Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the + phase field at 900 C. The microstructures obtained at room temperature after cooling from 900 C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the -phase, molybdenum and silicon to the -phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the -phase and the resultant microstructure. In quenched material containing transformed , substantial age hardening can be obtained in the range 350 to 600 C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.     

Effect of the weight of the admixture on the turbulence structure of a two-phase jet

The effect of gravity on the turbulence structure of an inclined two-phase jet is evaluated according to the Prandtl theory of mixing length.Notation C_x drag coefficient for a particle - D_p particle diameter - g_i components of the acceleration g due to gravity acting on a particle in the direction of jet flow (g_i=g sin ) and in the direction normal to it (g_i=g cos ) - V_poi ^±, V_goi ^± fluctuation components of the velocities of the particles and gas, respectively, at the end of a mole formation - V_fi free-fall velocity of a particle - l _u mixing length - m_p particle mass - t _p length of time of particle-mole interaction - V_pi ^±, V_gi ^± positive and negative fluctuation velocities of particles and of the gas respectively, with the components u_p ^±, u_g ^±, v_p ^±, v_g ^±, k=V_goi/V_fi - V_i ^± relative velocity of the gas - jet inclination angle relative to the earth's surface - empirical constant - _u, jet boundaries in terms of velocity and concentration - _u=y/ _u dimensionless velocity ordinate - =y/ dimensionless concentration ordinate - admixture concentration - u_m, _m velocity and the concentration of the admixture at the jet axis - _g dynamic viscosity of the gas - _s, _g densities of the particle material and of the gas - _g, _p shearing stresses in the gas and in the gas of particles - _m, ₀ shearing stresses in the mixture and in pure gas, respectively Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 3, pp. 422–426, March, 1981.     

Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si₃N₄ which was investigated involves the nitridation of Si:Si₃N₄ powder compacts containing additions of sintering aids (e.g. Y₂O₃ and Al₂O₃) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si₃N₄ forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si₃N₄ phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si₃N₄ grains. The grains are surrounded by a thin intergranular amorphous film.     

Morphological and crystallographic features of γ-γ′ Ni-Al and Ni-Al-Ti two-phase bicrystals

Several- Ni-Al and Ni-Al-Ti two-phase bicrystals were made by the solid-state diffusion couple method. Each couple consisted of a-phase single crystal and a pure-Ni polycrystal, and was annealed at 1473 K in an Ar gas atmosphere. Single crystal layers of-phase with uniform thickness always grow into the parent-phase single crystals. The resultant/ interface has no voids or facets regardless of the orientation of interface and the chemical composition of the-phase. Porosity formation due to the Kirkendall effect is observed in the diffused region. Concentration profiles exhibit nearly constant gradients in-phase. The orientation relationship between both phases is found to be 001//001, that is, the-phase grows epitaxially along the crystal orientation of-phase.     

The microstructure of rapidly solidified Ti-32wt% Fe melt-spun ribbon

A Ti-32wt% Fe alloy was rapidly solidified by the melt-spinning technique. Three microstructures are found in the melt-spun ribbon of the alloy: the first contains small FeTi particles in a matrix, the second is a cellular microstructure composed of and FeTi and the third is a eutectic of these phases. The correlation between various solidification and cooling rates across the ribbon width to the three microstructures is explained.     

Measurement of temperature in a non-steady-state gas flow

A method is described for measuring the temperature of a non-steady-state gas flow with a thermocouple which is an inertial component of the first order.Notation T*_f non-steady-state gas flow temperature - T_t thermosensor temperature - thermal inertia factor of thermosensor - time - C total heat capacity of thermosensor sensitive element - S total heat-exchange surface between sensitive element and flow - heat-liberation coefficient - temperature distribution nonuniformity coefficient in sensitive element - Re, Nu, Pr, Bi, Pd hydromechanical and thermophysical similarity numbers - P^* total flow pressure - P static flow pressure - T^* total flow temperature - d_t sensitive element diameter - w gas flow velocity - flow density - flow viscosity - _f flow thermal conductivity - k gas adiabatic constant - R universal gas constant - M Mach number - T thermodynamic flow temperature - _o, _o and values at T=288°K - A, m, n, p, r coefficients - _c heat-liberation coefficient due to colvection - _r heat-liberation coefficient due to radiation - _b emissivity of sensitive element material - Stefan-Boltzmann constant - T_e temperature of walls of environment - _c, _r, _tc thermosensor thermal inertia factors due to convective, radiant, and conductive heat exchange - L length of sensitive element within flow - a thermal diffusivity of sensitive element material - _t thermal conductivity of sensitive element material Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 59–64, July, 1984.     

Group Actions on Binary Resilient Functions

Let G_n,t be the subgroup of GL(n,₂) that stabilizes {x₂ⁿ:|x|t}. We determine G_n,t explicitly: For 1tn–2, G_n,t=S_n when t is odd and G_n,t=S_n, when t is even, where S_n<GL(n,₂) is the symmetric group of degree n and GL(n,₂) is a particular involution. Let _n,t be the set of all binary t-resilient functions defined on ₂ⁿ. We show that the subgroup ₂ⁿ(G_n,tG_n,n–1–t)<AGL(n,₂) acts on _n,t/₂. We determine the representatives and sizes of the conjugacy classes of ₂ⁿS_n and ₂ⁿS_n,. These results allow us to compute the number of orbits of _n,t/₂ under the above group action for (n,t)=(5,1) and (6,2). Keywords:General linear group, Affine linear group, Resilient function.     

Accuracy of temperature measurements in determining the thermal conductivity of substances by stationary methods in the range of moderate temperatures

An empirical comparison is made of the accuracy of platinum-rhodium-platinum and Chromel-Alumel thermocouples in determining the thermal conductivity of substances.Notation T, t temperature - temperature difference - y thermocouple readings - A_i parameters of approximating equation - sensitivity of thermocouple - sensitivity found from the generalized function - _st standard values of sensitivity - S₀ standard deviation of sensitivity for a given series - maximum deviation of from in different series - I, II indices indicating that the values pertain to platinum-rhodium —platinum and Chromel —Alumel thermocouples, respectively Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 306–310, August, 1980.     

Tensile properties and plastic deformation modes ofβ Zr-Nb alloys

Tensile properties and plastic deformation modes of zirconium-niobium alloys were investigated at 290 and 77 K in the wide composition range from metastable to stable phase. Three types of plastic deformation modes, {332}113 twinning, {112}111 twinning and slip, were observed depending on alloy composition and temperature. {332}113 twinning, which occurs in metastable zirconium alloys, is related to the stability of phase to decomposition and leads to low yield stress and large elongation. On the other hand, {112}111 twinning, which appears in stable zirconium alloys, results from high critical stress for slip due to solution hardening and high Peierls stress and does not affect tensile properties significantly. The results obtained for zirconium-alloys are similar to those for titanium alloys, strongly suggesting that {332}113 twinning is an important plastic deformation mode which is common to phase alloys containing athermal phase.     

Thermal expansion behaviour of sodium-beta-alumina

The thermal expansion of sodium -and '-alumina in thea andc directions has been determined by high temperature X-ray diffractometry using polycrystalline samples. The measured values show that internal stresses as a result of anisotropy in the thermal expansion of the two phases should have little adverse effect on the application of beta-alumina in electrochemical devices.     

Surface layers in polycrystalline sodium \-alumina

X-ray powder diffractometry using CrK radiation has revealed the existence of surface layers 5m deep in samples of high-density polycrystalline sodium -alumina. The surface layer is prominent by virtue of its crystal lattice distortion in which the c-axis is elongated by 1%. Its presence is due to ion exchange between Na⁺ and H₃O⁺ that occurs on exposure to air.     

Impulse approximation in solid helium

The incoherent dynamic form factorS _i(Q, ) is evaluated in solid helium for comparison with the impulse approximation (IA). The purpose is to determine theQ values for which the IA is valid for systems such as helium where the atoms interact via a potential having a steeply repulsive but not infinite hard core. For³He,S _i(Q, ) is evaluated from first principles, beginning with the pair potential. The density of statesg() is evaluated using the self-consistent phonon theory andS _i(Q, ) is expressed in terms ofg(). For solid⁴He reasonable models ofg() using observed input parameters are used to evaluateS _i(Q, ). In both casesS _i(Q, ) is found to approach the impulse approximationS _IA(Q, ) closely for wave vector transfersQ20 Å^–1. The difference betweenS _i andS _IA, which is due to final state interactions of the scattering atom with the remainder of the atoms in the solid, is also predominantly antisymmetric in (–_R), where _R is the recoil frequency. This suggests that the symmetrization procedure proposed by Sears to eliminate final state contributions should work well in solid helium.     

The setting of gypsum plaster

The - and -forms of calcium sulphate hemihydrate, CaSO₄·1/2H₂O, were characterized using thermal, X-ray diffraction, optical and electron microscopy techniques. The differences between the two forms arose from their mode of production, resulting in different crystal size and habit, and not from different crystal structures. The hydration of CaSO₄·1/2H₂O to CaSO₄·2H₂O was investigated using calorimetry, phase analysis and the changes in electrical resistance of the plaster slurries. The reaction occurred in three steps: dissolution, nucleation and precipitation, and finally completion due to depletion of reactants. The differences in reactivity of the - and -forms with water again appeared to be due to their different physical states.     

Stress-induced phase transformations and enhanced plasticity in copper-aluminium and copper-aluminium-zinc martensites

The effect of deformation on the structure of the-type martensite has been studied in copper-aluminium and copper-aluminium-zinc alloys by X-ray analysis and mechanical testing. This study has yielded information on the stability of the crystal structure of-type martensite, i.e. the tendency of the stress-induced transformation to the fcc structure, as zinc is added in increasing amount to copper-aluminium. The detailed study of the X-ray powder diffraction patterns of the as-quenched martensites showed that as zinc is added in increasing amounts, the structure of the-type martensite becomes distorted and shows an increasing degree of orthorhombic distortion (i.e. deviation from the trigonal symmetry). When these martensites are plastically deformed, the structure is transformed more and more easily into an fcc lattice, i.e. the volume fraction of the fcc phase increases with zinc content. Tensile tests showed that the martensite showed higher and higher elongation values, when plastically deformed, which is due to the progressive increase in fcc phase.     

Approximate treatment of plane channel flow

Summary The governing equation for steady two-dimensional channel flow are deduced from the equations of inviscid fluid dynamics. Streamline inclination and curvature effects are approximately accounted for. The integration procedure aims at the elimination of the spatially transverse coordinate; through it one arrives at a second order non-linear ordinary differential equation for the free surface profile having the form of an (extended) Bernoulli-equation and providing formulae for the pressure and velocity distributions. A second derivation, applicable to channels with horizontal Thalweg and arbitrary prismatic cross section, leads to a similar equation for the free surface profile. Illustrations concern solitary waves in rectangular and trapezoidal cross sections. Comparison of theoretical predictions of wave geometry with experiments indicate advantages of our formulation over previous more extensive ones.Notation a horizontal projection of radius of the flow section - b bottom width of the trapezoidal channel - d differential - f=Q ²/gb ² h ₀ ³ non-dimensional flow. Froude-number in rectangular channel - g gravity constant - h pressure head - k streamline curvature - m cotangent of side slope of trapezoidal channel - m ^* mass of a fluid particle - n coordinate, normal to streamlines - p pressure - q specific discharge (per unit width) - r quotient of boundary curvaturesr=R _o /R _u =K _u /K _o - s streamline coordinate - t water depth - u horizontal velocity component - v vertical velocity component - x horizontal coordinate - y non-dimensional pressure head - z bottom height function - z ^* vertical coordinate - F cross sectional area - H specific energy - N length of plane flow section measured alongn - Q discharge - R radius of curvature - S specific momentum - V algebraic velocity (along streamlines) - X non-dimensional horizontal coordinate - Z non-dimensional bottom profile - slope factor - non-dimensional geometrical factor for trapezoidal cross section =mh _o /b - specific weight - bottom amplitude - non-dimensional pressure head - non-dimensional horizontal coordinate - non-dimensional vertical coordinate - v non-dimensional normal coordinate - density - non-dimensional water depth - inclination angle of streamlines relative to horizontal direction - auxiliary variable - p _dyn dynamical pressure head With 6 Figures     

Hydroxyapatite-coated metals: Interfacial reactions during sintering

Electrophoretic deposition (EPD) is a low cost flexible process for producing HA coatings on metal implants. Its main limitation is that it requires heating the coated implant in order to densify the HA. HA typically sinters at a temperature below 1150C, but metal implants are degraded above 1000C. Further, the metal induces the decomposition of the HA coating upon sintering. Recent developments have enabled EPD of metathesis-synthesised uncalcined HA which sinters at 1000C. The effects of temperature on HA-coated Ti, Ti6Al4V, and 316L stainless steel were investigated for dual coatings of metathesis HA sintered at 1000C. The use of dual HA coatings (coat, sinter, coat, sinter) enabled decomposition to be confined to the undercoat (HA layer 1), with the surface coating decomposition free. The tensile strength of the three metals was not significantly affected by the high sintering temperatures (925C < T < 1000C). XRD/SEM/EDS analyses of the interfacial zones revealed that 316L had a negligible HA:metal interfacial zone (1 m) while HA:Ti and HA:Ti6Al4V had large interfacial zones (>10 m) comprising a TiO₂ oxidation zone and a CaTiO₃ reaction zone.     

Near-Infrared Spectral Emissivity of Cu, Ag, and Au in the Liquid and Solid States at Their Melting Points

Normal spectral emissivities of liquid and solid Cu, Ag, and Au have been determined at their melting points over a wavelength range 1000 to 2500 nm using an apparatus that consists of a cold crucible and a diffraction grating spectrometer. For the noble metals, the emissivities of liquid phases are systematically larger than those of solid phases over the measured wavelength range, and the wavelength dependence of the liquid is similar to that of the solid. The measured emissivities for the liquid metals are compared with those deduced from the optical constants measured by Miller and Krishnan et al. The present results for liquid Cu and Au are in good agreement with the data of Krishnan et al., but not with those of Miller for Cu, which suggests that the optical constants measured by Krishnan et al. for liquid Cu are more accurate than those of Miller. The present data for liquid Ag and Au are in excellent agreement with all previously reported data. For the solid metals at their melting point, a semi-empirical estimation of the emissivity was carried out based upon the Drude model incorporating the effects of interband absorption and a frequency-dependent scattering rate, ^–1()= ^–1 ₀+b ². The values of ^–1 ₀ and b at the melting point are obtained by fitting the modified Drude model to the measurement results for the solid noble metals.     

Determination of the random error when measuring the frequency-modulation index

Conclusions 1. The essence of the method of determining the random error when measuring the frequency-modulation index from the zeros of the Bessel function consists not in observing when the components of the spectrum become zero, but in measuring the relative zero value of the components of the spectrum, equal to the reduced value of the noise of the spectral instrument. 2. The relative zero value of the spectrum compnents is an objective criterion and a means for making a quantitative estimate of the reliability of the results of the measurements. 3. Equation (2) enables one to determine the maximum reduced zero value of the spectral components, i.e., a necessary and sufficient condition for obtaining an assigned random error in measuring the frequency-modulation index, and, conversely, enables one to calculate the random error in measuring from the measured reduced zero value of the components of the spectrum. 4. The use of a spectral instrument of limited frequency resolving power to determine the frequency-modulation index does not have any appreciable effect on the random error of the measurement provided thatU _r ( _v )/U ₀(0) 1.Translated from Izmeritel'naya Tekhnika, No. 4, pp. 22–24, April, 1978.