysis, X-ray diffraction and transmission electron microscopy. It was found that the crystallization of the amorphous phase begins at 382 K and proceeds via nucleation and growth of metastable phases MS1 and MS2. The first of these phases was assigned the primitive cubic structure with lattice parameter a=3.2Å. The second phase, being a Te (Sn) solution, was assigned the hexagonal structure with lattice parameters a=4.45Å and c=5.85Å. During heating at 410 K the remaining amorphous phase decomposed into a mixture of Te crystals together with metastable phase MS3, which probably has the ZnS type structure with lattice parameter a=6.05Å. Within the temperature range 450 to 550 K the MS1 phase was transformed into SnTe, and the MS2 phase into Te. The metastable intermediate phase MS3 decomposed only near the solidus temperature, and the alloy attained its equilibrium structure.     

Metastable structures of liquid-quenched and vapour-quenched antimony films

Antimony foils/films have been prepared by liquid-quenching and by vapour-quenching. (1) Simple cubic (a=3.16 Å), (2) f c c (a=4.61 Å), (3) tetragonal (a=3.01 Å, c=4.96 Å and c/a=1.64) structures, and (4) a new rhombohedral (a _H=4.12 Å, c _H=10.26 Å and c _H/a _H=2.49) metastable structure have been observed in splat-quenched foils. The metastable rhombohedral phase transforms to the equilibrium rhombohedral phase (a _H=4.30 Å, c _H=11.27 Å and c _H/a _H=2.62) on heating. The simple cubic, f c c and tetragonal phases are converted in to a hexagonal close-packed (a=3.33 Å, c=5.23 Å and c/a=1.57) structure on heating in the electron microscope. The simple cubic and h c p phases correspond to the known high-pressure phases of antimony. The h c p phase does not transform on further heating. Films prepared by vapour deposition on substrates held at room temperature show the same metastable f c c and rhombohedral phases as observed in splat-quenched foils, in addition to the equilibrium rhombohedral phase. On heating, these metastable phases transform in the same way as the splat-quenched specimens. The films deposited at liquid air temperature are amorphous; on electron-beam heating, these crystallize first to an unidentified complicated structure and then to the equilibrium structure.     

Transmission electron microscopic observation of a metastable phase on the thermal decomposition process of Ca-deficient hydroxyapatite

Calcium-deficient hydroxyapatite (Ca-def HAp) decomposes to stoichiometric hydroxyapatite (HAp) and β-tricalcium phosphate (β-TCP) at high temperature. In a previous study, we reported that a metastable phase with a high Ca/P molar ratio appeared in the temperature range from 700 to 800°C. In the present study, the formation process of a metastable phase and the crystallographic relationship between the Ca-rich metastable phase and HAp matrix were investigated by high-resolution transmission electron microscopy (HRTEM). Ca-def HAp was annealed at 600–850°C for 2 or 6 h in air. TEM observations were performed before and after annealing Ca-def HAp. Based on analysis of image of Ca-def HAp before annealing, several HAp crystals with different aspect ratios agglomerated. The metastable phases grew thicker by long-term annealing. HRTEM image suggested that the Ca-rich metastable phase was formed by migration to the interface and continuous accumulation of calcium ions from HAp crystals with a small aspect ratio. From HRTEM images and results of the analysis of selected area electron diffraction patterns along the [010], [110] and [001] zone axes, lattice constants of the metastable phases were determined to be a = 2.86 nm, b = 0.94 nm, and c = 0.69 nm with orthorhombic crystals system.     

Crystallization behaviour of amorphous Sc100−x Fe x alloys near x=25 composition

A detailed study of the process of crystallization in amorphous Fe₂₅Sc₇₅, Fe₂₈ Sc₇₂ and Fe₂₅Sc₄₅Zr₃₀ alloys has been carried out using differential scanning calorimetry, X-ray diffraction and Mössbauer spectroscopic techniques. The complex multi-stage transformation process in Fe₂₅Sc₇₅ and Fe₂₈Sc₇₂ is understood in terms of polymorphous formation of an intermediate metastable crystalline phase of orthorhombic structure (a = 0.521 nm, b = 0.648 nm and c = 1.212 nm) and of likely Sc₃Fe stoichiometry, before final eutectic crystallization to stable Fe₂Sc and -Sc.     

Properties of Bi-Sr-Ca-Cu-O highT c superconductor

The effect of sintering in air and annealing in oxygen flow at various temperatures and times on the superconducting transition of Bi₂Sr₂Ca₁Cu₂O₈ ceramic was studied. Samples sintered at 1138 K for 24 h in air and slowly-cooled to room temperature are single-phase with the highest zero resistivity temperature of 88 K. XRD pattern showed that the single-phase was characteristic of tetragonal structure witha=0.5398 nm andc=3.0620 nm. Both a.c. susceptibility and resistivity measurements give similar values for the onset (100 K) of superconducting transition.     

Decomposition behaviour of rapidly quenched Al–Nb alloys– TEM study

Abstract

Two binary Al-rich Al–Nb alloys containing 4·5 and 7·7 wt-%Nb have been rapidly quenched from the liquid state using the ‘gun’ technique. Transmission electron microscopy and diffraction techniques have been extensively employed to characterize the as quenched as well as externally heat treated alloy foils. Each alloy in the as-quenched state shows only a supersaturated solid solution, indicating a substantial increase in the solid solubility limit of Nb in Al by rapid quenching. Elongated solid solution grains and defect structures have also been observed in the as-quenched foils. While no precipitation was found to occur on annealing the quenched dilute alloy (Al–4·5Nb) foils for 1 h at 673 K, precipitation along grain boundaries was observed in the concentrated alloy (Al–7·7Nb). Higher temperature annealing (≥773 K) resulted in the formation of a new metastable phase having an ordered fcc GeCa₇ type structure with a lattice parameter a ~ 0·8 nm. This phase forms predominantly with a rodlike morphology and is arranged in a Widmanstätten pattern inside the grains, although fine precipitate particles have also been observed. On prolonged annealing at or above 773 K, the metastable phase transforms with the formation of the equilibrium tetragonal Al₃Nb phase.

MST/391     

Thermobaric treatment induced changes in the structure and magnetic properties of manganese antimonide

An orthorhombic phase of Mn₂Sb (a=0.562 nm; b=0.432 nm; c=0.757 nm) was obtained for the first time in the manganese antimonide samples treated at a pressure of P=7 GPa and a temperature of T=1800 K. The high-pressure phase is metastable under normal conditions and breaks on heating above 420 K. A characteristic feature of this phase is the absence of a total magnetic moment at T>90 K.     

Microstructural development of Zn–40Al alloy during aging

Abstract

The microstructure development of a Zn–40 wt-%Al alloy during aging was studied using transmission electron microscopy and electron diffraction. The supersaturated solid solution α′_s phase was formed after solution quenching treatment, which exhibited a substructure with a structure factor contrast in some areas after aging for shorter times at ambient temperature. Electron diffraction results show that there are satellite reflections in the 〈110〉 _α′s direction. The α′_sphase decomposes during aging, forming an (α+η) lamellar structure by the discontinuous reaction or cellular decomposition and an equiaxed fine grained structure by the continuous reaction, which are α″_m and α″ or α′_m and α′ with twin relationships to each other. A very fine tweedlike structure was observed to spinodally decompose ahead of the cellular zone. The value of the modulation wavelength λ is about 70–85 nm. The bowing and migration of the grain boundary would occur in the evolution of the discontinuous precipitates. Precipitate free zones of the vacancy denuded type were found in the vicinity of the grain boundary and within grains. It has been observed that the formation of the metastable R phase is always associated with dislocations.     

Structure observations by high-resolution electron microscopy of Ni-B melt-spun alloys (B<30 at%)

The microstructure of a series of Ni-B alloys (15 at%3B phase (a=0.44 nm,b=0.52 nm,c=0.66 nm) is expected. High-resolution electron microscopy images of Ni-25 at% B rapidly quenched indicate that a two-phase structure is observed. A metastable phase is observed with a periodicity of 1.0 nm. HREM images and EELS analyses led us to propose the structure of this metastable phase. This phase (Ni₅B₂) is correlated with the monoclinic Hägg carbide Fe₅C₂. Depending on the velocity of the substrate and on the boron concentration, a variety of microstructures has been observed from a faulted to a semi-amorphous structure. The structures are related to the solidification behaviour and the heat flow in the undercooled melt. Particular attention was paid to the eutectic Ni-Ni₃B composition.     

Two phase co-existence in YBa2(Cu1−x Co x )3O7−δ superconductor with x=0.03

Co-doped impurity-free YBa₂(Cu_1–x Co _x )₃O_7–(123Co_x) with x=0.03 superconducting samples have been synthesized using conventional solid state reaction techniques. A Rietveld analysis of X-ray diffraction data for these specimens reveals that two structures (one tetragonal represented by T-YBCO, and the other orthorhombic represented by O-YBCO) co-exist at this composition of the cuprate. The use of a single tetragonal or single orthorhombic structure as the model for the refinements does not produce acceptable fits to the X-ray diffraction pattern. The refinements show that the T-YBCO phase at 298 K has tetragonal symmetry (P4/mmm) with a=0.387879(4) nm, c=1.17314(1) nm, and that the O-YBCO phase has at 298 K an orthorhombic symmetry (Pmmm) with a=0.387555(4) nm, b=0.389400(4) nm, c=1.17363(1) nm, respectively.     

A metastable phase Al3Cu2

A new metastable intermediate phase having a trigonal unit cell belonging to the space groupP¯3ml has been detected in a rapidly solidified aluminium-45 at. % copper alloy. The unit cell dimensions area=4.106 andc=5.094 å. With five atoms per unit cell, the observed structure can be regarded as an isotype of Al₃Ni₂.     

Epitaxial growth and stabilization of the compound Ag3Sb on silver {1 1 1}

The vapour deposition of antimony on silver {1 1 1} under ultra-high vacuum conditions has been studied by means of reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). At room temperature, no evidence for sub monolayer structures was obtained and a deposit of an apparent thickness equivalent to six monolayers was necessary before any diffraction features typical of the overlayer could be detected. With increasing film thickness, the formation of islands with random rotational orientation was detected with Sb 〈100〉 normal to the substrate surface. Annealing of the deposit at 573 K led to the formation of the intermetallic compound Ag₃Sb in epitaxial relation to the substrate. Lattice parameters for pseudo-cubic antimony (a=0.613 nm) and hexagonal Ag₃Sb (a=0.295 nm andc=0.482 nm) were obtained.     

Transmission electron microscopy on the microstructure of 7050 aluminium alloy in the T74 condition

The microstructure of 7050 aluminium alloy in the T74 condition has been investigated by transmission electron microscopy. It was found that the alloy contains the superlattice Al₃Zr phase, η′ phase and Al₇Cu₂Fe constituent phase. The η′ phase is proposed to have an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The orientation relationship between the matrix and η′ phase is [11−2]_m//[100]_η′; [1−]_m//[010]_η′;[−1−1−1]_m//[001]_η′. The phases on the small-angle grain boundary are found to be mainly η′ phase and Cu/Si-rich phase, whereas on the large-angle grain boundary there is only η phase.     

The solid solution structure of BiSr x Ca1−x O2.50

The formation and solid-solution structure of BiSr _x Ca_1–x O_2.50 were studied by X-ray powder diffraction analysis, and the crystallography data of the phase are given. The crystal structure of BiSr _x Ca_1–x O_2.50 belongs to the monoclinic system with space group P1m1 or P12/m1, and the cell parameters for BiCaO_2.50 (x=0.00) are a=1.8363 nm, b=0.5366 nm, c= 1.4670 nm and =100.26°. BiSr _x Ca_1–x O_2.50 is a solid solution of Bi-Ca-O and Bi-Sr-O. When the strontium solid solubility Sr/Sr+Ca=0.50, the strontium dissolved in the phase reaches saturation, while the strontium solid solubility limit of the phase is between 0.67 and 0.75, and beyond this limit the crystal structure is greatly distorted.     

CO-DEPOSITION OF FULLERENES AND SULFUR FROM DILUTED SOLUTION

ABSTRACT

A new method to synthesize fullerene and sulfur compounds has been developed. Using this method, C₆₀S₁₆ and C₇₀S₁₆ compounds were grown from dilute fullerene and sulfur toluene solution. Their atomic structures were analyzed by x-ray diffraction with the single crystal. The C₆₀S₁₆ crystal is C-centered monoclinic structure of a=2.0874?nm, b=2.1139?nm, c=1.05690?nm and β=111.93°, and the C₇₀S₁₆ has a primitive monoclinic, P2₁/c, with lattice parameters of a=1.5271?nm, b=1.49971?nm, c=2.18024?nm and β=109.791°. In this compound, the structure of fullerenes is maintained and sulfur atoms form S₈ rings placed around the fullerenes.     

The structure of Al x FeNi phase in Al-Cu-Mg-Fe-Ni alloy (AA2618)

The structure of the aluminide phase (Al _x FeNi) in cast AA2618 has been investigated by means of electron probe microanalysis and transmission electron microscopy techniques. The Al _x FeNi phase has been determined to have a C-centred monoclinic structure with lattice parametersa=0.8673 nm,b=0.9000 nm,c=0.8591 nm, and =83.504°. It is also shown that the structural formula of the phase varies from particle to particle and is influenced by the aluminium content.     

Structure and mechanical properties of polyethylene-fullerene composites

The microhardness of films of fullerene-polyethylene composites prepared by gelation from semidilute solution, using ultrahigh molecular weight polyethylene (PE) (6×10⁶), has been determined. The composite materials were characterized by optical microscopy and X-ray diffraction techniques. The microhardness of the films is shown to increase notably with the concentration of fullerene particles within the films. In addition, a substantial hardening of the composites is obtained after annealing the materials at high temperatures (T _a=130 °C) and long annealing times (t _a=10⁵s). The hardening of the composites with annealing temperature has been identified with the thickening of the PE crystalline lamellae. Comparison of X-ray scattering data and the microhardness values upon annealing leads to the conclusion of phase separation of C₆₀ molecules from the polyethylene crystals within the material. The temperature dependence is discussed in terms of the independent contribution of the PE matrix of the C₆₀ aggregates to the hardness value.