Determination of the Gibbs energy of diamond using a solid state cell

he chemical potential of carbon in diamond, relative to its value in graphite, has been directly determined using a solid state electrochemical cell incorporating single crystal CaF2 as the solid electrolyte. The cell can be represented as Pt, C(graphite) + CaC2 + CaF2double vertical barCaF2double vertical barCaF2 + CaC2 + C(diamond), Pt The reversible emf of this cell is directly related by the Nernst equation to the Gibbs free energy change for the conversion of diamond to graphite. The difference in the chemical potential of carbon in the two crystal structures varies linearly with temperature in the range 940 to 1260 K ?C(diamond) ? ?C(graphite) = 1100 + 4.64T (±50) J mol?1 On the average, the values given by the equation are 320 J mol?1 less positive than the currently accepted ones based on calorimetric studies. The difference is primarily in the enthalpy term.

Differences in the glass-forming ability of rapidly solidified and mechanically alloyed Ti-Ni-Cu alloys

Melt spinning of Ti50Ni50 ? xCux (x = 10, 25, 40) alloys showed that the glass-forming ability is good for Cu-rich compositions and poor for Ni-rich compositions. The results of mechanical alloying experiments in the same system showed a reverse trend as far as the glass-forming ability is concerned. These contradictory results are explained in the light of thermodynamic and kinetic considerations. Crystallization results of the melt spun alloys are also presented.

Representation of thermodynamic properties of ternary systems and its application to the system silver-gold-copper at 1350 K

The article presents a generalized analytical expression for description of the integral excess Gibbs free energy of mixing of a ternary system. Twelve constants of the equation are assessed by the least mean squares regressional analysis of the experimental integral excess data of the constituent binaries; three ternary parameters are evaluated by a regressional analysis based on the partial experimental data of a component of the ternary system. The assessed values of the ternary parameters describe the nature of the ternary interaction in the system. Activities and isoactivities of the components in the Ag-Au-Cu system at 1350 K are calculated and found to be in good agreement with the experimental data. This analytical treatment is particularly useful to ternary systems where the thermodynamic data are available from different sources.

Study of fracture features in foam bearing glass epoxy composites subjected to repeated impacts

The paper reports the failure features observed in low mass repeatedly (pendulum) impacted glass epoxy composites with and without the mid section having either 2-layers or 3-layers of flexible foam. Features such as through width and inclined cracks as well as adhering of foam observed in the experiments are explained. The significance of the foam material in modifying the impact response of the composite is stressed.

Solid state amorphization in binary Ti---Ni, Ti---Cu and ternary Ti---Ni---Cu system by mechanical alloying

An amorphous phase has been synthesized by mechanical alloying in a planetary mill over a nickel content range of 10�70 at.% in the Ti---Ni system and a copper content range of 10�50 at.% in the Ti---Cu system. In the case of ternary Ti---Ni---Cu alloys the glass-forming composition range has been found to be given by x = 10�20 for Ti60Ni40 ? xCux, x = 10 � 30 for Ti50Ni50 ? xCux and x = 10 � 40 for Ti40Ni60 ? xCux alloys. The difficulty in the amorphization of copper-rich compositions is explained in the light of enthalpy composition diagrams calculated for the ternary solid solution and the amorphous phase.

Microstructural stability and nanoindentation of an electrodeposited nanocrystalline Ni-1.5wt.%P alloy

Microstructural stability of nanocrystalline Ni-1.5wt.%P alloy with an initial grain size of 3 nm processed by pulsed electrodeposition was studied using differential scanning calorimetry (DSC) and annealing. Microstructural characterization suggests that the observed exothermic peak during heating in DSC is related to both concurrent grain growth and Ni3P formation. Nanoindentation on samples with grain sizes from 3 to 50 nm revealed a breakdown in Hall-Petch strengthening in nano Ni-P alloy at grain sizes <= 10 nm, consistent with some previous observations. It is concluded that there is a grain boundary weakening regime for grain sizes < 10 nm, based on analysis which show that the data cannot be rationalized in terms of microstrain relaxation, variation in elastic modulus, texture evolution and duplex structure formation.

Electron microscopy and diffraction of ordering in Ni---W alloys

Electron microscopy and diffraction studies of ordering in stoichiometric Ni-20%W and off-stoichiometric Ni-15%W alloys have been carried out. The specimens of Ni-20%W were first disordered at 1398 K for 4 h and then quenched rapidly into water. Short range order (SRO) spots were observed at {1 1/2 0}* positions. Two hitherto unknown metastable phases: D-2h(25)-Ni2W and DO22-Ni3W were observed in the diffraction patterns. Long range order (LRO) transformations were studied at 1103 and 1213 K. Kinetics and mechanism of transformations have been identified. Ni-15%W specimens were solution treated at 1523 K for 1 h followed by quenching in water. SRO spots similar to those found in Ni-20%W were observed in this alloy as well. The transition to LRO was studied at 1093 K. Distinct Ni4W precipitates could be observed after 5 h of annealing at this temperature. After 100 h of annealing precipitates were found to grow into faceted shape coherent with the disordered matrix. After prolonged annealing for over 150 h the Ni4W precipitates began to lose coherency by the generation of misfit dislocations. The microstructural observations have been compared for the stoichiometric and off-stoichiometric alloys.

Enhanced magnetoresistance in as?deposited oxygen?deficient La0.6Pb0.4MnO3-y thin films

The La0.6Pb0.4MnO3(LPMO) thin films were in situ deposited at different oxygen partial pressure and at a substrate temperature of 630 degrees C by pulsed laser deposition. The films grown at lower oxygen partial pressures showed an increase in lattice parameter and resistivity and a decrease in the insulator-metal transition temperature as compared to the stoichiometric LPMO thin film grown at 400 mTorr. Further, these oxygen-deficient thin films showed over 70% giant magnetoresistance (GMR) near the insulator-metal transition temperature against the 40% GMR in the case of stoichiometric thin films. (C) 1995 American Institute of Physics.

The influence of pH on the corrosion of medium strength aerospace alloys 8090, 2091 and 2014

The influence of pH on the corrosion behaviour of two aluminium-lithium-copper-magnesium-zirconium (8090 and 2091) alloys was studied and compared with a standard aircraft alloy, 2014 (Al-4.4% Cu) and 99.9% pure Al. In constant exposure and potentiodynamic polarization studies conducted in 3.5% NaCl solution having different pH values, all the alloys exhibited high corrosion rates in acidic and alkaline environments, with a minimum in less hostile environments close to neutral pH. The pitting potentials for aluminium-lithium alloys were slightly lower than those for 2014 and pure Al. The effect of pH on the passive current density was also less for aluminium-lithium alloys.

On the decomposition of ? phase in some rapidly quenched titanium-eutectoid alloys

The decomposition of the beta phase in rapidly quenched Ti-2.8 at. pet Co, Ti-5.4 at. pet Ni, Ti-4.5 at. pet, and 5.5 at. pet Cu alloys has been investigated by electron microscopy. During rapid quenching, two competitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The beta phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pet Co and Ti-4.5 pet Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pet Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid alpha and the intermetallic compound in the nonlamellar type as well as between the proeutectoid alpha and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti2Cu phase during rapid quenching involved the formation of a metastable phase closely related to an ''omega-type'' phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti2Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid alpha crystals formed during rapid quenching.

Effect of Strain Rate on Ductile-to-Brittle Transition Temperature of a Free-Standing Pt-Aluminide Bond Coat

The ductile-to-brittle transition temperature (DBTT) of a free-standing Pt-aluminide (PtAl) bondcoat was determined using the microtensile testing method and the effect of strain rate variation, in the range 10(-5) to 10(-1) s(-1), on the DBTT studied. The DBTT increased appreciably with the increase in strain rate. The activation energy determined for brittle-to-ductile transition, suggested that such transition is most likely associated with vacancy diffusion. Climb of aOE (c) 100 > dislocations observed in analysis of dislocation structure using a transmission electron microscope (TEM) supported the preceding mechanism.