Microstructure and mechanical properties of transient liquid phase bonds between NiAl and a Nickel-Base superalloy

An investigation of microstructural development in transient liquid phase bonds between the B2 type intermetallic compound NiAl and a nickel-base superalloy MM–247 is presented in this paper. The bonds discussed in the paper employed pure copper interlayers. Based on edge-on transmission electron microscopy investigations, the paper examines both microstructural development at the bond-line and the influence of bonding on the adjacent substrates. The paper considers the epitaxial growth of the B2 type (nominally NiAl) phase into the joint and the formation of non-epitaxial -phase layers. The paper also examines the formation of second-phases, including the L1₂ type -phase (nominally Ni₃(Al, Ti)), MX type carbides, -phase intermetallics and elemental chromium and tungsten.Bond-line and adjacent substrate microstructures for the NiAl/Cu/MM–247 bonds are correlated with joint mechanical properties determined by room-temperature shear testing. The paper compares the microstructure and mechanical properties of NiAl/Cu/MM–247 bonds with those of NiAl/Cu/Ni joints.     

Effect of atmosphere on the electrical properties of TiO2–SnO2 varistor systems

This work illustrates the advancement of research on TiO₂-based electroceramics. In this work will be presented that the addition of different dopants, as well as thermal treatments at oxidizing and inert atmosphere, influences of the densification, the mean grain size and the electrical properties of the TiO₂-based varistor ceramics. Dopants like Ta₂O₅, Nb₂O₅, and Cr₂O₃ have an especial role in the barrier formation at the grain boundary in the TiO₂ varistors, increasing the nonlinear coefficient and decreasing the breakdown electric field. The influence of Cr_Ti is to increase the O and O₂ adsorption at the grain boundary interface and to promote a decrease in the conductivity by donating electrons to O₂ adsorbed at the grain boundary. In this paper, TiO₂ and (Sn,Ti)O₂-based studies of polycrystalline ceramics, which show a non-linear I–V electrical response typical of low voltage varistor systems are also presented. All these systems are potentially promising for varistor applications.     

Doping Dependence of the Effective Mass Anisotropy and Oxygen-Isotope Effect on the Magnetic Penetration Depth: The Role of Lattice Vibrations in High-Temperature Superconductivity

Studies of the critical behavior of high-temperature superconductors (HTSC) show the importance of the strong doping dependence of the effective mass anisotropy for the generic (T, x) phase diagram of these materials. A possible microscopic model that can explain the doping dependence of is based on polaronic charge carriers. That lattice effects may play a role for high-temperature superconductivity is further supported by measurements of the oxygen-isotope effect on the in-plane penetration depth _ab (0) in underdoped La_2–x Sr _x CuO₄ single crystals that are reported in this paper. The oxygen-isotope effect on ^–2 _ab (0) is found to be around –9% for the samples investigated. It arises mainly from the oxygen–mass dependence of the in-plane effective mass m* _ab . The experimental facts presented in this paper suggest that lattice vibrations are important for the occurrence of high-temperature superconductivity.     

Probing High-Temperature Superconductivity with Positive Muons

Muon-spin rotation (SR) is a unique tool to investigate internal magnetic fields in superconductors on a microscopic scale. In particular, the complex vortex structure in high-temperature superconductors can be explored with this technique. As an example, SR experiments on the vortex phase diagram of single-crystal Bi₂Sr₂CaCu₂O₈₊ are briefly described. A novel SR technique using low-energy muons allows the measurement of internal magnetic fields near the surface of a superconductor with a few nanometers' resolution. First results obtained with this technique on a thin YBa₂Cu₃O_7- film are presented.     

Bicriterial Design of Digital Filters

A problem of technical interest is the solution of approximation problems which make a tradeoff between the L ₂ norm and the L norm error criteria. This problem is investigated in the framework of filter design with respect to two conflicting optimality goals. The particular interest in L ₂-L norm compromise filters has been raised by a paper of Adams (IEEE Trans. on Circuits and Systems vol. 39, pp. 376–388, 1991), who suggested to compute such FIR filters by solution of certain constrained L ₂ approximation problems which require a proper choice of weights. It is shown in this paper that bicriterial filter design problems can be approached by classical methods from multicriteria optimization and that especially reference point approximation with the ideal point as reference point is a suitable tool to deal with Adams' problem. Solutions from this latter approach do especially not depend on the choice of weights and yield the best possible compromise filters with respect to a prescribed measure. The resulting optimization problems can be solved with (semi-infinite) programming methods having proven convergence under standard assumptions. Examples of L ₂-L norm compromise designs of a linear-phase FIR and an IIR filter are presented.     

Refinement on the theoretical analysis of multifiber ceramic capacitor

In a previous paper, a new type of ceramic capacitor, multifiber ceramic capacitor (MFC), was designed and analyzed to compare the properties with multilayer ceramic capacitors (MLC). Refinement on the theoretical analysis of the MFC is presented in this paper for the capacitors made from dielectric film less than 2 m thick. A critical _c exists above which the specific capacitance of an MFC is higher than an MLC, where is the ratio of the dielectric thickness of the MLC and the MFC. Such a _c value is readily attainable because a high-quality dielectric coating on fiber substrates can be easily produced by modern thin-film technology. In other words, MFC has the potential to surpass MLC in term of the specific capacitance. Core fibers (inner electrodes) with a small diameter should be selected, whenever possible, for improved specific capacitance. The choice of possible materials for the MFC is also discussed.     

Buckling of thin-walled compression members with shear lag using spline finite member element method

Based on the displacement variational principle a systematic semi-discrete method, called as spline finite member element method, is developed for buckling analysis of thin-walled compression members with arbitrary cross-sections in the present paper. The displacements at two ends of the member element are adopted as basic variables in the method. A transformed B ₃-spline function presented in this paper is used to simulate the warping displacements along the cross-section of the thin-walled member. The analysis takes into account the effect of shearing strains of the middle surface of walls on the buckling, which reflect the shear lag phenomenon. Compared with the results from classical theory and the COSMOS/M finite element analysis program, the numerical results proposed in this paper demonstrate the versatility, accuracy and efficiency of the proposed method. The fast convergency shown in numerical examples predicts the reliability of the results.     

Evaluation of dynamic fracture toughness KId by Hopkinson pressure bar loaded instrumented Charpy impact test

A novel method for measuring the dynamic fracture toughness, K_Id, using a Hopkinson pressure bar loaded instrumented Charpy impact test is presented in this paper. The stress intensity factor dynamic response curve (K_I(t)−t) for a fatigue-precracked Charpy specimen is evaluated by means of an approximate formula. The onset time of crack initiation is experimentally detected using the strain gauge method. The value of K_Id is determined from the critical dynamic stress intensity factor at crack initiation. A K_Id value for a high-strength steel is obtained using this method at a stress-intensity-factor rate () greater than 10⁶ MPa .     

Wafer-level site-controlled growth of silicon nanowires by Cu pattern dewetting

An approach for the wafer-level synthesis of size- and site-controlled amorphous silicon nanowires (α-SiNWs) is presented in this paper. Microscale Cu pattern arrays are precisely defined on SiO₂ films with the help of photolithography and wet etching. Due to dewetting, Cu atoms shrink to the center of patterns during the annealing process, and react with the SiO₂ film to open a diffusion channel for Si atoms to the substrate. α-SiNWs finally grow at the center of Cu patterns, and can be tuned by varying critical factors such as Cu pattern volume, SiO₂ thickness, and annealing time. This offers a simple way to synthesize and accurately position a SiNW array on a large area.

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Melt spinning as a microstructural analogue for aluminide diffusion coating

The feasibility of using melt spinning, of a high alloy nickel aluminide, as a microstructural analogue for aluminide diffusion coated nickel base superalloys is investigated in this paper. Transmission electron microscopy studies are used to characterize the relationship between coating and melt spun analogue microstructures. Attention is focused on three phases that are of principal importance in coating mechanical properties, namely: the B2 type phase coating matrix, L1₂ type precipitates and M ₂₃ X ₆ carbides.The microstructure of the matrix of the melt spun analogue is shown to closely resemble that of the coating. Evidence is presented that the formation of in the melt spun alloy generally occurs in a similar manner to that in the coating. The formation of M ₂₃ X ₆ in close association with in the melt spun materials is compared to similar events in the coatings. Limitations in the ability of the melt spun materials to recreate the microstructures resulting from incorporation of substrate M ₂₃ X ₆ into the coating and formation of M ₂₃ X ₆ within precursor phases are discussed.     

Effects of Orthorhombic Distortion on Magnetic Excitation in t-J Model

Neutron scattering experiments on La_2–x Sr _x CuO₄ (LSCO) have revealed the incommensurate antiferromagnetic peaks do not lie exactly on the symmetry axes (q _x=± and q _y=±), but, are slightly shifted from them. In this paper, a scenario is presented for such shift in terms of the anisotropy of t (next-nearest-neighbor hopping integral on the square lattice) in the slave-boson scheme of the two-dimensional t-J model. Since the predictions of the present theory are different from those based on the spin-charge stripes hypothesis, further studies of the shift may clarify the factor responsible for the incommensurate antifcrromagnetic fluctuations in LSCO systems.     

Alkali dimers on the surface of liquid helium

A recent paper by Ancilottoet al. (Zeitschrift für Physik B, in press), presented calculations of adsorption energies and the geometry of a surface dimple for alkali atoms bound to the surfaces of quantum liquids (⁴He,³He, H₂). Here we present a study of the adsorption of two alkali dimers (Li₂, Na₂) on the surface of liquid helium. The calculations employ a model of an abrupt interface formulated by Ancilotto et al. as well as one using a diffuse interface. Our conclusion its that the dimers are bound to the surface more strongly than their respective monomers. In the case of dimers there is an additional degree of freedom-the orientation of the molecular axis relative to the surface. We study the influence of molecular anisotropy on adsorption by comparing the cases of erect and spinning flat orientations and conclude that the latter is energetically favored.     

Closed-form buckling analysis of stiffened composite plates and identification of minimum stiffener requirements

Longitudinal stiffeners attached to composite plates may significantly increase the overall buckling loads of the resultant stiffened structure. As long as the bending stiffness EI of the stiffener remains beneath some a priori unknown threshold value EI_min (with E being Young’s modulus and I being the bending moment of inertia), the buckling pattern will usually be of a more or less global nature, meaning that both the plate and the stiffener will exhibit some certain buckling modes. Once a threshold value EI_min for the bending stiffness, also called minimum stiffness, is exceeded, the stiffener more or less remains in its original position in the state of the onset of buckling while the buckling pattern of the stiffened plate is dominated by local buckling modes of the plate itself. The knowledge of this minimum bending stiffness EI_min of longitudinal stiffeners of composite plates is of high practical importance and a predominant design criterion and will be considered in this paper. For the basic load cases uniform compression and pure shear and their combination, simple closed-form analytical approaches will be presented which enable a straightforward and quick yet accurate estimation of the buckling loads (compression) and (shear) of stiffened composite plates on the one hand, and the minimum bending stiffness EI_min of the attached stiffeners on the other hand.     

SiO2-Al2O3 metastable phase equilibrium diagram without mullite

A metastable binary phase diagram between SiO₂ (cristobalite) and-Al₂O₃ (corundum) in the absence of any mullite phase is presented. A eutectic is indicated at a temperature of 1260° C and a composition of 18 wt% ( 12 mol%) Al₂O₃. The liquidi of the proposed metastable system were positioned on the basis of the thermodynamic data calculated from the stable equilibrium diagram of Aksay and Pask [2]. Experimental evidence is also presented. A SiO₂-Al₂O₃ melt containing 80 wt% Al₂O₃ cooled at a slow rate in sealed molybdenum crucibles shows crystalline Al₂O₃ plus a glass phase whose composition followed the calculated extension of the stable Al₂O₃ liquidus to lower temperatures. Compacts of cristobalite—corundum mixtures were fired at subsolidus temperatures to estimate the eutectic temperature experimentally. The proposed metastable phase diagram effectively explains the formation of non-crystalline phases in subsolidus reactions, and microstructure obtained on solidification of high alumina melts.     

The generating groups of geometrically uniform spherical signal sets

An [M, n] spherical signal set is a collection ofM unit-norm vectors in the Euclideann-dimensional space ⁿ . Itsconfiguration matrix C is the matrix of the scalar products between pairs of vectors. isgeometrically uniform if, given any two vectors x _i , x _j there exists an isometry that transforms x _i to x _j while leaving invariant. Agenerating group of is a group of isometries of ⁿ that transform any given vector of into each of the vectors in while leaving invariant. In this paper we characterize the configuration matrix of a geometrically uniform spherical signal set and we show how its generating groups can be obtained.Parts of an earlier version of this paper were presented at the IEEE International Symposium on Information Theory, Ithaca, NY, 1977     

General features of fatigue fracture diagrams of metals

In this paper the results of investigations of fatigue fracture diagrams (v-K diagrams) of materials or products (depicting the relationship between the fatigue crack growth rate, v, and stress intensity factor range in a cycle, K) performed in the Physico-mechanical Institute are summarized. Typical diagrams are described and their main features in normalized coordinates are clearly demonstrated. On this basis the defining parameters of crack growth rate curves (v-K curves), K_th, K_fc, m, and K^*, characterizing the crack extension resistance of material are validated. Convenient expressions for analytical approximating v-K curves by splines are proposed. The main observed deviations of v-K curves from typical ones are shown.Paper presented at the XIIth International Colloquium on Mechanical Fatigue (Miskolc, Hungary, 10–12 March, 1994).Published in Problemy Prochnosti, No. 1, pp. 30–35, January, 1996.     

Nonlinear transient response and its sensitivity using finite elements in time

The bilinear formulation proposed earlier by Peters and Izadpanah to develop finite elements in time to solve undamped linear systems, is extended (and found to be readily amenable) to develop time finite elements to obtain transient responses of both linear and nonlinear, and damped and undamped systems. The formulation is used in the h-, p- and hp-versions. The resulting linear and nonlinear algebraic equations are differentiated to obtain the sensitivity of the transient response with respect to various design parameters. The present developments were tested on a series of linear and nonlinear examples and were found to yield, when compared with results obtained using other methods, excellent results for both the transient response and its sensitivity to system parameters. Mostly, the results were obtained using the Legendre polynomials as basis functions, though, in some cases other orthogonal polynomials namely, the Hermite, the Chebyshev, and integrated Legendre polynomials were also employed (but to no great advantage). A key advantage of the time finite element method, and the one often overlooked in its past applications, is the ease with which the sensitivity of the transient response with respect to various system parameters can be obtained.List of symbols B bilinear term in the variational formulation - a linear term in the variational formulation - variational operator - T kinetic energy - V potential energy - Q _i nonconservative forces not included in the variational operation - generalized coordinates: displacement, velocity - T ₀ initial time - T _f final time - p _k design parameters - q _j generalized coordinates - {} column vector - <> row vector Portions of this research were supported by a grant from Army Research Office, Grant No. DAALO3-90-G-0134, with Dr. Gary Anderson as the grant monitor.Portions of this paper were presented at the ASME 15th Biennial Conference on Vibrations and Noise, held in Boston, September, 17–21, 1995.Parts of this paper were first presented at the 15th ASME Biennial Conference on Vibration and Noise. Permission by ASME to adopt and publish this paper is gratefully acknowledged.     

Electron transport in selenium-based amorphous xerographic photoreceptors

The present paper is the sequel to a previous report. In this paper, electron transport in various selenium-based amorphous xerographic photoreceptors is described. As before, measurements were carried out using xerographic time-of-flight (XTOF) and conventional time-of-flight (TOF) experiments. From the observed flight time the electron drift mobility _e was deduced. Transient photocurrent signals were also recorded at low fields to determine the decay time constant. For a given composition, the sample thickness, light intensity, substrate material and top contacts (in TOF) were varied to investigate whether the observed decay time constant _e is a meaningful bulk parameter as in the case of hole transport. Detailed results will be presented to show that the interpretation of _e is complicated. The more plausible explanation for the observed decay is the presence of a uniform distribution of positive space charge in the bulk. _e and _e were also measured as a function of composition and applied field. Experimental data suggest that electron transport is shallow trap-controlled. Light doping of selenium with arsenic or tellurium creates shallow traps and hence reduced _e. The field dependence of electron mobility is in the form _e E ⁿ whereE is the applied field andn is a constant less than unity. No electron response can be detected in chlorine-doped Se-As or Se-Te alloys.     

Physical motivations for thermal detectors

The resistivity of two-dimensional electrons on the surface of liquid helium can be measured using the Sommer-Tanner technique in which a rectangular array of electrodes is placed beneath the liquid surface. An ac voltage is applied to one electrode and the phase of the signal at another electrode is measured. In zero magnetic field the phase shift ₀ from that of purely capacitative coupling is proportional to the resistivity ₀ of the electron sheet. A simple expression is given for the phase shift in a magnetic field (B), which depends on both _xx and _xy , the components of the magnetoresistivity tensor, and on the aspect ratio of the electrodes. Experimental results are presented and analysed using this expression.