Defect and Diffusion Forum Vol. 263

Abstract: Tracer diffusion experiments have historically furnished much of the information about fundamental diffusion processes as embodied in such quantities as tracer correlation factors and vacancy-atom exchange frequencies. As tracer diffusion experiments using radiotracers are rather less often performed nowadays, it is important to be able to process other diffusion data to provide similar fundamental information. New procedures that are primarily based around the random alloy model have been established recently for analyzing chemical diffusion data in binary and ternary alloy systems. These procedures are reviewed here. First, we review the random alloy model, the Sum-rule relating the phenomenological coefficients and three diffusion kinetics formalisms making use of the random alloy. Next, we show how atom-vacancy exchange frequency ratios and then component tracer correlation factors can be extracted from chemical diffusion data in alloy systems. Examples are taken from intrinsic diffusion and interdiffusion data in a number of binary and ternary alloys.

Abstract: The simulation of diffusion-controlled transformations in multi-component systems is presented using the software DICTRA. It is shown that not only stable phases precipitate during heat treatments, but also metastable phases. These phases appear in the microstructure for a certain interim time period, which can be predicted by the simulation. Various reaction regimes are possible: local equilibrium which is a slow reaction, local equilibrium with no partitioning which is a fast reaction, para-equilibrium which is a regime with no diffusion at all of the substitutional elements. The transition between the various regimes leads to the development of precipitation stases.

Abstract: First principles calculations have been applied in various fields in Materials Science. The authors have been attempting to reproduce a binary phase diagram by combining FLAPW electronic structure total energy calculations with Cluster Variation Method of statistical mechanics. Such a first principles calculation for static equilibrium has been quite successful for a series of Febased alloy systems. Recently, main attention is directed towards the extension of the first principles calculation to phase transformation dynamics by incorporating Phase Field Method. A series of preliminary calculations on disorder-L10 ordering in Fe-Pd and –Pt are satisfactory and the evolution process of Anti-Phase Boundaries was reproduced. In the present report, first-principles calculations of phase equilibria and phase transformation are briefly reviewed. Particular focuses are placed on coarse graining operation which authors developed and and time scaling as a remaining problem.

Abstract: Many Aluminium alloys use the precipitation of metastable phases to generate optimum properties. The effect of including additional structures such as θ’ and GP zones is described in the context of a hierarchy of metastable structures. Extending a Thermodynamic data base that has been designed solely to deal with equilibrium conditions is a vital prerequisite to handling the heattreatment of aluminium alloys. It is then possible to generate TTT and CCT diagrams, using the Johnson-Mehl-Avrami treatment previously applied in to other materials providing provision is made for the presence of supersaturated quenched-in vacancies. Calculations using JMatPro are given for the expected behavior of commercial aluminium alloys of increasing complexity, including AA319, AA6061 and AA7075.

Abstract: Ferromagnetic L10 ordered alloys are extensively studied nowadays as good candidates for high density magnetic storage media due to their high magnetic anisotropy, related to their chemical order anisotropy. Epitaxial thin bilayers NiPt/FePt/MgO(001) have been grown at 700 K and annealed at 800 K and 900 K. At 800 K, the L10 long-range order increases without measurable interdiffusion. At 900 K, the interdiffusion takes place without destroying the L10 long-range order. This surprising observation can be explained by different diffusion mechanisms that are energetically compared using molecular dynamics simulations in CoPt in the second moment tight binding approximation. In addition, the frequencies of the normal modes of vibration have been measured in FePd, CoPt and FePt single crystals using inelastic neutron scattering. The measurements were performed in the L10 ordered structure at 300 K. From a Born-von Karman fit, we have calculated the phonon densities of states. The migration energies in the 3 systems have been estimated using the model developed by Schober et al. (1981). The phonon densities of states have also been used to calculate several thermodynamic quantities as the vibration entropy and the Debye temperature.

Abstract: The self-diffusion of nitrogen is studied in amorphous silicon nitride, which is a model system for a covalently bound amorphous solid with a low atomic mobility where reliable diffusion data are still lacking. Comparative experiments on Si14Nx/Si15Nx (x ≈ 1.33) isotope multilayers were carried out with secondary ion mass spectrometry (SIMS) and neutron reflectometry (NR), respectively. It was found that experiments with SIMS are not very well suited for the determination of diffusivities in a broad temperature range. The minimum diffusion length of about 5-10 nm detectable with this method is too large. At high temperatures (> 1200 °C) the amorphous solid crystallizes before any diffusion is measured and at low temperatures (< 1100 °C) the diffusivities are too low to be detected. In contrast, with neutron reflectometry diffusion lengths in the order of 1 nm and diffusivities down to 10-24 m2 s-1 were measured between 950 and 1250 °C. The potential of this method for the determination of ultra slow diffusion processes is discussed.

Abstract: In the course of reactive diffusion of hydrogen in metal-doped glasses, at some conditions, metallic nanoparticles grow forming quasi-periodic layered structure. We have developed a model defining conditions necessary for the formation of the layered structures. The model indicates relatively narrow range of parameters providing the quasi-periodic growth of the nanoparticles. The layered structure arises at relatively low over-saturation by neutral metal in the diffusion zone, due to the competition of two processes: enrichment of the glass by neutral metal atoms via reducing of metal ions by diffusing hydrogen and depletion of the glass by the metal atoms caused their diffusion to the nanoparticles. The model can be also applied to other situations where reactive diffusion inducing the formation and growth of nanoparticles occurs.

Abstract: In this work we investigated the mobility of hydrogen in amorphous ceramics with the composition Si13B13C60N13 (AM26C). The material was derived from a pre-ceramic polymer and thermolyzed at 1000 °C. After thermolysis the AM26C ceramics are assumed to be separated in an amorphous SiC phase and an amorphous C(BN)x phase. To measure the diffusivities we used deuterium as a tracer, which was introduced via isotope exchange from the gas phase at temperatures between 700 °C and 1100 °C. Depth profiling was done with secondary ion mass spectrometry (SIMS). The profiles could be fitted with complementary error functions. The diffusivities obey an Arrhenius law. The activation enthalpy is 0.8 eV, the pre-exponential factor is 5×10-12 m2 s-1. These values are close to those found for glassy carbon and thin amorphous C-B-N films as reported in the literature. We therefore conclude that the amorphous C(BN)x phase is the transport path for hydrogen in AM26C ceramics. A direct interstitial diffusion mechanism can account for the activation enthalpy of 0.8 eV. The low value for the pre-exponential factor is attributed to an entropy factor arising from the temperature dependence of the chemical potential of hydrogen.

Abstract: The properties of grain boundary cores and adjacent areas in polycrystalline Nb manufactured by rolling with subsequent recrystallization annealing and nanocrystalline Nb obtained by high pressure torsion have been investigated by the Mössbauer emission spectroscopy on 119mSn nuclei.

Abstract: A physico-chemical consideration of the interfacial interaction and diffusion resulting in the formation of chemical compound layers at the interface of initial substances A and B is presented. The layer-growth kinetics is shown to be much more complicated than it follows from conventional diffusional views neglecting interfacial reactions. In the majority of multiphase binary systems, layer occurrence appears to be sequential rather than simultaneous. Under conditions of diffusion control, the number of simultaneously growing compound layers at the A–B interface cannot exceed two. Multiple layers (three and more) can only form as a result of secondary processes connected with the rupture of a diffusion couple. In such cases, great care is necessary when calculating diffusion coefficients to avoid obtaining their physically meaningless values.