Critical strength criteria for DB/SPF processing of Al-Li 8090 alloy

Peel strengths are reported for solid state or transient liquid-phase diffusion-bonded (DB) joints between aluminium-lithium 8090 alloy sheets. The joints were tested under superplastic forming (SPF) conditions at 530 °C and with a progressively increasing peel angle,, in the range 0°–60°. The sheet deformed superplastically with or without peel fracture of the joints. A deformation model is proposed which predicts a critical combination of peel strength and superplastic flow stress for DB/SPF processing of the 8090 alloy and indicates peel fracture will occur when sheet thicknesses exceed 2 and 0.8 mm in solid state and transient liquid-phase diffusion-bonded joints, respectively.     

Calculation of Structural Characteristics Evolution during Superplastic Forming of Shells by

The applications of commercial computer program set "SPLEN-O" based on finite elements method (FEM) for the simulation of superplastic forming (SPF) technology of shells are described. The changes in characteristics of ultrafine-grained (UFG) structure during deformation are taken into account in function of viscosity and solving algorithm. Some examples are shown to illustrate the possibilities of FEM simulation for prediction of numerical grain-size data on different SPF stages of shells.     

Calculation of Structural Characteristics Evolution during Superplastic Forming of Shells by "SPLEN-O" Application Computer Program Set

The applications of commercial computer program set "SPLEN-O" based on finite elements method (FEM) for the simulation of superplastic forming (SPF) technology of shells are described. The changes in characteristics of ultrafine-grained (UFG) structure during deformation are taken into account in function of viscosity and solving algorithm. Some examples are shown to illustrate the possibilities of FEM simulation for prediction of numerical grain-size data on different SPF stages of shells.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

Ultrasonic determination of the elastic and nonlinear acoustic properties of transition-metal carbide ceramics: TiC and TaC

Pulse-echo overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic transition-metal carbides TiC and TaC as functions of temperature in the range 135–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. The carbon concentration of each ceramic has been determined using an oxidation method: the carbon-to-metal atomic ratios are both 0.98. In general, the values determined for the adiabatic bulk modulus (B ^S), shear stiffness (), Young's modulus (E), Poisson's ratio () and acoustic Debye temperature (_D) for the TiC and TaC ceramics agree well with the experimental values determined previously. The temperature dependences of the longitudinal stiffness (C _L) and shear stiffness measured for both ceramics show normal behaviour and can be approximated by a conventional model for vibrational anharmonicity. Both the bulk and Young's moduli of the ceramics increase with decreasing temperature and do not show any unusual effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic pressure derivatives of elastic stiffnesses and the acoustic-mode Grüneisen parameters. The values determined at 295 K for the hydrostatic pressure derivatives (C _L/P)_{P = 0}, (/P)_{P = 0} and (B ^S/P)_{P = 0} for TiC and TaC ceramics are positive and typical for a stiff solid. The adiabatic bulk modulus B ^S and its hydrostatic pressure derivative (B ^S/P)_{P = 0} of TiC are in good agreement with the results of recent high pressure X-ray diffraction measurements and theoretical calculations. The longitudinal (_L), shear (_S) and mean (^el) acoustic-mode Grüneisen parameters of TiC and TaC ceramics are positive: the zone-centre acoustic phonons stiffen under pressure. The shear _S is much smaller than the longitudinal _L. The relatively larger values estimated for the thermal Grüneisen parameter ^th in comparison to ^el for the TiC and TaC ceramics indicate that the optical phonons have larger Grüneisen parameters. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic TiC and TaC.     

Coherent Brillouin Spectroscopy in Solid Parahydrogen

We applied coherent Brillouin spectroscopy to solid parahydrogen, and measured the Brillouin spectra of longitudinal acoustic modes at 5.6K. It was found that the linewidth of these spectra is 1.5MHz. From the observed Brillouin shift and the crystal orientation, the elastic stiffness was determined as C ₁₁=0.355±0.016GPa and C ₃₃=0.432±0.022GPa.     

The effect of sheet thickness on the magnetic properties of 77 wt% Ni permalloys

The effect of sheet thickness,t, on the magnetic properties of some Ni-Fe alloys of approximate composition 77% Ni-14% Fe-5% Cu-4 wt% Mo for various heat treatments in the temperature range 300 to 1250° C has been investigated. The range of sheet thickness used was 50 to 375m and was obtained by cold-rolling without interstage annealing. It was found that permeability increases with decreasing sheet thickness, attains a maximum att75m and starts to fall as the thickness is further reduced. The increase in permeability with decreasing sheet thickness (t>75m) is thought to be due to a decrease in eddy current losses, changes in texture formation and possibly the degree of short range order (SRO) developed in the material. The permeability obeys a 1/t ² relationship with sheet thickness in the range 75 to 375m. Below a sheet thickness of 75m permeability starts to decrease with decreasing sheet thickness. The reason for this fall in permeability is thought to be due to a thickness dependent magnetostatic energy contribution to the wall energy associated with free poles along the domain wall and also to the loss ratio,, which increases rapidly at small thicknesses. This rapid increase in is due to factors such as the ratio of domain wall spacing to sheet thickness, which varies with sheet thickness, the spin relaxation term, which has a 1/t ² dependence on sheet thickness, and the increase in the wall energy which increases the domain wall spacing. Finally a decrease in permeability could be caused by a different degree of SRO in the thinner specimens due to different cooling rates which could lead to an increase in first anisotropy constant,K ₁, and saturation magnetostriction, _s which then produces a decrease in _i att<75m when the degree of SRO in the specimens is not optimum.     

Effect of joint stiffness on peel strength of diffusion bonded joints between Al–Li 8090 alloy sheet

Abstract

The peel behaviour of diffusion bonded joints between Al–Li 8090 alloy sheet depends upon joint geometry, sheet thickness, and the local stiffness of the bonded joint. The local stiffness was increased by bonding 8090 metal matrix composite onto the faces of the joint. At the superplastic forming temperature of 530°C the peel strengths of solid state or liquid phase diffusion bonded joints at peak load were increased from 5–7 N mm^?1 to >8 N mm^?1. This led to superplastic deformation of the sheet without peel fracture at the bonded joint. After air cooling and aging, the corresponding room temperature peel strengths were 174–252 N mm^?1, compared with 30–54 N mm^?1 for an unstiffened joint, an increase by a factor of 3·2–8·4. It was concluded that stiffened bonded joints would enable multiple thin sheet structures to be manufactured in Al–Li 8090 alloy via a diffusion bonding/superplastic forming (DB/SPF) technique. A DB/SPF technique for a three sheet structure is described.

MST/1687     

The influence of stacking fault energy on ductile fracture micromorphology

The influence of stacking fault energy on microvoid coalescence in pure materials has been studied. It was shown that as a material's stacking fault energy (SFE) decreased, the extent of microvoid coalescence that occurred during ductile fracture also decreased. The decrease of microvoid coalescence in low SFE materials was attributed to a hindrance in the development of dislocation cells associated with the restricted motion of dislocations. In pure materials, microvoids are believed to initiate and grow along dislocation cell walls formed during deformation. As such, the absence or scarcity of cells in lower SFE materials limits the formation of these voids during ductile fracture.     

Rheology of stimulated whole saliva in a typical pre-orthodontic sample population

The dynamic viscosity () of stimulated whole saliva in a typical pre-orthodontic sample population was characterized as a function of temperature (T). Samples were collected from 30 adolescents or young adults, after screening for factors that are known to have an effect on salivary viscosity. Using a cone and plate viscometer, 1.5 ml of stimulated whole saliva was evaluated at a constant shear rate of 450 s^-1 from T=20°C to T=40°C. Data from the -T plots showed a negative dependence of the form, =a–bT, over a range of from 1.08 to 2.45 centipoise (cps) at 34°C. Most of the samples fell into a narrow envelope, where the mean of the saliva samples ranged from 2.42±0.61 cps at 20°C to 1.57±0.32 cps at 37°C. With regard to sample stability, viscosity-time plots indicated that a small but predictable decrease in occurred during the 3 h period. The -T plots generated from fresh and frozen saliva samples demonstrated an appreciable change in as a result of refrigeration. With regard to sample reproducibility, viscometric data obtained from a typical pre-orthodontic patient over a 1-week period fluctuated within a fairly broad envelope of values.Presented, in part, at the 21st Annual Meeting and Exhibition of the A.A.D.R., Boston, Massachusetts, March, 1992.     

Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 m to 10 m after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers     

Strength and fracture behaviour of diffusion bonded joints in Al-Li (8090) alloy

Peel strengths at room temperature and under superplastic forming conditions at 530 °C were measured for diffusion-bonded joints in Al-Li 8090 alloy sheet. The bonds were made in the solid state, or via a transient liquid phase using interlayers. The effect of strain rate, sheet thickness and heat treatment were investigated. The significance of these results for the testing of DB joints and for their use in DB/SPF structures is discussed.     

Production of superplastic aluminium composites reinforced with Si3N4 by powder metallurgy

Superplastic aluminium composites were processed from these fine aluminium powders of less than 20 m and reinforcements of either Si₃N₄ whiskers or Si₃N₄ particulates by hot extrusion at temperatures between 733 and 793 K with a reduction ratio of 1001. The dispersion of the reinforcements was homogeneous, and the size of the grains of this matrix alloy after extrusion was fine, at less than 3 m for the composites reinforced with either Si₃N_4W or Si₃N_4P. All composites showed large superplastic elongations of more than 300% in a relatively high strain-rate range from 4×10^–22 s^–1 at testing temperatures between 788 and 833 K. These superplastic composites also exhibited excellent mechanical properties at room temperature, which are supposedly attributable to both the homogeneous dispersion in reinforcements and the fine-grained structures.     

Effect of prior cold rolling and test temperature on stress-strain rate behaviour of a Zr-2.5Nb alloy

Zr-2.5 wt% Nb alloy sheet, obtained by unfolding and straightening a pressure tube, was further cold rolled upto 39% reduction in thickness to investigate the effect of cold working on the stress ()-strain rate () behaviour over a strain rate range of 2 × 10–5 to 5 × 10–3 s–1 and a temperature range of 625 to 700 °C. Irrespective of the amount of rolling, the log vs log plots exhibit superplastic behaviour with strain rate sensitivity index, m, as high as 0.8, which decreases to 0.2 at higher strain rates. On the other hand, the activation energy for deformation, Q, increases from 171.1 kJ/mol for superplastic deformation to 249 kJ/mol in Region III. The tendency for improved superplasticity (m) is seen upon cold working by 22% or more at the test temperatures 675 and 700 °C.     

Genetische Modellierung von Künstlichen Neuronalen Netzen

Zusammenfassung Künstliche Neuronale Netze (KNN) haben sich für eine automatisierte Klassifikation, wie sie etwa bei der Kreditwürdigkeitsprüfung durchzuführen ist, als Alternative zu klassischen statistischen Methoden etabliert. Während die KNN-Typ-spezifischen Lernverfahren, wie etwa das Back-Propagation-Verfahren bei Multi-Layer-Perceptron-Netzen dabei anwendungsdomänenunabhängig und auch instanzunabhängig definiert sind, stellt die Konfiguration des spezifischen KNN für eine konkrete Anwendung ein Entscheidungsproblem dar, das jeweils in Abhängigkeit der Anwendungsdomäne (Kreditwürdigkeitsprüfung) und der konkreten Instanz (unternehmenspezifische Kennzahlensysteme) zu lösen ist. Für dieses Konfigurationsproblem, das in der Literatur noch nicht umfassend behandelt wurde, wird in dieser Arbeit mit dem Genetic Modeling ein Ansatz vorgestellt, bei dem eine passende Netzkonfiguration mittels eines Genetischen Algorithmus ebenfalls aus historischem Wissen gelernt wird. Wir berichten über erste empirische Ergebnisse beim Einsatz zur Kreditwürdigkeitsprüfung mittels LVQ-KNN.     

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.     

Superplastic Forming of Friction Stir Welded AA6061-T6 Alloy Sheet with Various Tool Rotation Speed

This study attempts to investigate the superplastic forming (SPF) of friction stir welded (FSW) AA6061-T6 alloy sheet at various tool rotation speeds in the range of 500 to 2000 rpm. The effect of FSW on SPF free blow forming of AA6061-T6, pole height, pole thickness, equivalent strain rate, and equivalent flow stress were investigated at constant pressure and constant temperature. Using the Cheng model the pole thickness, the equivalent strain rate, and equivalent flow stress were determined from superplastic free blow forming experiment. The finite element modeling and simulation is performed over the SPF of FSW specimens using selective superplasticity method. Experimental results indicate that tool rotation speed is the critical parameter during friction stir welding that has a greater influence on SPF. The theoretical modeling results exhibit that the SPF of friction stir welding can be practically applied to determine pole thickness, strain rate, flow stress, and strain rate sensitivity index. The finite element modeling results were found to be fairly agreeing with the experimental results. Hence, superplasticity can be significantly enhanced by friction stir welding by varying the FSW tool rotation speed.     

Wetting of nickel by silver

Sessile drop experiments of molten silver and nickel were performed in air and helium at 970 C. A NiO layer formed at the interface in air; silver formed a 90 contact angle. In helium silver formed a 9 contact angle on nickel. The role of solution reactions in forming these angles is discussed.     

Effect of phase proportions on deformation and cavitation of superplastic α/β brass

Studies have been made, using metallographic and precision density techniques, of the deformation and cavitation behaviour during superplastic tensile straining at 873 K of three microduplex/ brasses which, as a consequence of varying composition, contained varying proportions of and phases. It was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present. Cavitation, on the other hand, decreased rapidly as the volume fraction of phase was increased. The cavitation behaviour was attributed to the relative abilities of the phases to accommodate grain boundary sliding. When a high proportion of phase is present accommodation is minimal and cavity nucleation. occurs readily. Evidence is presented to show that grain-boundary sliding plays a predominant role in cavity growth. When a high proportion of phase is present accommodation is almost complete and cavity nucleation is minimal.     

Dielectric relaxation in polychlorotrifluoroethylene

The effect of increasing lamellar thickness in bulk polychlorotrifluoroethylene (PCTFE) by various methods (annealing and increasing time and temperature of crystallisation) on the parameters of the and relaxations has been studied.For the a relaxation consistent increase in temperature of maximum loss and activation energy with increasing lamellar thickness confirms the strong suggestion of earlier work [6] on oriented specimens and shows that the relaxation arises mainly in the interior of the lamellae with little contribution from the chain folds.No systematic changes in these parameters with increasing lamellar thickness was found for the relaxation and this is explained by the presence of _c and _a components in crystalline and amorphous regions respectively. The component _a (as well as the _c one) shows a marked anisotropy in magnitude with respect to the draw direction in oriented specimens relative to the direction of the applied electric field. This implies a lining up of the chain molecules parallel to one another in amorphous regions.The non-existence of analogous short chain compounds and experimental scatter prevented detailed checking of the consistency of the data for the relaxation with the theory for the _c-C _c model [5].