Crystallization of amorphous Fe78B13Si9

Crystallization of amorphous Fe₇₈B₁₃Si₉ has been investigated using a combination of differential scanning calorimetry (DSC) and conventional and high-resolution transmission electron microscopy. The crystallization mechanisms and crystalline products are sensitive to the annealing temperature. At 450C, crystallization takes place by the growth of b c c -Fe (Si) dendrites, while at 510 and 515C there are three simultaneous reactions to form dendritic b c c -Fe (Si), elliptical crystals of b c t Fe₃B and lamellar eutectic spherulites of b c c -Fe (Si) and b c t Fe₃B. Quantitative TEM shows that the b c c -Fe (Si) dendrites and b c c -Fe (Si)-b c t Fe₃B spherulites both form with constant nucleation and growth rates, in agreement with previous. DSC measurements of an Avrami exponent of 4.     

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mössbauer spectra of epitaxial -Fe₁₆N₂ and -Fe₈N films have been studied and their differences are discussed in detail. The Mössbauer spectrum of -Fe₁₆N₂ can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mössbauer spectrum of -Fe₈N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for -Fe₁₆N₂ than for -Fe₈N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for -Fe₁₆N₂. The iron moments tend to locate in the film plane for -Fe₁₆N₂ and to arrange perpendicularly to the film plane for -Fe₈N.     

Mössbauer study on the magnetic state of iron particles in Fe-N and Fe-Zr-N soft magnetic thin films

The magnetic state of -Fe particles and the behaviour of nitrogen and zirconium during annealing in Fe₉₆N₄ and Fe_85.6Zr_7.6N_6.8 magnetic thin films have been studied by conversion electron Mössbauer spectroscopy for ⁵⁷Fe. The crystalline phases present in the Fe-N annealed films were -Fe and -Fe₄N, and those in the Fe-Zr-N annealed films were -Fe and ZrN. In the Fe-N films annealed below 300°C, about 60% nitrogen is incorporated interstitially into -Fe and the rest is used for the formation of -Fe₄N. In the Fe-N film annealed at 500°C, almost all nitrogen participates in the formation of -Fe₄N, leading to the grain growth of -Fe particles and an increase in coercive force. The values (291–325 kOe) of internal magnetic field of iron sites in -Fe in the Fe-Zr-N films are much smaller than that (333 kOe) of the iron site in pure -Fe. Even if the Fe-Zr-N films were annealed at 500–700°C, some zirconium and nitrogen is still incorporated substitutionally and interstitially into -Fe, respectively. In particular, the substitutional zirconium depresses the grain growth of -Fe particles, perhaps due to a chemical interaction between zirconium and iron.     

The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable phase was identified in the first stage of dissolution nucleating at the - boundaries. Interface dislocations at the - boundaries forming a perpendicular net in the 110 directions were identified as of screw character and of Burgers vector b=1/2a 110. During dissolution a change of shape of precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 110 directions.     

Iron dispersed carbon composites formed from iron-polyvinylalcohol complexes

Iron-polyvinylalcohol (Fe-PVA) complexes have been pyrolysed at the temperatures up to 1000 K, and the iron-carbon composites formed have been characterized. The yield of carbon was much higher for the complexes than for PVA alone. The degree of carbon graphitization and the chemical form of iron species were dependent on the pyrolysis temperature. About 30 wt% fine particles of Fe₃O₄ or -Fe were dispersed in the matrix of amorphous carbon at 800 or 900 K, respectively. At 1000 K, -Fe was partly transformed to Fe₃C, and the agglomeration of -Fe was not so significant. At this temperature the carbon was graphitized, which resulted in a lowering of the surface area of the composite. It is suggested that the graphitization proceeds through the mechanism involving the formation and subsequent decomposition of Fe₃C. Thus, the use of Fe-PVA complexes achieves a high yield of carbon and a high dispersion of a large amount of iron species throughout the carbon matrix.     

Flux-line cutting in type II superconductors

Experimental evidence for flux-line cutting in superconductors (intersection and cross-joining of singly quantized vortices) is briefly reviewed. The interaction energy between two straight vortices tilted at an angle ( 0)is then shown to be finite in the London model, i.e., in the limit of vanishing core radius. Next, the activation energy and maximum interaction force are calculated for the vortices in an analytic approximation to the Ginzburg-Landau theory. Here two competing interactions determine the behavior. Electromagnetic repulsion (0 < < /2) varies as cos and decays over distances scaled by the penetration depth , while core attraction is independent of and varies over distances scaled by the coherence length . The force is always repulsive at large flux-line separation (0 < < /2) and its maximum decreases rapidly as decreases, so that flux-line cutting isexpected to be more probable in low- materials. The calculations provide a basis for explaining longitudinal flux-flow resistance as well as some intriguing magnetization behavior in the same configuration.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences Division, and by the Deutsche Forschungsgemeinschaft.On leave from Max-Planck-Institut für Metallforschung, Institut für Physik, Stuttgart, West Germany.On leave from New University of Ulster, Coleraine, Northern Ireland.     

Phase transformation behavior in a multipurpose dental casting gold alloy during continuous heating

Phase transformation in a multipurpose dental casting gold alloy during continuous heating was studied by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. The behavior can be explained by the following reaction sequences in the nodule: ₁(FCC) + ₂ (L1₂) ₁(fcc) + ₂(L1₂) + (L1₀), where fcc is face centred cubic. A discontinuous precipitation with very fine nodules contributed to the hardening and the growth produced the softening. This multipurpose gold alloy is characterized by the introduction of a PtZn ordered phase with L1₀ structure instead of a CuAu I phase.     

The morphology of α-Fe crystals which grow in the amorphous Fe80(C1−xBx)20 alloys

Crystallization behaviour of amorphous Fe₈₀(C_1–x B _x )₂₀ alloys, obtained by splat-cooling technique, for x values ranging from zero to unity has been investigated mainly by transmission electron microscopy. The crystalline phase which first appeared in the amorphous matrix was -Fe for all alloys studied. However, the morphology of -Fe phase changed from a spherical shape for low x values to a watch-glass shape for intermediate x values and to dendritic for large x values. The nucleation of -Fe crystals was homogeneous for low x samples while preferred nucleation on edges and surfaces was noted for samples with higher x values. The final volume fraction of -Fe phase before the appearance of the second crystalline phase increased with the increase in x.     

Preparation of ultrafine Fe-Si-C powders in a radio-frequency thermal plasma and their catalytic properties

Ultrafine Fe-Si-C powders were prepared from mixed gases of SiH₄-CH₄-Fe(CO)₅ using a newly developed thermal-plasma apparatus with a dual-radio-frequency-coupled-plasma-torch system. The phases of the powders prepared were classified into two groups; one composed of -FeSi₂ which was prepared by rapid quenching, while the other was composed of -Fe, Fe₃Si, Fe₅Si₃, FeSi, -SiC and amorphous Si, which were prepared by slow quenching. The diameters of these powders were in the range 5–50 nm. The catalytic activities of these powders for Fischer-Tropsch synthesis were examined. The ultrafine -FeSi₂ powder was very active for the selective formation of olefin. On the other hand, powders of -Fe, Fe₃Si and Fe₅Si₃ were low in their selectivity but gave high CO conversion. Relations between the plasma conditions for the preparation of ultrafine Fe-Si-C powders and their catalytic properties are discussed.     

Preferentially orientated precipitation in MnTiO3 single crystals

Preferentially orientated precipitation of -TiO₂ (rutile) needles in single crystals of MnTiO₃ grown by a floating-zone method under a controlled atmosphere of oxygen fugacity was studied. Optical microscopy and X-ray analysis revealed that the orientation relation between the -TiO₂ precipitates and the MnTiO₂ matrix is: {000 1}MnTiO₃// {111}-TiO₂ and 11¯20MnTiO₃// 110-TiO₂. The precipitation phenomenon was explained by introducing a nucleation and growth mechanism which was experimentally supported by heat treating experiments.     

Analytical electron microscopy of Al2O3 implanted with iron ions

Single crystals of -Al₂O₃ were implanted with iron ions at room temperature to fluences ranging from 4×10¹⁶ Fe cm^–2 to 1×10¹⁷ Fe cm^–2. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques. Special emphasis was placed on monitoring the microstructural changes which take place during post-implantation annealing. Clusters of metallic -Fe were identified in the specimen after implantation to a dose of 1×10¹⁷ Fe cm^–2. Analytical electron microscopy of implanted specimens annealed in oxygen revealed the redistribution of the implanted iron and the formation of surface precipitates of -Fe₂O₃, subsurface precipitates of various forms of spinel, and, in some cases, subsurface precipitates of iron, depending on the annealing temperature. Examination of implanted specimens annealed under reducing conditions revealed the presence of precipitates of -Fe.     

Application of coherent-optical methods in physical studies of evaporation

The application of the methods of laser and holographic interferometry to the problems of measuring evaporation rates is examined. Mass transfer accompanying evaporation in the region of extradiffusion monitoring is studied.Notation j_v specific mass flux of the vapor, kg/(m²·sec) - _c coefficient of condensation - _v coefficient of evaporation - D coefficient of diffusion - m²/sec, wetting angle - R radius of the vessel, m - time, sec - h height of the segment, m - wavelength of the laser radiation, m - N order of the interference fringe or the number of fringes passing through a given point - density of the matter, kg/m³ - length of the cell with the liquid of interest, m - T temperature, K - n refractive index of the medium - m mass - kg, V volume of the spheroidal segment, m³ - _liq and _g molar mass, kg/mole - C₀ and C_p concentration of vapor in the gas, kg/m³ - a and b semiaxes of the spheroid, m - _e ellipsoidal coordinate of a point on the surface, m² - c/n derivative along the normal - r coordinate of a point on the surface of evaporation, m - n change in the refractive index of the medium Indices st standard liquid - liq liquid - g gas - 0 starting state - p equilibrium pressure Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 4, pp. 605–611, October, 1988.     

On the analysis of tunneling data for A15 superconductors

We examine the applicability of the standard McMillan inversion of Eliashberg's equations for superconductors with a nonconstant electronic density of statesN(). We do this usign simple models forN() and a realistically shapedN() taken from recent band structure work for Nb₃Sn. It turns out that peak structure inN() near _F may lead to gross errors in the derived Eliashberg function ²F() when the energy dependence ofN() is omitted in the inversion procedure. For Nb₃Sn, this leads to a 40% overestimate of when ²F() is evaluated via the standard McMillan program.     

Mössbauer study of nanocrystallization in amorphous Fe-P-Si-V alloys

Nanocrystallization of amorphous Fe-P-Si-V alloys during annealing is studied by Mössbauer spectroscopy and x-ray diffraction. The resulting nanostructured materials are shown to contain not only equilibrium phases ( -Fe and Fe₃P) but also a significant amount of the metastable paramagnetic phase Fe₂P and trace levels of the and D phases. Nanocrystallization markedly enhances the strength of the alloys but has an adverse effect on their magnetic properties.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 427–433.Original Russian Text Copyright © 2005 by Baldokhin, Vavilova, Kovneristyi, Kolotyrkin, Palii, Perfilev.     

Anisotropic grain growth of R-α-sialon (R = Nd and Er)

Two R--sialon (R_0.6Si_9.3Al_2.7O_0.9N_15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the phase transformation. Elongated -sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R--sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R--sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain.     

Characterization of ultrafine γ-Fe(C), α-Fe(C) and Fe3C particles synthesized by arc-discharge in methane

Ultrafine -Fe(C), -Fe(C) and Fe3C particles were prepared by arc-discharge synthesis in a methane atmosphere. The phases, morphology, structure and surface layer of the particles were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) techniques and X-ray photoelectron spectroscopy (XPS). It was found that the mean particle size ranged from 9.8 to 12.8 nm. The surface of particles mostly consisted of a carbon layer and a little oxide. Phase transformation from -Fe(C) to -Fe(C) was studied by annealing in vacuum and by differential thermal analysis and thermogravimetry (DTA–TGA) measurement. The abundance of -Fe(C) was determined by a magnetization measurement to be approximately 30%. Phase transformation occurred between 300 and 500 °C in a flowing argon atmosphere. The Fe3C particles oxidized to -Fe2O3 and carbon dioxide at 610 °C or so. © 1998 Chapman & Hall     

Thermal diffusivity of inhomogeneous systems

The possibility of analyzing the nonsteady temperature fields of inhomogeneous systems using the quasi-homogeneous-body model is investigated.Notation t, t_I, t_i temperature of quasi-homogeneous body inhomogeneous system, and i-th component of system - a, , c thermal diffusivity and conductivity and volume specific heat of quasi-homogeneous body - a_{i i}, c_i same quantities for the i-th component - q heat flux - S, V system surface and volume - x, y coordinates - macrodimension of system - dimensionless temperature Fo=a/² - Bi=/ Fourier and Biot numbers - N number of plates - =h/ ratio of micro- and macrodimensions - _V, volumeaveraged and mean-square error of dimensionless-temperature determination - time - m_i i-th component concentration Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 1, pp. 126–133, July, 1980.     

Studies on phase transformations of Cu-phthalocyanine thin films

Copper phthalocyanine (CuPc) thin films were obtained by a sublimation technique on Si wafer substrates maintained at room temperature. As-deposited CuPc films with less than 0.1 m thickness crystallize primarily in the -form with a preferential orientation of the crystallites in the [2 0 0] direction. The effect of randomizing of the orientation of the CuPc crystallites is observed as the film thickness increases, whereas the preference in appearance of the -form remains. The changes in phase composition, structure, morphology and surface chemical composition of as-deposited CuPc thin films due to different heat treatment conditions were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).     

An investigation of the ordered DO19 phase formation in the Ti-Al system

For the Ti-rich, Ti-AI system, the- and- ₂ phase boundaries have been obtained through differential thermal analysis for alloy compositions ranging from 10.2 to 25.2 at% Al (6 to 16 wt% Al). It has been shown that in the above mentioned composition range, a metastable disordered-phase can be quenched in. This metastable then transforms to the ordered ₂(DO)₁₉ phase upon heating and/or isothermal ageing. It has also been observed that the kinetics of this ordering phenomenon is composition dependent.     

Effect of Intense Self-Irradiation on the Phase Composition of Metallic Plutonium

The crystal structure of film samples of "high-level" (based on ²³⁸Pu) and low-level (based on ²³⁹Pu) metallic plutonium during their prolonged (up to 343 days) storage (self-irradiation) at room temperature was studied by X-ray diffraction. In the samples of high-level plutonium, the -Pu and -Pu lattices coexist. In the period of 40-60 days, the other known crystal modifications of plutonium (-Pu, -Pu, -Pu, and -Pu) are also present. Low-level plutonium had only the -Pu lattice. A possible origin of this phenomenon is discussed.