The increasing importance of powder materials fabrication by use of the HIP has led to recent emphasis on analytical techniques for describing and understanding the process. Modeling of particle adhesion and consolida...
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The increasing importance of powder materials fabrication by use of the HIP has led to recent emphasis on analytical techniques for describing and understanding the process. Modeling of particle adhesion and consolidation during the HIP process has been achieved through the characterization of the particle system densiflcation behavior by the deformation of a representative particle due to forces transmitted through its interparticle contacts. Instead of assuming that all of these forces are equal, a model is developed based on forces which vary to produce a uniform rate of contraction (densiflcation) across a compact. Initial parameters of the systems are obtained from radial distribution data available for monosized and bimodal-sized particle systems. Deformation mechanism maps are predicted for superalloys under temperature conditions of 873–1373 K and pressure conditions of 1–1000 MPa by coupling the equations for athermal yield and power-law creep densification mechanisms. The deformation of large vs small particles in a bimodal-sized particle system is also analyzed and assumptions important in these models are discussed. L'importance croissante de la fabrication de matériaux à partir de poudres en utilisant la pression isostatique à chaud (PIC) a conduit à un intérêt récent pour les techniques analytiques décrivant et expliquant ce procédé. La modélisation de la consolidation et de l'adhésion de particules pendant le procédé PIC à été réalisée grâce à la caractérisation du comportement du système de densification des particules par a déformation d'une particule représentative, due aux forces transmises par contacts interparticulaires. Au lieu de considérer toutes ces forces égales, on développe un modèle basé sur des forces qui varient de façon à générez un taux uniforme de contraction (densification) sur le système compact. Les paramètres initiaux des systèmes sont obtenus à partir des distributions radiales des systèmes de particules à une seule taille et b
A new laboratory technique has been developed in an effort to simulate the deterioration processes occurring in dental amalgam restorations while they are exposed to attack by the oral environment and simultaneously s...
A new laboratory technique has been developed in an effort to simulate the deterioration processes occurring in dental amalgam restorations while they are exposed to attack by the oral environment and simultaneously subjected to biting forces during mastication. This combined mechanical wear and corrosion action may be one of the major contributors to the degradation of dental amalgam restorations. The technique provides the capability of varying and measuring electrochemical and mechanical parameters during a sliding-wear process in a corrosive environment. A high-copper dental amalgam was selected and tested to demonstrate the applicability of the method in evaluating and studying the effects of the combined action of wear and corrosion processes on dental materials. For the particular amalgam material tested in the present study, it was found that sliding-wear significantly lowered its corrosion potential and increased corrosion rates by at least one order of magnitude.
A thermodynamic treatment for a second-order superconducting phase transformation has been developed which incorporates crystallographic effects. The temperature dependence of x-ray-measured lattice constants in polyc...
A thermodynamic treatment for a second-order superconducting phase transformation has been developed which incorporates crystallographic effects. The temperature dependence of x-ray-measured lattice constants in polycrystalline YBa2Cu3O7−δ has been used to determine the orthorhombic, crystallographic thermal-expansion coefficient matrix. The principal thermal strains were curvefit above and below the superconducting transition temperature to deduce the thermal-expansion jumps at the phase boundary. The thermodynamic arguments show that the three elastic shear stiffness elements do not contribute to the superconducting specific-heat jump. Also, the three independent shear stresses at the superconducting transition point are zero. The specific-heat jump is, however, related to the three normal stresses at the phase boundary. The experimental thermal-expansion data suggest that a compressive stress aligned with the oxygen-deficient crystallographic â axis changes the transition temperature by 0.3 K/kbar; the b^ axis stress is 0.07 K/kbar, while the ĉ is 0.1 K/kbar, and all stresses are in compression. The isothermal elastic compliance jumps can be predicted by assuming zero jumps in the adiabatic elastic compliances. The specific-heat jump is calculated from this assumption and crystallographic thermodynamics.
Recent measurements of the discontinuity of the volume expansivity of Y//1Ba//2Cu//3O//7// minus // delta across the superconducting transition have been shown to be self-consistent with the measured pressure dependen...
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Recent measurements of the discontinuity of the volume expansivity of Y//1Ba//2Cu//3O//7// minus // delta across the superconducting transition have been shown to be self-consistent with the measured pressure dependence of the superconducting transition temperature. Ehrenfest second-order transitions have jumps in specific heat, bulk modulus and thermal expansivity. Supercondutivity is generally considered to occur as an Ehrenfest second-order phase transition. The jump in thermal expansivity has been found by measuring the crystallographic cell volume by high resolution neutron diffraction at selected temperatures and assuming that the volume was continuous across the superconducting transition. This data was curve fit above and below the transition temperature. The thermal expansion coefficient jump of normal less superconducting values deduced using this curve fit data is DELTA a equals minus 6. 4 plus or minus 9. 3 multiplied by 10** minus **7/K. In this paper it is shown that this value is consistent with other thermodynamic data while reducing the large error bars on the data. This also suggests that the lattice has only a minor role in the transition. 11 Refs.
Conventional powder processing methods have been used to fabricate superconducting cermets, using non-noble metals. Two distinct classes of superconducting cermets have been made. The first expels magnetic fields at l...
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Conventional powder processing methods have been used to fabricate superconducting cermets, using non-noble metals. Two distinct classes of superconducting cermets have been made. The first expels magnetic fields at liquid N 2 temperatures. The second exhibits zero resistivity on a macroscopic scale. We report preliminary success in the fabrication of these cermets.
The stress field around the tip of a semi-infinite crack with an edge dislocation parallel to the tip but located in any nearby position has been obtained in closed form. Using this, the equilibrium positions under mo...
The stress field around the tip of a semi-infinite crack with an edge dislocation parallel to the tip but located in any nearby position has been obtained in closed form. Using this, the equilibrium positions under mode I loading of an array of edge dislocations emitted from the crack tip on a slip plane oriented at an angle to the crack plane were obtained numerically. A dislocation-free zone (DFZ) was found to exist if the slip plane was not saturated with dislocations. The DFZ disappeared at saturation. The case of two arrays emitted on two symmetrically oriented slip planes was studied also. The effects of the applied stress, the angle of inclination of the slip plane, the lattice friction for dislocation motion and the number of dislocations in the plastic zone are presented.
Negative creep, defined as a contraction of the sample length on the application of a tensile stress at elevated temperatures, was observed in the Metglas® alloys 2826B (Ni 49 - Fe 29 P 14 B 6 Si 2 ) and MBF 20/2...
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Negative creep, defined as a contraction of the sample length on the application of a tensile stress at elevated temperatures, was observed in the Metglas® alloys 2826B (Ni 49 - Fe 29 P 14 B 6 Si 2 ) and MBF 20/20A (Ni 68.8 Cr 6.6 - Fe 2.6 B 14.1 Si 7.9 ). Thermal annealing of the sample prior to testing was necessary for the phenomenon to occur. For the MBF 20/20A material, heat treatments at 213 and 268°C for 500 min maximized the degree of contraction on testing at 123°C and 570 MPa. A creep treatment before annealing enhanced the peak contraction at 268°C and reduced that at 213°C. Samples predeformed and then annealed at 258°C for 500 min showed maximum negative creep at testing temperatures of 123, 143 and 162°C for a stress of 570 MPa and exhibited an overall maximum at a stress of 570 MPa for a testing temperature of 127.7°C. The contraction recurred for repeated testings of the same sample although, beyond the first test, the maximum negative creep usually decreased in magnitude. Analysis of the strain-time curves in terms of the Johnson-Mehl equation yielded time exponents of either 0.7–0.8 or 1.0–1.2, the latter range describing a majority of the data.
Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470&#...
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Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470° C a large number of cycles could be obtained before failure, and the cyclic stress-strain response showed a period of rapid hardening followed by a period of decreasing hardening rate. TEM investigations of the lower temperature samples show structures of isolated dislocation dipoles, multipoles and debris. At 470° C dense bundles of dislocations were observed aligned perpendicular to the Burgers vector direction. The regions between the bundles were relatively dislocation free, but they contained a high density of debris. Bowed out screw dislocations are observed between the edge dislocation bundles, suggesting that screw dislocations were largely mobile. Comparisons are made with the cyclic deformation and structure of fcc metals and other NaCl structure single crystals.
Shear bands introduced into the as-quenched metallic glass alloy, MetglasR 2826 (Fe40Ni40P14B6), were etched by immersion in a solution of CUS04 and HCI. Elimination of the etchability property through isothermal anne...
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Shear bands introduced into the as-quenched metallic glass alloy, MetglasR 2826 (Fe40Ni40P14B6), were etched by immersion in a solution of CUS04 and HCI. Elimination of the etchability property through isothermal annealing had an apparent activation enthalpy of 250 kJ mol?1, similar to that of stress relaxation. Direct microscopic observations indicated that the etching of shear bands may be a result of stress corrosion cracking. The electrochemical etchability of shear bands in the nickel-based alloy, BNi2 (Ni68.8Cr6.6Fe2.6B14.1Si7.9), polarized in a solution of perchloric acid and acetic acid at 12° C, could also be eliminated by thermal annealing. The loss of etchability had an apparent activation enthalpy of 580 kJ mol?1, a value indicative of more complex atomic rearrangements taking place within the deformed material. Finally, shear bands introduced into the as-received BNi2 material retained the ability to reverse shear after heat treatment at temperatures in the range of 192 to 375° C even if the electrolytic etchability was apparently diminished.
The low-temperature (120°-245 °C) structural relaxation of Metglas® 2826B (Ni49Fe29P14B6Si2) amorphous alloy was investigated for samples subjected to a tensile stress in the range of 20–400 MPa during...
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