A TEM investigation of M23C6 carbide precipitation behaviour on varying grain boundary misorientations in 304 stainless steels

Transmission electron microscopy (TEM) along with electrochemical potentiokinetic reactivation (EPR) testing was performed on different grades of 304 stainless steel (0.01, 0.025, 0.05, and 0.07%C) in order to assess the sensitization and precipitation behaviour on different grain boundary misorientations. The materials were heat treated at 670°C for 50 h to subject the materials to the sensitization regime. The EPR data and TEM observations revealed that when the amount of carbon was increased the degree of sensitization increased along with the density of precipitates. Large angle misorientations (>15°) were prevalent in all the carbon content materials and the {1 1 0} grain surface orientation was found to be the major texturing orientation. The steels with lower carbon contents nucleated a few small precipitates on high angle grain boundaries, while larger amounts of carbides were observed on lower angle grain boundaries for the higher carbon contents. It was deemed that higher carbon contents required lower energies to nucleate and grow precipitates. A carbon content threshold was found (above 0.05% C) in which precipitates fully saturate the grain boundary. Precipitation followed the energies of different types of boundaries. The highest energy boundary (general random grain boundary) nucleated precipitates first, then precipitation followed on non-coherent twin boundaries, and was not observed on coherent twin boundaries. A critical nucleation energy, gb(crit.), was therefore found to exist at which precipitation will occur on a boundary. This value was found to be in the range of 16 mJ m-2相似文献   

Laminar thermal boundary layer on a continuous accelerated sheet extruded in an ambient fluid

Summary Exact boundary layer similarity solutions are developed for flow, friction and heat transfer on a continuously accelerated sheet extruded in an ambient fluid of a lower temperature.Melt-spinning, polymer and glass industries and the cooling of extruded metallic plates are practical applications of this problem.Results for skin-friction and heat-transfer coefficients are given. Larger acceleration is accompanied by larger skin-friction and heat-transfer coefficients. Rapid cooling of the sheet is accompanied by a larger Nusselt number.Nomenclature sheet width - c dimensionless constant - c _f local skin friction coefficient - F dimensionless transformed stream function - G dimensionless transformed temperature - local heat transfer coefficient - fluid thermal conductivity - length of deformation zone - m exponent of surface speed variation - q exponent of surface temperature variation - T dimensionless temperature - sheet surface temperature - solidification temperature - ambient temperature - sheet thickness - u velocity component along the sheet - u _s sheet surface velocity - wind up velocity - v velocity component normal to the sheet - x dimensionless coordinate along the sheet - y dimensionless coordinate normal to the sheet - Nu Nusselt number, - Pr Prandtl number, - Re Reynolds number, - =Re^–0.5 - dimensionless similarity coordinate - dynamic viscosity - kinematic viscosity - fluid mass density - sheet mass density - wall shear stress - dimensionless stream function With 3 Figures     

Thermal-stress resistance and fracture toughness of two tool ceramics

The thermal-stress resistance and fracture strength behaviour of two oxide ceramics (a hot-pressed pure Al₂O₃ and a composite ceramic NTK-HC2) subjected to severe thermal shocks have been investigated. The damage resistance parameter (K _IC/ _f)² for both ceramics is also determined for a wide range of temperatures (25° to 800° C) and cross-head rates (10^–2 cm min^–1 2.5 m sec^–1). Fracture strength behaviour of these two oxide ceramics is shown to follow Hasselman's model where the instantaneous strength loss at the critical quenching temperature may be calculated using appropriate (K _IC/ _f)² values to give good agreement with experimental results. Repeated shocks show some further degradation in the retained strength for both ceramics so that these materials are susceptible to thermal fatigue. It is found that both materials possess similar resistance to crack initiation (i.e. similar T _c and retained strength after shocking through T _c) but the pure oxide ceramic has higher resistance to crack propagation.     

Molded Carbon–Carbon Composites Based on Microcomposite Technology

A one-step, cost-effective processing methodology based on compression molding of a mixture of graphite particles and short fibers, both coated with a soluble polyimide adhesive was developed. This technique shows a considerable potential in decreasing the complexity of the current carbon–carbon fabrication procedures. The new process eliminates additional infiltration and densification steps following the initial carbonization, which reduces the processing time from 5 weeks to 3–5 days and saves energy. The structure and properties of the new carbon–carbon composites were characterized using optical and electronic microscopy, thermal analysis, density and porosity measurements, and mechanical properties (hardness and flexural strength).The flexural strengths ranged from 20–45 MPa. The densities ranged from 1.9 to 2.2 g/cc (which is close to pure graphite) while the porosity was as low as 3%. The CTE was approximately ± 1 ppm/C (R.T. to 550C). The thermal stability of the carbonized and graphitized specimens when heated in flowing air up to 500C and flowing nitrogen up to 1000C showed no observable weight loss.There are numerous applications for these materials which include: optical mirrors, medical implants, thermal radiators and parts for rotating equipment, etc.A car piston was successfully molded using a mixture of polymer coated graphite powder, flakes and chopped fibers.     

Two-Band Eliashberg Theory in Doped MgB $_{2}$_{2}: Experimental Tc and Superconductive Gaps

The variation of the critical temperature and of the superconductive gaps as functions of doping (Al, C) in the diboride MgB has been studied in the framework of the two-band Eliashberg theory and traditional phonon coupling mechanism. We have solved the two-band Eliashberg equations using first-principle calculations or simple assumptions for the variation of the relevant physical quantities. We have found that the experimental curves can be exactly explained only if the Coulomb pseudopotential changes with x by tuning the Fermi level toward the σ band edge. We also found that a small amount of impurities changes the structural properties of the material, so we cannot treat the Mg and MgB systems as a contamination with Al or C of MgB, but as new materials. Finally, we compare the predictions of our theory with the available experimental data.     

Submicron diameter nickel filaments and their polymer-matrix composites

Discontinuous nickel filaments of diameter 0.4 m and having a carbon core of diameter 0.1 m were fabricated by electroplating nickel on discontinuous carbon filaments. They exhibited a grain size of 0.016 m and electrical resistivity of about 5 × 10^–6 ·cm. In an amount as low as 7 vol.% in a polymer (polyether sulfone) matrix, they resulted in a composite exhibiting electromagnetic interference shielding effectiveness of 87 dB and reflection coefficient 0.95 at 1–2 GHz, tensile strength 52 MPa, tensile ductility 1.0%, and density 1.87 g/cm³.     

Effect of carbon monoxide partial pressure on the high-temperature decomposition of Nicalon fibre

The high-temperature equilibrium partial pressures of the predominant gaseous species over Nicalon were determined thermochemically. It was calculated that the most prevalent gaseous species in equilibrium with Nicalon at 1300 °C is carbon monoxide. Subsequently, fibres of Nicalon (NLM 202) were heat treated at 1300 °C in various partial pressures of carbon monoxide gas and analysed via single filament strength testing, scanning electron microscopy, X-ray diffraction, and scanning Auger microscopy. The heat treatments in carbon monoxide had a significant effect on the strength retention and composition of the fibres (75% retained) compared to the treatments in argon where only 25% of the initial strength was retained. The Auger analysis revealed that the treatment in argon evolved carbon and oxygen from the fibre while in carbon monoxide atmospheres a carbon layer was deposited on the fibre surface. X-ray diffraction showed that grain growth had not occurred in any of the heat treatments. This study shows the important role of thermochemical reactions in the strength degradation of Nicalon, and its possible relationship to the formation of carbon surface/interface layers.     

Temperature dependent torsional properties of high performance fibres and their relevance to compressive strength

A simple arrangement for the measurement of torsional moduli of high performance fibres as a function of temperature has been reported. Torsional moduli and damping factors have been measured on a number of polymeric [Kevlar, poly(p-phenylene benzobisoxazole) (PBO), poly(p-phenylene benzobisthiazole) (PBZT) and Vectran] and carbon fibres [pitch and PAN based, and one bromine intercalated pitch based carbon fibre] as a function of temperature (room temperature to 150 °C, range) and as a function of vacuum level (1.1–80 ×10³ Pa). At these vacuum levels damping in the fine fibres is mainly due to aerodynamic effects. In general PAN based carbon fibres have higher torsional moduli than pitch based carbon fibres. Kelvar 149, PBO and PBZT fibres have comparable room temperature torsional moduli, while the torsional modulus of Vectran fibre is very low, probably due to the torsional flexibility of the -COO- group. In the above temperature range, torsional moduli of both pitch and PAN based carbon fibres do not change significantly, while for polymeric fibres they decrease; a small decrease is observed for PBO and PBZT, and a significantly higher decrease is observed for Vectran. Relationships between compressive strength and torsional moduli have been discussed     

Microstructure and mechanical properties of nanoscale SiC/Ca α-SiAlON composites

Ceramic composites comprising nanoscale (less than 200 nm) silicon carbide particles distributed in a matrix of calcium -SiAlON () have been prepared by uniaxial hot pressing, and the reaction sequence, during densification of a sample containing 20 wt% SiC, has been investigated in the temperature range 1400–1800°C. Samples containing up to 20 wt% SiC were produced to near full density by pressure sintering at temperatures as low as 1600°C for 1 h. For samples with higher SiC contents subjected to a similar treatment, there was residual porosity which was detrimental to the mechanical properties. The SiC was preferentially distributed intergranularly within the -SiAlON and was effective in controlling grain growth of the -SiAlON during processing. There was an increase in the uniformity of the grain structure and a significant refinement of the grain size of the composite microstructures with increasing SiC content. The hardness and the three-point bending strength of the composite samples increased markedly with increasing SiC content up to a level of 20 wt%. For a sample containing 20 wt % SiC, the bending strength was about 1.5 times that of single-phase -SiAlON. For samples with higher SiC contents, the rate of increase in hardness was diminished and the bending strength decreased because of incomplete densification. The initial improvement in fracture strength with increasing SiC content is plausibly attributable to the uniform refined grain structure of the composite materials, assuming that the maximum flaw size scales with the grain size.     

Elevated temperature deformation of several quaternary L12Al3Ti based alloys reinforced with TiB2 particles

Concurrent with an investigation of quaternary cast and forged L1₂ modified Al₃Ti's containing 9 at% Cr, Fe and/or Mn, a similar series of four alloys was produced via XD^TM technology, whereby 20 vol% of 0.5 m TiB₂ particles was incorporated as a reinforcement. Following densification by hot pressing and slow isothermal forging, small diameter compression test samples were machined from each compact and tested. The 0.2% yield strength measurements revealed a strength advantage for the particulate reinforced materials over the unreinforced ingot metallurgy matrices to about 1175 K. Furthermore, 900 and 1100 K constant velocity testing indicated that the TiB₂ containing materials were stronger than the ingot metallurgy matrices down to strain rates of 10^–7s^–1. None of the quaternary L1₂ alloys + 20 TiB₂ consistently displayed a strength advantage over others. However, extrapolation of the present mechanical property data into slower strain regimes indicated that the composites will not possess any strength advantage overthe unreinforced versions. Such losses in strength appeared to be caused by small grains in the particulate reinforced alloys, which promoted grain boundary weakening mechanisms.     

Growth of Carbon Nanowires and Nanotubes via Ultrarapid Heating of Ethanol Vapor

A method is proposed for producing carbon nanomaterials by ultrarapid heating of vapors of organic compounds to high temperatures, and an experimental setup for implementing this method is described. The starting reagents used are ethanol and mixtures of ethanol with water, glycerol, and ferrocene. At heater temperatures of 1500–2000°C and substrate temperatures of 600–1000°C, carbon nanowires and nanotubes are obtained. The nanowires attain 100 m in length and range in thickness from 30 to 150 nm. The nanotubes have bamboo or fish-bone structures, with a thickness from 20 to 50 nm. The deposition of nanotubes on supported catalysts (iron, nickel, gold, and others) is also examined. It is shown that, under certain conditions, selective deposition of carbon nanotubes (nanowires) onto catalyst-coated parts of the substrate is possible.     

Nonlinear liquid oscillations in spherical systems under zero-gravity

Summary Nonlinear free oscillations of the interface of a concentric frictionless immiscible liquid system in a spherical container are investigated in a zero-gravity environment. The natural frequencies are determined for the axisymmetric and asymmetric oscillations of the interfacial surface with the diameter ratio and density ratio as parameters. It was found that for small outer- to inner liquid density ratio the oscillations exhibit softening, while for large density ratios it renders hardening oscillation. The asymmetric oscillations exhibit in the softening range softer and in the hardening range harder liquid oscillations. For a liquid layer around a rigid center sphere the oscillations of the free liquid surface yields softening behavior, where for thinner layers the softening effect is more pronounced.Nomenclature a radius of spherical container, or radius of rigid center sphere - b radius of undisturbed interfacial surface, or radius of undisturbed free liquid surface - k=a/b diameter ratio - pressure - pressure (dimensionless) - , , spherical coordinates - dimensionless radius - R _i main radii of curvaturei=1, 2 time - dimensionless time - v _i liquid velocity (j=1 spherical layer region,j=2 inner liquid sphere region) - V volume of the liquid - Y _nm tesseral surface harmonics - _i density of liquids - velocity potential - dimensionless velocity potential - interfacial surface- or free surface elevation - dimensionless interfacial surface- or free surface elevation - ₀ maximum elevation - circular frequency - circular frequency - _n0 axisymmetric natural frequency - _n1 asymmetric natural frequencym=1 - _nm ⁽⁰⁾ natural frequency of linearized liquid system - mean curvature - _nm Kronecker symbol With 10 Figures     

The tensile properties of carburized and uncarburized low carbon mild steel

The properties of a low carbon mild steel in monotonic tension loading were compared in the plain normalized and the carbo-nitrided slowly cooled conditions. The application of a carbo-nitriding process raised the yield strength of the steel to that of the nominal tensile strength in the uncarburized samples, and increased the nominal tensile strength to a value 45% above that of the plain normalized steel, whilst still retaining a good measure of ductility. The fractures for the plain samples were cup and cone type whilst those for the reinforced samples revealed slant mode fractures.     

On a stability theory of non-selfadjoint mechanical systems

Summary The concept of the Hamiltonian functional is generalized in such a way that a bilinear functional results, which plays the role of the Hamiltonian for non-selfadjoint systems. For this generalized Hamiltonian the condition leads to the so called hybrid Galerkin's equations, and the condition , to the load-frequency reationship. This relationship can be interpreted as a surface in the load-frequency space, the projection of which on the load-planes yields the stability boundaries, i.e. the buckling loads.

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Zu einer Stabilitätstheorie nicht-selbstadjungierter mechanischer Systeme

Zusammenfassung Der Begriff des Hamiltonschen Funktionals wird in solcher Weise verallgemeinert, daß ein bilineares Funktional bei nicht-selbstadjungierten Systemen an seine Stelle tritt. Für dieses verallgemeinerte Hamiltonsche Funktional führt die Bedingung auf die sogenannten hybriden Galerkinschen Gleichungen und die Bedingung auf die Last-Frequenz-Funktion. Diese Funktion kann im Last-Frequenz-Raum als eine Fläche aufgefaßt werden, deren Projektion auf die Last-Ebenen die Stabilitätsgrenzen und damit die Knicklasten liefert.

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Dedicated to Professor Kurt Magnus in honor of his sixtieth birthday.     

Higher approximations for supersonic flow past slowly oscillating bodies of revolution

Summary Supersonic flow past slowly oscillating pointed bodies of revolution is studied. Starting from the complete nonlinear potential equation an elementary linearized solution is discussed and it is shown how this solution together with the method of matched asymptotic expansions can be used to derive an elementary second-order slender body theory. This approach is further demonstrated for the oscillating cone and its range of validity is evaluated by comparison with other theoretical methods.

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Zusammenfassung Es wird die Überschallströmung um langsam schwingende spitze Rotationskörper untersucht. Ausgehend von der vollständigen nichtlinearen Potentialgleichung wird zuerst eine elementare linearisierte Lösung besprochen und gezeigt, wie diese Lösung im Verein mit der Method of matched asymptotic expansions zur Herleitung einer elementaren Schlankkörpertheorie zweiter Ordnung verwendet werden kann. Die Theorie wird am Beispiel des schwingenden Kegels näher erläutert und mit anderen Methoden verglichen.

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Symbols a Velocity of sound - c _N Normal force coefficient - Damping coefficient - F _(x) Dipole distribution - k Reduced frequency - M Mach number - R (x) Meridian profile - t Time - x, r, Cylindrical coordinates - - Ratio of specific heats - Amplitude of oscillation - Thickness ratio - Perturbation potential - Zero angle of attack potential - æ - Velocity potential - Out-of-phase potential - - In-phase potential - - Source coordinate With 4 Figures     

Studies on the structure of light-cured Glass-ionomer cements

The behaviour of two commercial light-cured glass-polyalkenoate (Glass-ionomer) cements has been studied in terms of changes in strength following storage under various conditions. Unlike conventional glass-polyalkenoates, these materials cure partly by a photochemical polymerization process, in addition to the normal acid-base setting reaction. Results presented in the current paper show that these hybrid materials are able to take up a considerable amount of water when stored in either water or physiological saline solution. This leads to an observed change of failure mode in compression from purely brittle to partly plastic. These changes are accompanied by a reduction in compressive strength. Overall, the behaviour of the light-cured Glass-ionomers was found to be similar to that of hydrogels. This finding is discussed in light of the information it gives about the underlying microstructure of these materials.     

Preparation and properties of TaC/C/TaC∼TaC composite micro-tubes by vapor phase tantalizing of the regular carbon micro-coils/micro-tubes

TaC/C/TaCTaC composite micro-tubes were prepared by the vapor phase tantalizing of the regular carbon micro-coils/micro-tubes, and the preparation conditions and some properties were examined. The carbon micro-coils with a tube-like morphology were tantalized from the surface to the core of the carbon fibers with full preservation of the tube-like morphology to form TaC/C/TaCTaC composite micro-tubes. The bulk electrical resistivity and specific surface area of the TaC/C/TaCTaC composite micro-tubes were 4 × 10^–3 to 5 × 10^–4 ·m and 5 × 10³ to 2 × 10⁴ m²/kg, respectively, depending on the tantalized ratio and the bulk density.     

Removal of residual stresses by low-temperature annealing to prevent stress-corrosion cracking of carbon steels

The effect of low-temperature annealing on the relaxation of residual stresses and on the stress-corrosion resistance of carbon steels was investigated. It was shown that stress-corrosion cracking of welded joints in carbon steels under the influence of boiling nitrate solutions and alkaline solutions is prevented by annealing at temperatures 450° C and 500° C, respectively.     

Off-eutectic composite solidification and properties in Al-Ni and Al-Co alloys

To investigate the range of coupled eutectic growth in Al-Co alloys from 1 to 4 wt% Co and Al-Ni alloys from 5 to 10 wt% Ni directional solidification, using rates from 0.8×10^–2 cm/sec to 10.6×10^–2 cm/sec, was employed. Both alloy systems exhibited coupled zones skewed towards the hypereutectic compositions. Fully eutectic structures were obtained in the ranges Al-1 wt% Co to Al-3 wt% Co and Al-5.7 wt% Ni to Al-9.2 wt% Ni.The off-eutectic alloys which exhibit a fully eutectic structure behave as reinforcing composite materials, with the tensile strength and microhardness increasing as the volume fraction of the strengthening phase increases.     

Influence of lithium on the mechanical properties of metallic composite materials based on the eutectic Al-5.7% Ni alloy

The effect of lithium addition was examined on the structure and the mechanical properties of metallic composite materials, prepared from the binary eutectic Al-5.7 wt % Ni alloy by casting and isostatic extrusion. A small amount of lithium addition significantly increases the tensile strength. The beneficial effect of lithium addition is due to solid-solution hardening for materials with low lithium content, i.e. those with less than about 1.7 wt %; in addition, precipitation hardening due to the formation of small spherical particles of -Al₃Li phase in the aluminium matrix has an increased effect for materials of higher lithium content.