Microstructural development in cast and aged Ni–Al–Cr-based alloys derived from the B2 type β-NiAl structure

The formation of intermetallic compounds consisting of nickel-rich B2-type NiAl (-phase) ductilized by two-phase A1 ()/L12 () regions provides the possibility of combining ductility and high-temperature performance. Similar microstructures can also form the basis of high-temperature shape memory alloys, due to martensitic transformation of the -phase to an L10-type product. One route by which –/ microstructures can be produced involves the use of chromium as a -stabilizer. However, microstructural development in such a case is complicated by the formation of -Cr precipitates.This paper examines microstructural development and stability in cast Ni-25 at % Al-14 at % Cr, Ni-29 at % Al-22 at % Cr and Ni-27 at % Al-8 at % Cr alloys, together with a more complex material, namely, Ni-20 at % Al-13 at % Co-9 at % Cr-4 at % Ti-1 at % Mo-1 at % V. Both the as-cast condition and samples aged at 850 and 1100 °C for 140 h are examined using transmission electron microscopy. The paper discusses the formation of L10 martensite, intradendritic ', interdendritic /' and -Cr precipitation.     

Thermal stability of advanced Ni-base superalloys

Exposures consisting of 1 to 900 h at 1000 and 1100 °C after an ageing treatment of 16 h at 870 °C were used to study the thermal stability of selected -strengthened Ni-based superalloys representing conventional, directional solidification, and single-crystal castings. Various techniques of microscopy, spectroscopy and diffraction were used to characterize the microstructure. Primary MC carbides in the alloys studied were found to be stable toward decomposition into lower carbides. In the aged condition, the strengthening phase assumed a cuboidal morphology; however, all alloys also contained varying proportions of coarse lamellar and hyperfine cooling . On an atomic scale, the nature of the cuboidal -matrix interface was found to vary from coherent to partially coherent. However, the overall lattice mismatch varied from one alloy to another depending upon its composition and the distribution of various elements in carbide phases and lamellar phase. Directional growth of the cuboidal phase upon exposure to higher temperatures was found to be accelerated by a large initial lattice mismatch leading to a considerable loss of coherency, as indicated by the observation of dislocation networks around the particles. Although the composition of the phase remained essentially unchanged, there was a marked change in matrix composition. Sigma phase was found to precipitate in all alloys, but its thermal stability was a function of alloy composition. The initial decrease in hardness followed by a hardening effect during exposure could be explained in terms of the partial dissolution of the phase and precipitation of sigma phase.     

Ion-implantation technology for improved GaAs MESFETs performance

Electrical properties of Si-implanted and co-implanted with Mg or Be in semi-insulating GaAs was studied. The Si-implanted MESFETs with and without buried p-layer (formed by Mg or Be) have been fabricated and characterized by their d.c. and r.f. performance. The experimental results showed that the device with a buried p-layer can effectively suppress the substrate leakage current (thus good pinch-off characteristic) and obtained higher gain linearity than these without a buried p-layer. For 1 m×100 m MESFETs device with co-implantation of Si (8×10¹² cm^–2) and Be (6×10¹¹ cm^–2) demonstrated uniform transconductance (g_m) of 115 mS mm^–1 with the gate voltage ranging from –1 to 1 V and reduced pinch-off voltage compared to those with co-implantation of Si and Mg (6×10¹¹ cm^–2). The measured f_T and f_max of a 1 m×25 m MESFET with co-implantation of Si and Be are 10 and 39 GHz, respectively. However, FETs with increased Mg dose (from 6×10¹¹ cm^–2 to 2×10¹² cm^–2) in a buried p-layer can obtain higher transconductance and saturation current.     

Solution of thermal conductivity problems with boundary condition of the third kind and arbitrary coordinate and time dependence of the biot number

An analytical solution of the thermal conductivity problem with boundary conditions of the third kind and arbitrary coordinate and time dependence of the Biot number is found in the form of a converging series of quadratures.Notation , z dimensionless coordinates - dimensionless temperature - Q dimensionless volume heat-liberation density per unit time - Fo=/² Fourier number - Bi₁(, Fo)=(, Fo) · / Biot number - thermal diffusivity coefficient - plate thickness - time - (, Fo) heat-liberation coefficient - thermal conductivity coefficient - i summation index - Jo zero order Bessel function of the first kind Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 3, pp. 536–540, September, 1981.     

Singularity parametersε andκ for interface cracks in transversely isotropic piezoelectric bimaterials

Two parameters, and (Suo et al., 1992), are of key importance in fracture mechanics of piezoelectric material interfaces. In this paper, it is shown, for any transversely isotropic piezoelectric (TIP) bimaterial, that one of the two parameters and always vanishes but the other one remains non-zero. Physically, it means that the non-oscillating crack-tip generalized stress field singularity exists for some TIP bimaterials (with vanishing ). Consequently, TIP bimaterials can be classified into two classes: one with vanishing performed crack tip generalized stress field oscillating singularity and the other one with vanishing is independent from the oscillating singularity. Some numerical results for and are given too.     

The role of starting powder size on the compressive response of stand-alone plasma-sprayed alumina coatings

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 m, 19 m and 32 m alumina; the resulting effective lamella diameters were 10 m, 28 m, and 55 m, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 m powder exhibited greater moduli than tubes sprayed from either 19 or 32 m powders. This difference was attributed to the greater percentage of unmelted -Al₂O₃ in the coating.     

A precipitate phase in AA2124

A new precipitate phase (designated X phase) other than the S(Al₂CuMg) precipitate phase has been discovered in the 2124 aluminium alloy. Using selected-area electron diffraction analysis, dynamical diffraction simulation, energy dispersion analysis of X-rays and high-resolution electron microscopy, it is suggested that the X phase has an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The space group of the phase is Cmmm. There are 10 atoms in a cell with AlCuMg=244. The orientation relationship between the X phase and the matrix is determined as (1–10)_m(010)_x, [–1–1–1]_m [001]_x.     

Energy loss distribution of charged particles in thin absorbers

Conclusions 1. For thin absorbers, the Landau theory incorporating certain above-listed corrections is a general-purpose one and is in good agreement with experiment both for heavy and light charged particles 0.01. 2. For intermediate layers (0.01 1 and 1), exact solutions are provided by the Vavilov theory [11]. 3. The numerical-analytic method of plotting the energy loss distribution function proposed in [13] is suitable for very thin ( 0.01) absorbers.Translated from Izmeritel'naya Tekhnika, No. 3, pp. 60–62, March, 1970.     

The effects of chemical polishing on the strength of sapphire whiskers

Tensile tests at 20 C have been carried out on forty-four sapphire whiskers after chemical polishing in hot orthophosphoric acid. The orientations tested were 0001, 11¯20, 10¯10, and 10¯11. The results show that chemical polishing increases the strength of large whiskers by a factor of up to 10, but not the strength of small ones. Good correlation is obtained between fracture strength, _f, and whisker diameter,d. The relevant size-strength equations, _f=Kd ^m l ⁿ (wherel is gauge length, andK, m, andn are constants depending on whisker orientation), predict strengths in good agreement with the theoretical strength of sapphire at unit-cell dimensions and with the measured strengths of macroscopic flame-polished crystals.The observations are contrasted with those for unpolished sapphire whiskers [1]. They show a transition in the fracture nucleation mechanism of unpolished whiskers at a certain stress.In unpolished, A-type (11¯20 and 10¯10) whiskers, with _f<1000 kg/mm², fracture initiates at surface flaws, and strength is dependent on surface area. But, for whiskers with _f>1000 kg/mm₂, and for all polished whiskers (both A and C type), fracture is due to dislocation pile-ups or interactions, and strength is dependent only upon diameter. In unpolished, C-type (0001) whiskers, however, with _f<800 kg/mm₂, fracture initiates at surface flaws which are related to whisker diameter; while, for _f> 800 kg/mm², it occurs at dislocation pile-ups or interactions and is again related to diameter. In contrast, therefore, to A-type whiskers, the strength of C-type whiskers is always diameter-dependent, although there is a clear transition in the size-strength curve at _f800 kg/mm².     

A rank-frequency model for scientific productivity

If f(r) is the number of contributions of an author or rank r, then it is shown that f(r) is proportional to r^–, where >0. The model is dependent on the definitions of a contribution and rank of an author. Three estimation procedures are illustrated and four other scientific productivity studies and two data sets on Canadian Mathematicians are shown to adequately fit this rank-frequency relationship.Research partially supported by National Research Council Grant No. A9229.     

Nonlinear asymptotic theory of hypersonic flow past a circular cone

Summary The hypersonic small-disturbance theory is reexamined in this study. A systematic and rigorous approach is proposed to obtain the nonlinear asymptotic equation from the Taylor-Maccoll equation for hypersonic flow past a circular cone. Using this approach, consideration is made of a general asymptotic expansion of the unified supersonic-hypersonic similarity parameter together with the stretched coordinate. Moreover, the successive approximate solutions of the nonlinear hypersonic smalldisturbance equation are solved by iteration. Both of these approximations provide a closed-form solution, which is suitable for the analysis of various related flow problems. Besides the velocity components, the shock location and other thermodynamic properties are presented. Comparisons are also made of the zeroth-order with first-order approximations for shock location and pressure coefficient on the cone surface, respectively. The latter (including the nonlinear effects) demonstrates better correlation with exact solution than the zeroth-order approximation. This approach offers further insight into the fundamental features of hypersonic small-disturbance theory.Notation a speed of sound - H unified supersonic-hypersonic similarity parameter, - K hypersonic similarity parameter, M - M freestream Mach number - P pressure - T temperature - S entropy - u, v radial, polar velocities - V freestream velocity - shock angle - cone angle - density - density ratio, /() - ratio of specific heats - polar angle - stretched polar angle, / - (), (), () gage functions     

Flow visualization glass-ceramic: Preliminary experimental and modelling results

A glass-ceramic material was developed to act as a flow visualization material. Preliminary experiments indicate that aperiodic, thermally induced, convective flows can be sustained at normal processing conditions. These flows and the stress and temperature gradients induced are most likely responsible for the anomalous behaviour seen in these materials and the difficulties encountered in their development and in their production on industrial and experimental scales. A simple model describing the dynamics of variable-viscosity fluids was developed and was shown to be in qualitative agreement with more sophisticated models as well as with experimental results. The model was shown to simulate the dependence of the critical Rayleigh number for the onset of convection on the viscous properties of the fluid at low T, and also to simulate quenching behaviour when the temperature differences were high.Nomenclature C _p Heat capacity - D, E, F Expansion coefficients - H Height of the roll cell - Pr Prandtl number - R _a Rayleigh number - R _c Critical Rayleigh number for the onset of convection in a constant-viscosity fluid - S Dimensionless stream function - T Temperature - T _m Mean temperature - T ₀ Bottom surface temperature - T _r Reference temperature - a Aspect ratio of cell - g Acceleration due to gravity - k Thermal conductivity - k ₁ Function related to ²v/T ² - k ₂ Function related to ⁴v/T ⁴ - r Rayleigh number ratioR _a/R _c - t Time - w Dimensionless vertical coordinate - w _m Mean cell height - x Horizontal coordinate - y Dimensionless horizontal coordinate - z Vertical coordinate - , Constants - _t Thermal expansion coefficient - Constant in viscosity function - T Temperature difference between top and bottom surfaces - _i Viscosity coefficients - Kinematic viscosity - _m Mean kinematic viscosity - Dimensionless kinematic viscosity - Thermal diffusivity - Non-linear temperature function - Dimensionless non-linear temperature function - _o - Stream function - Dimensionless time - Eigenvalues     

Detailed investigation of the temperature dependence of ionic transport parameters of a new composite electrolyte system (1−x) (0.75AgI : 0.25AgCl) : xSnO2

Detailed investigations of a new Ag+-ion-conducting two-phase composite electrolyte system (1–x) (0.75AgI : 0.25AgCl) : xSnO2 are reported, where 0x50 in weight per cent. A quenched-and-annealed (0.75AgI : 0.25AgCl) mixed system–solid solution was used as the first phase instead of the commonly used host matrix salt AgI. Micron-sized particles (about 10 m) of SnO2 were dispersed in the first (matrix) phase. The composition 0.8(0.75AgI : 0.25AgCl) : 0.2SnO2 exhibited conductivity enhancements of more than eight times over the annealed host and about three times over the quenched host at room temperature and has been referred to as optimum composition. The existence of two separate phases has been ascertained by X-ray diffraction and differential thermal thermal analysis techniques. The temperature dependence of the electrical conductivity, , ionic mobility, , mobile ion concentration, n, ionic transference number, tion, and ionic drift velocity, vd, are also reported. The enhancement in the conductivity in this two-phase composite electrolyte has been attributed to the increase in ionic mobility at room temperature.     

Method for heat transfer calculation using isothermal coordinates

Equations for steady-state heat transfer are considered in curvilinear coordinates. The equations are shown to be simplest when one of the families of coordinates are isotherms. Conditions are obtained for which these coordinate systems and some exact solutions of the heat conduction equations must satisfy.Notation a ₁, a₂, ..., a_n coefficients determining the temperature dependence of thermal conductivity (see formula (8)) - f() function of the -coordinate (see formula (4)) - H ₁,H ₂,H ₃ coefficients of the first differential form (Lamé coefficients) (see formula (2)) - n number of a term of the series in formula (8) - q heat flux - Q power of volume heat release - x, y, z Cartesian coordinates - , , general curvilinear orthogonal coordinates - ₁, ₂ coordinates of the boundary surfaces on which the temperatures are prescribed - x ₀₁ thermal conductivity att=0 - () function of the -coordinate (see formula (4)) - () -function determining temperature distribution in the case of constant heat flux along the coordinate lines Scientific-Research and Design-Technological Institute of Machine Manufacture, Scientific-Production Subbranch Novator, Kramatorsk, Ukraine. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 4, pp. 651–659, July–August, 1995.     

Shear viscosity of liquid4He and3He-4He mixtures,especially near the superfluid transition

High-resolution measurements of are reported for liquid⁴He and³He-⁴He mixtures at saturated vapor pressures between 1.2 and 4.2 K with particular emphasis on the superfluid transition. Here is the mass density, the shear viscosity, and in the superfluid phase both and are the contributions from the normal component of the fluid ( _n and _n ). The experiments were performed with a torsional oscillator operating at 151 Hz. The mole fraction X of³He in the mixtures ranged from 0.03 to 0.65. New data for the total density and data for _n by various authors led to the calculation of . For⁴He, the results for are compared with published ones, both in the normal and superfluid phases, and also with predictions in the normal phase both over a broad range and close to T. The behavior of and of in mixtures if presented. The sloped/dT near T and its change at the superfluid transition are found to decrease with increasing³He concentration. Measurements at one temperature of versus pressure indicate a decreasing dependence of on molar volume asX(³He) increases. Comparison of at T, the minimum of _n in the superfluid phase and the temperature of this minimum is made with previous measurements. Thermal conductivity measurements in the mixtures, carried out simultaneously with those of , revealed no difference in the recorded superfluid transition, contrary to earlier work. In the appendices, we present data from new measurements of the total density for the same mixtures used in viscosity experiments. Furthermore, we discuss the data for _n determined for⁴He and for³He-⁴He mixtures, and which are used in the analysis of the data.     

Study of the Temperature Fields in Oil and Gas Strata with Water Injection Based on the Perturbation Method

The problem of the thermal action on oil and gas strata by injection of a heating medium using expansion in a small parameter is reduced to an infinite sequence of boundaryvalue problems that are solved by the method of integral transforms. It is shown that, with an appropriate selection of the small parameter, the zerothorder approximation corresponds to a spaceaveraged (across the stratum thickness) solution of the main problem and leads to a concentratedcapacity scheme that is constructed assuming that the stratum temperature is independent of the vertical coordinate. The first approximation permitted marked refinement of calculations according to the concentratedcapacity scheme and an evaluation of its error. Spacetime temperature distributions are presented that have been calculated using the analytical solutions obtained.     

Chemical Transformations of γ-Al(OH)3during Closed-System Heat Treatment

The chemical transformations of -Al(OH)₃during closed-system heat treatment in a self-generated gaseous atmosphere (water vapor) were studied. At t 200°C, -Al(OH)₃was found to convert into -AlOOH, which, in turn, converts into the equilibrium phase -Al₂O₃at t 400°C. The processes underlying the effect of water vapor on the kinetics and mechanisms of the chemical transformations of -Al(OH)₃and -AlOOH in a closed system are discussed.     

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.     

Precipitation hardening of cast SiCp reinforced Mg-6Zn

Age hardening behaviour of 20 vol % SiC particulate reinforced Mg-6Zn has been studied using hardness, eddy current and transmission electron microscopy techniques. The results show that the precipitation processes in this composite, although accelerated, are similar to those in the unreinforced Mg-Zn alloys; i.e., supersaturated solid solution GP zones Kinetic analyses of the former indicate that above 423 K, bulk diffusion of Zn controls precipitation, whereas, below 423 K precipitate formation is associated with vacancy migration.     

Theory of periodically driven,current-carrying,superconducting filaments. I. Spatially homogeneous states

Starting from the nonequilibrium theory of dirty superconductors in the Ginzburg-Landau regime, spatially homogeneous states with an applied currentI=I ₀+I ₁ cos (t) are considered. Expressions for the linear response (I₁ small) valid up to high frequencies (k _BT_c) are derived and evaluated analytically for the experimentally important case of smallI ₀ and ₀(T). Then the nonlinear response is treated for frequencies with _E1. Interesting new behavior is found for frequencies ₀ 1, where ₀ is essentially the GL relaxation time.