Defect and Diffusion Forum Vols. 297-301

Abstract: The intrinsic diffusion coefficients in diffusion aluminide coatings based on Fe-30Cr were determined at 1000oC. The diffusion fluxes were given by the Nernst Planck formulae and the Darken method for multicomponent systems was applied. This paper summarizes some numerical results to determine the composition dependent diffusivities in Fe-Cr-Al systems. The method presented in this study to obtain average intrinsic diffusion coefficients is as an alternative to the Dayananda method. Our method based on empirical parameters allowed us to predict the concentration profile during the interdiffusion process.

Abstract: We investigated the oxidation of nanocrystalline aluminum surfaces by using variable charge molecular dynamics at 600 K under three oxygen pressures: 1, 10 and 20 atm. The interaction potential was described by the electrostatic plus (Es+) model that allows dynamical charge transfer among atoms. We mainly focused on the effect of the oxygen pressure on the oxidation kinetic, the chemical composition and the microstructure of the oxide films formed. The results show that oxidation kinetics as well as chemical composition and microstructure depend on the applied oxygen pressure. The oxide film thickness tends to a limiting value equal to ~3 nm. Finally, we obtained a partially crystalline oxide films for all oxygen pressures and we observed that the degree of crystallinity increases with time.

Abstract: This numerical study focuses on the determination of macroscopic (effective) properties from pore scale calculation. These results will be applied to heat exchangers design. The computational domain -representative of heat exchanger section- is a parallelepiped filled with metallic foam, heated on one face and crossed by a forced fluid flow. Conjugate heat transfer and fluid flow are computed using finite volume approach on the actual solid matrix and pore space topology obtained from X-ray tomograms. Calculated heat transfer coefficient and flow law parameters are in good agreement with literature data. An active foam length is defined and measured in order to provide optimal design characteristic for foamed heat exchanger.

Abstract: In this paper, we describe two experimental set-ups which enable the measurement of electrical properties and intrinsic diffusion coefficients in UO2. Electrical conductivity measurements are insured by a standard four point Kelvin-Bridge method. In parallel, the gas-solid isotopic exchange method is used to load the samples with 18O tracer atoms, the concentration profile of which are subsequently characterized using SIMS and chromatic confocal microscopy. An application of both types of measurements on a UO2 single crystal is given. The diffusion study was carried out at 750°C, and the electrical conductivity study was performed between 1000°C and 1300°C at oxygen potentials at which the material exhibits extrinsic behaviour. We show how a careful use of both measurements in conjunction can be an indication of the operative migration mechanism.

Abstract: The aim of this work is to join -TiAl intermetallics to Ni based superalloys by solid state diffusion bonding. The surface of the -TiAl alloys and Ni superalloys to be joined was prepared by magnetron sputtering with a few microns thick Ni/Al reactive multilayer thin films with nanometric modulation periods. Sound joining without cracks or pores is achieved along the central region of the bond, especially at 800°C and when a 14 nm period Ni/Al film is used as filler material. During the diffusion bonding experiments interdiffusion and reaction inside the Ni/Al multilayer thin film and between the interlayer film and the base materials is promoted with the formation of intermetallic phases. The final reaction product in the multilayer films is the B2-NiAl intermetallic phase. The interfacial diffusion layers between the base materials and the multilayer films should correspond to: 3-NiTiAl and 4-Ni2TiAl phases from the -TiAl side; Ni-rich aluminide and -phase from the Inconel side. These intermetallic phases are responsible for the hardness increase observed on the diffusion layers.

Abstract: A method is presented to measure lattice and grain boundary diffusion coefficients using secondary ion mass spectroscopy and 2-dimensional diffusion simulations. SIMS is used to measure concentration profiles of implanted species before and after annealing. The as-implanted concentration profile is used as the initial condition for 2-dimensional diffusion simulations using the finite element method. The geometry of the simulation is based on the microstructure of the sample observed by transmission electron microscopy. Both lattice and grain boundary diffusion are simulated. The final 2-dimensional concentration distribution is projected on the depth axis to obtain a simulated depth profile. The diffusion coefficients are adjusted to fit the profiles measured after annealing. We find that this method allows to determine simultaneously and independently the lattice and grain boundary diffusion coefficients from the same profiles. This method is used to measure the diffusion coefficients of As in polycrystalline Ni2Si thin films. The simulations are found to fit the measured profiles with accuracy. The coefficients are measured between 550 and 700°C. An activation energy ratio Qgb/Qv is found greater than one. This result is corroborated by existing data in silicides and is compared to results in other materials for discussion.

Abstract: Boron Nitride nanotubes (BNNTs) together with carbon nanotubes (CNTs) have attracted the wide attention of the scientific community and have been considered as promising materials due to their unique structural and physical properties. In this paper, the behavior of BNNTs of different diameters under compressive loading has been studied through molecular dynamic (MD) simulations. We have used a Lennard-Jones pair potential to characterize the interactions between non-bonded atoms and harmonic potentials for bond stretching and bond angle vibrations. Results of the MD simulations determine the critical buckling loads of the BNNTs of various diameters under uniaxial compression, and indicate that for the simulated BNNTs of length L = 6 nm, the critical buckling loads increase by increasing the nanotube diameters.

Abstract: The thermal conductivity of unfilled polypropylene foams produced using different foaming processes has previously been demonstrated to be mainly affected by the foam’s bulk density [1]. The influence of adding inorganic particles is now studied, with the thermal conductivity of the mineral-filled PP foams being determined using the Transient Plane Source Method (TPS). To this end, two different fillers were used. The incorporation of high amounts (50 and 70 wt.%) of magnesium hydroxide resulted in considerably higher thermally conductive foamed materials, with interesting thermal anisotropies being observed for the higher expansion ratio foams. On the contrary, adding montmorillonite (MMT) nanoparticles did not considerably alter the thermal conductivity of the foams, their value being mainly affected by the relative density.

Abstract: Carbon nanofibre-reinforced polypropylene nanocomposites containing from 5 to 20 wt.% of carbon nanofibres and a chemical blowing agent were melt-compounded and later foamed using compression-moulding. Alongside their foaming behaviour analysis and cellular characterization, foams showing an increasingly finer isometric cellular structure with increasing the amount of nanofibres, their thermal conductivity was determined using the Transient Plane Source Method (TPS). Contrarily to the electrical conductivity, which has previously been shown to rise with increasing the amount of carbon nanofibres [1], the addition of the nanofibres did not significantly alter the thermal conductivity of the PP foams, their value being mainly affected by the relative density, only slight differences being assessed for the higher expansion ratio PP-CNF foams.

Abstract: Mechanical properties, microstructure of the Sn–38wt. %Pb eutectic and the development of deformation - induced diffusion processes on interphase boundaries (IB) were investigated. Experiments were carried out both in deformed and annealed states of eutectic using micro- and nanoindentation, SEM, AFM and optical microscopy techniques. It was found that the deformation of the annealed alloy is localized at the Pb/Sn interphase boundaries and occurs by grain boundary sliding (GBS) accompanied by sintering micropore processes under the action of the capillary forces on the Pb/Sn IB. During severe plastic deformation of Sn-Pb eutectic phase transition in the Sn grain boundary occurs. This deformation-induced process takes place due to the wetting of tin with Pb. These diffusion accommodation processes (sintering and wetting) are facilitated by the low values of the Pb/Sn interphase energy (0.07 J/m2). Wetting is thermodynamically favourable because the condition γgb > 2 γib is satisfied and it is also kinetically allowed due to the relatively high homologous temperature (> 0.5•Tm). The obtained values of the nanohardness and elastic modulus evidence that the IBs in the Sn–Pb eutectic have to be considered as a separate quasi-phase with its own properties.