Kinetic interpretation ofα- andβ-Si3N4 formation from oxide-free high-purity silicon powder

A kinetic analysis of the isothermal nitridation of high-purity oxide-free silicon powder is described. The kinetic analysis suggests that the and polymorphs of Si₃N₄ are formed by separate and parallel reaction paths. This analysis provides for the decoupling and quantitative kinetic interpretation of- and-Si₃N₄ formation reactions. Consistent with existing microstructural and thermodynamic evidence, the-forming reaction is shown to obey a first-order rate law, whereas a phase-boundary controlled rate law describes the-forming reaction. A kinetic model employing these rate laws is developed and is used to predict the/ phase ratio as a function of isothermal reaction temperature and extent of reaction. The/ phase ratios so obtained are shown to be in good agreement with experimental observations made under a variety of reaction conditions.     

Microstructural and dimensional stabilities of a potential γ/γ′-α(Mo) directionally solidified eutectic superalloy under cyclic thermal exposure to 1000° C

A potential heat-resistant ductile eutectic composite,/-, in the Ni-Al-Mo ternary system has been thermally cycled in the temperature range 200 to 1000° C for up to 1000 cycles in an attempt to examine dimensional as well as microstructural stability of the composite under thermal fatigue conditions. The composite examined has two types of initial microstructure; in one, blocky -Ni₃Al encircles individual-Mo fibres (as-grown condition) whereas in the other, is in the form of fine cuboidal particles uniformly distributed in an Ni-rich fcc matrix (heat-treated condition). Dilatometric measurements upon temperature cycling show that the composite is stable against thermal ratchetting irrespective of initial microstructural conditions. However, the cycling induces microstructural change, which is characterized by segmentation of-Mo fibres or formation of a detrimental brittle phase identified as an intermetallic-NiMo that consumes-Mo fibres whether the fibres are encircled by or not. Post-cycling tensile tests at room temperature show that the fibre damage in the former has no fatal effect on tensile strength and ductility. A beneficial effect of the-encircling configuration is discussed on the basis of the recognition of a peritecto-eutectoid reaction:++ that has been disregarded.     

Microstructure and mechanical properties of metastable lath martensite wires in the Fe-Ni-Cr-Al-C system produced by melt spinning in rotating water

Metastable lath martensite ( _L ) phase wires with high strengths have been produced in the Fe-Ni-Cr-Al-C alloy system by melt spinning in rotating water. These wires have a circular cross section and a white lustre and the wire diameter is in the range of 100 to 140m. The width and length of each lath in the _L phase are as small as about 0.3 and 2m, respectively. The _y, _f and _p are about 900 and 1650 MPa and 2.0% for the _L wires. The subsequent annealing causes an increase in _p as well as _y and _f and the attained values are about 1000 and 1700 MPa and 3.0% for Fe-10Ni-10Cr-6.5 Al-1.0C wire annealed at 773 K for 1 h owing to the precipitation strengthening of a very fine unidentified carbide and to a high density of dislocations and lath boundaries in the _L phase. Further annealing causes a significant decrease in _p through decomposition of _L to+M₇C₃+M₂₃C₆. Therefore, the high strength combined with relatively good ductility for the _L wires is interpreted as due to the suppression of the phase transformation of _L to a mixed structure of+M₇C₃+M₂₃C₆ by melt quenching.     

The effect of explosive loading on the microstructure of some metals and alloys

The effects of explosive pulses on the microstructure of Al-4.5% Cu duralumin, tough pitch copper, pure magnetic iron and 18 chromium 8 nickel stainless steel have been studied using transmission electron microscopy. An explosive loading assembly was designed to investigate the effect of both the flanks and the length of shock pulses on the material microstructure. In copper and duralumin it was found that the microhardness increased with the duration of the rarefaction flank. The size of the shock transformed martensite particles in stainless steel also increased with rarefaction time. Three wave interactions could be related to the microstructure of baratol-loaded iron and were attributed to the double shock front caused by the a to phase transformation. Twinning was found to occur when the reflected elastic wave interacted with the first part of the double shock front. The abrupt hardness increase caused by the interaction of the first reflected wave and the second transformation shock front was observed. In addition, a second more gradual hardness increase is attributed to the interaction between the low pressure and the high pressure rarefaction waves which caused a relatively slowly increasing rarefaction shock. The microstructure of shocked stainless steel showed that hexagonal plates and twins could occur in the same grain and that martensite particles were formed at the intersection of two twins, or of plates and twins. Both Kurdjumov-Sachs and Nishyama-Wassermann orientations were observed and sometimes occurred on the same fault band. The short rise and fall times in the pulse flanks made it possible to discuss formation mechanisms for twins and martensite particles in terms of the available formation times.     

A nonlinear composite shell element with continuous interlaminar shear stresses

A numerical model for layered composite structures based on a geometrical nonlinear shell theory is presented. The kinematic is based on a multi-director theory, thus the in-plane displacements of each layer are described by independent director vectors. Using the isoparametric apporach a finite element formulation for quadrilaterals is developed. Continuity of the interlaminar shear stresses is obtained within the nonlinear solution process. Several examples are presented to illustrate the performance of the developed numerical model.List of symbols reference surface - convected coordinates of the shell middle surface - ⁱ coordinate in thickness direction - ⁱ h thickness of layer i - X_o position vector of the reference surface - ⁱX_o position vector of midsurface of layer i - t _k orthonormal basis system in the reference configuration - ⁱ a _k orthonormal basis system of layer i - ⁱW axial vector - R_o orthonormal tensor in the reference configuration - ⁱ R orthonormal tensor of layer i - ⁱ Cauchy stress tensor - ⁱ P First Piola-Kirchhoff stress tensor - ⁱ q vector of interlaminar stresses - ⁱ n, ⁱ m vector of stress resultants and stress couple resultants - v _x components of the normal vector of boundary - ⁱ N, ⁱ Q, ⁱ M stress resultants and stress couple resultants of First Piola-Kirchhoff tensor - stress resultants and stress couple resultants of Second Piola-Kirchhoff tensor - ⁱ , ⁱ , ⁱ strains of layer i - _K transformation matrix - u_o displacement vector of layer 1 - ⁱ local rotational degrees of freedom of layer i     

Kinetics of the α → γ phase transformation in LiAlO2 under various atmospheres within the 1073–1173 K temperatures range

Stability of the LiAlO₂ crystallographic forms within given temperature limits, especially the occurrence of -LiAlO₂ phase transformation, is one of the crucial problems of application of this compound as solid phase matrix for electrolyte in molten carbonate fuel cells (MCFC). In this study, kinetics of this transformation has been investigated under atmospheres of CO₂ and N₂, both dehydrated and water vapor containing, at constant temperatures 1073, 1123 and 1173 K, in the -LiAlO₂ powder prepared in a molten salts ambience. The increasing -LiAlO₂ phase concentration in the -LiAlO₂ sample was determined by X-ray diffractometry as dependent on time of heating the sample. Similar measurements have been performed with -LiAlO₂ samples after 5 h of heating at temperatures within 923–1223 K. The results of this investigation suggest the existence of some chemical steps of the overall phase transformation process, probably the decomposition of the double lithium-aluminum oxide at the -LiAlO₂ particle surfaces via a surface-nucleation mechanism. This step determines the overall transformation rate, strongly depending on gas atmosphere and on molten carbonate ambience (solution-deposition mechanism). This transformation does not take place below 973 K, which is the upper temperature limit of the MCFC operation.     

Methanation of carbon deposited directly from CO2 on rhodium-bearing activated magnetite

Methanation reactivity was studied for the surface carbon deposited from CO₂ on the surface of Rh-bearing activated magnetite. The most active material (Rh=0.83 wt %) for methanation was prepared by the impregnation method at 60°C and showed 98% conversion at 300°C. The surface carbon was composed of elemental carbon (-carbon) and polymerized carbon (-carbon), the proportion being dependent on the density of carbon deposited. In temperature-programmed surface reaction, the extent of conversion of the - and -carbon to CH₄ was 0.34 (-carbon) and 0.53 (-carbon), respectively, and the total conversion was 0.87. This result indicates that not only elemental carbon but polymerized carbon (-carbon) could be converted to CH₄ on the Rh-bearing activated (-carbon) magnetite, whereas -carbon is not hydrogenated on activated magnetite.     

Analysis of the effect of the decomposition of char-forming heat-insulating materials on the free discharge of a gas mixture

The article presents results of a numerical solution of a nonsteady problem on the free discharge of a mixture of gases from a hemispherical volume with allowance for thermal decomposition of heat-insulating materials.Notation V volume - S area - t - P p - T - u v - Q q, dimensional and dimensionless time, pressure, temperature, TIM decomposition rate, and heat flux - adiabatic exponent - R gas constant - density - H specific enthalpy - c specific heat - thermal conductivity - , , _s dimensionless complexes - coefficient expressing the radiative properties of the gas medium and the heat-transfer surface - Stefan-Boltzmann constant Indices 0 initial state and scale factors - s surface - coke - M TIM material - P pyrolysis front - A ablation front - v volatile degradation products - adiabatic conditions - c completion of discharge Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 54, No. 5, pp. 787–793, May, 1988.     

Boundary layer similarity flow driven by power-law shear

Summary Similarity solution of the Prandtl boundary layer equations describing wallbounded flows and symmetric free-shear flows driven by rotational velocitiesU(y)=y are determined for a range of exponents and amplitudes . Asymptotic analysis of the equations shows that for <–1 no similarity solutions with proper algebraic decay exist. For wall-bounded flow, exact solutions found at =–1/2 and =1 correspond to an Airy function wall jet and uniform planar Couette flow. Numerical integration of the governing similarity equation reveals singular behaviour for wall-bounded flows as ₀ = –2/3, and no solutions are found in the range –1<–2/3. For >–2/3 the shear stressf(0) parameter is determined as a function of and . Symmetric free-shear flow solutions become singular as ₀ = –1/2 and no solutions are found in the range –1<–1/2. For >–1/2 the centerline velocityf(0) is determined as a function of and . An asymptotic analysis of the singular behavior of these two problems as ₀, given in a separate Appendix, shows excellent comparison with the numerical results. Similarity solutions at the critical values ₀ have exponential decay in the far field and correspond to the Glauert wall jet for wall-bounded flow and to the Schlichting/Bickley planar jet for symmetric free-shear flow.     

A Method for Calculating Thermal Radiation Properties of Multilayer Films from Optical Constants

A calculation method for the incident angle dependence of the solar absorptance _S and the temperature dependence of the total hemispherical emittance _H of multilayer films is proposed. The method is based on calculation of _S and _H from optical constants in the wavelength region from 0.25 to 100 m for thin polymer films and deposited metal. In this paper we provide values of _S in the incident angle region from 0 to 90° and _H in the temperature range from 173.15 to 373.15 K for two-layer samples of aluminum-deposited polyimide film. The results obtained for _S and _H by the present method are compared with experimental results measured by both spectroscopic and calorimetric methods. The calculated results of _S and _H agree well with the experimental results.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Dislocations and faults in some alloys containing copper and aluminum

By using the method of variance of the respective X-ray diffraction line profile, the effective particle size (p) and r.m.s.. strain (S ^21/2) of four alloys containing copper and aluminium (Al-0.08, 4 and 10 wt% Cu and Cu-5.87 wt% Al) at different annealing stages (starting from the cold filed stages) have been determined. From the peak shift of these samples, their average stacking fault probability = – where is the intrinsic stacking fault probability and the corresponding extrinsic stacking fault probability, respectively, were determined. Similarly, from the angular distance between the peak and the centroid of the diffraction profiles, the magnitude of + 4.5, where is the twinning fault probability, was measured. From these, the true particle sizeT, the stacking and twinning fault probabilities , and and the minimum stacking fault widthD _min have been determined. The hardness of alloys at different annealing stages was found to be dependent on the dislocation density and the stored energy in the alloys. The relationT S ^21/2 = constant was found to be valid for the alloys and from this a mechanism of grain growth with annealing has been suggested. With annealing the dislocation density and stored energy per unit volume were found to decrease until finally, in the fully annealed stages, they disappeared. The same was found to be valid for stacking and twinning fault probabilities as well as the minimum stacking fault width. A mechanism for the creation and annihilation of the intrinsic and extrinsic stacking faults has been suggested.     

Scattering in a Spin-Charge Separated Non-Fermi Liquid

The electron self-energy due to the scattering on nonmagnetic impurities in a spin-charge separated non-Fermi liquid is calculated. The frequency dependence of the inverse scattering time is proportional to , for the case when the anomalous parameter = 0, and to ¹⁺ for 0. The scattering time is cutoff dependent when the anomalous parameter is nonzero.     

An exact calculation of Price's law for the law of Lotka

Price's law asserts — in its simpliest version — that authors produce half of the papers made by the total ofN authors. More generally: the topN (0<<1) authors produce a fraction (0<<1) of the papers made by the total ofN authors and the Price's law says that . In this paper — using Lotka's law — we prove a mathematical relationship of in function of and the parameter (the mean number of papers per author) and investigate when . More-over our reasoning uses the theory of the 80/20 rule as developed in: L. EGGHE, On the 80/20-rule,Scientometrics, 10 (1986) 55–68, thereby also showing the relation betwwen the 80/20-rules (being an arithmetical form of measuring elitarism) and Price's law (being a geometric form of measuring elitarism).     

The athermal α → ω transformation in Zr-2 at% Nb alloy

The athermal transformation in Zr-2 at.% Nb alloy has been investigated by transmission electron microscopy. Analysis of the selected-area diffraction pattern has shown that the orientation relationships between the omega and the parent-phase in quenched Zr-2 at.% Nb alloy are the same as have been previously observed for the reaction in pure zirconium. Thus it was deduced that the direct transition has taken place in the alloy during cooling. The-originated -particles were visualized using the dark-field technique. The formation of the athermal omega in the-region of-stabilized Zr-Nb alloy is discussed in terms of the relative positions of the free energy equilibrium curvesT ₀ ,T ₀ ,T ₀ and the correspondingM _s ,M _s andT _s start curves. It is concluded that the omega phase can occur over a much wider range of alloy compositions than is usually recognized on the basis of transformation data.     

Die Herleitung von Schalengleichungen über ein erweitertes Variationsprinzip bei Berücksichtigung der Querschubverzerrungen

Ohne ZusammenfassungBezeichnungen L Bezugsgrößen für dimensionslose Koordinaten - L charakteristische Schalenabmessung - t Schalendicke - Schalenparameter - körperfeste, krummlinige, dimensionslose Koordinaten der Schalenmittelfläche - Dimensionslose Koordinate in Richtung der Schalennormalen - i, j,...=1,2,3 Indizierung des dreidimensionalen Euklidischen Raumes - ,,...=1,2 Indizierung des zweidimensionalen Riemannschen Raumes - (...), Partielle Differentiation nach der Koordinate - (...), Kovariante Differentiation für Tensorkomponenten des zweidimensionalen Raumes nach der Koordinate - (...)| Kovariante Differentiation für Tensorkomponenten des dreidimensionalen Raumes nach der Koordinate - Variationssymbol - a ,a ₃ Basisvektoren der Schalenmittelfläche - V Verschiebungsvektor - U ,U ₃ Verschiebungskomponenten des Schalenraumes - v ,w,w ,W Verschiebungskomponenten der Schalenmittelfläche - Verhältnis der Metriktensoren des Schalenraumes und der Schalenmittelfläche - _ik Verzerrungstensor des Raumes - _{(, )}, Symmetrische Verzerrungstensoren der Schalenmittelfläche - _{[, ]} Antimetrischer Term des Verzerrungsmaßes - ^, Spannungstensor - n ,m ,q Tensorkomponenten der Schnittgrößenvektoren - p ,p,c Tensorielle Lastkomponenten     

The origin of heteroepitaxy in the system of uniaxially oriented isotactic polypropylene and polyethylene

The epitaxy of polyethylene (PE) on oriented and -form of isotactic polypropylene (PP) at different crystallization temperatures and rates have been investigated. The result shows that the epitaxy of PE on -form of PP does not differ from that on the -form of PP. The epitactic alignment of the PE lamellae on the PP films becomes more perfect with higher crystallization rates of PE. The epitaxy of PE on - or -form of PP does not occur at crystallization temperatures higher than 123 °C. A nucleation controlled process for the alignment of the PE is proposed.     

Activated slip systems during yielding of α-β brass two-phase bicrystals

- brass two-phase bicrystals, consisting of fcc () single crystals and bee () single crystals, which were made by the solid state diffusion couple technique, were tensile-tested at room temperature in order to clarify the role of phase-interface on the deformation. The two-phase bicrystals had small concentration gradients in the- and-phases and satisfied the Kurjumov-Sach's orientation relationships i.e. {1 1 1} {1 1 0} and [1 1 0] [1 1 1] at the interface. The slip traces observed in bicrystals deformed to about 3% plastic strain showed a striking contrast between the- and-phases; the slip traces in the-phase were clear and straight, while those in the-phase were fine and wavy. The slip systems in the bicrystals were attributed to those observed in and single crystals, and were explained by a plastic strain incompatibility mechanism. The slip systems, originating at the interface or propagating from another phase, were observed on matching planes.On leave from Mechanical Engineering Department, Faculty of Engineering, Al-Azhar University, Cairo, Egypt.     

Spray atomization of two Al–Fe binary alloys: solidification and microstructure characterization

The effect of solidification history on the resultant microstructures in atomized Al–3Fe and Al–7Fe powders is studied, with particular emphasis on the relationships between droplet size, undercooling and phase stability. The atomized Al–Fe powders exhibit four microstructural features, i.e. Al3Fe phase, Al + Al6Fe eutectic, -Al dendrite and a predendritic structure. The presence of these is noted to depend on a kinetic phase competitive growth mechanism, which was determined by the initial undercooling experienced by the powders. The results of scanning electron microscope analysis demonstrate that the content of Fe in the -Al phase increases with decreasing powder particle size, i.e. for Al–3 wt% Fe powders, the content of Fe in the -Al phase is 2.21 and 2.56 wt% corresponding to powder particle sizes of 90 and 33 m, respectively; for Al–7 wt% Fe powders, the content of Fe in the -Al phase is 5.51 and 5.98 wt% corresponding to powder sizes of 90 and 33 m, respectively. In the present study, homogeneous nucleation undercooling, corresponding to the -Al phases, is also estimated using an existing correlation.     

The optical properties and a.c. conductivity of magnesium phosphate glasses

Magnesium phosphate [X MgO-(100–X) P₂O₅] glasses in the composition range [X=20, 25, 30, 40, 45, 50 mol %] have been made. The optical properties and a.c. conductivities were measured and their amorphous nature confirmed by X-ray diffraction technique. The variation of relative density with x was anomalous. In the ultraviolet/visible regions it was found that the fundamental absorption edge is a function of glass compositions and lower absorption coefficients, () follow the so-called Urbach edge. At lower absorption levels (1<<10⁴cm^–1), the width of the tail of localized states in the band gap, E _g, did not vary significantly with glass composition and lay in the range (0.26–0.343) eV. In the high absorption region (()>10⁴ cm^–1), the behaviour of () suggests that there are two different transition energies for electrons in k-space, namely direct allowed transitions and non-direct transitions. In the infrared region at wavelengths =2.5–30 m, the transmission spectrum has four absorption bands. Using the Kramers-Kronig theory, the optical constants (refractive index n and extinction coefficient k) have been determined from the transmission spectrum. The a.c. conductivity, (), real and imaginary dielectric constants, ₁, ₂, and loss factor, tan , have been determined at room temperature in the frequency region, = 2×10⁴–10⁶ Hz. It has previously been established theoretically that () ^s and s was found to be in the range 0.64–0.73, depending on glass composition.