Diffusion in Materials DIMAT-2017

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Authors: Jing Zhong, Kai Wang, Li Jun Zhang
Abstract: A coupling interface between phase-field model with finite interface dissipation and the CALPHAD (CALculation of PHAse Diagram) thermodynamic and atomic mobility databases is developed. It robotizes the procedures that provides the composition and temperature dependent properties in multicomponent and multi-phase systems. Based on the developed coupling interface, different CALPHAD properties can be directly coupling in the phase-field simulation.
Authors: Daniil M. Levin, Yury N. Kolmakov
Abstract: Examination was made of the early stages of binodal or spinodal solid solutions decay connected with the growth of local zones of concentration fluctuations. The theory of Markovian process of random walk of the individual atoms describes how particle transport with short diffusion paths causes the growth of these zones. The resulting equation predicts the growth and transformation of initial zones of concentration fluctuations into growing clusters with surface boundary or formation of the quasiperiodic concentration distribution inherent in spinodal decomposition.
Authors: Christophe Girardeaux, Brice Sarpi, Sébastien Vizzini
Abstract: Generation of ultra-thin oxide layers (in the nanometer range) is currently a technological lock for numerous applications such as microelectronics, spintronics or even molecular electronics. A precise study of the stages of growth of Mg is essential before studying the growth of the oxide. In this work we report and discuss an experimental study of the very first stages of Mg growth onto Si(100) by Scanning Tunneling Microscopy-Spectroscopy (STM-STS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). First, we have shown that an amorphous underlayer is formed onto the silicon substrate for Mg deposits of 0.25 monolayers (ML). This underlayer is attributed to a Mg2Si silicide formed at RT during Mg deposition. Then, using an original growth method based on alternate cycles of magnesium monolayer adsorption and room temperature (RT) oxidation, we did grow ultra-thin magnesium oxide films onto Si(100). Our study revealed that the ultra-thin Mg2Si layer at the MgO/Si(100) interface acts as a diffusion barrier and prevents oxidation of the highly-reactive silicon during magnesium oxide growth.
Authors: Bence D. Gajdics, János J. Tomán, Fanni Misják, György Radnóczi, Zoltán Erdélyi
Abstract: For revealing internal atomic processes in bimetallic nanoparticles, individual hemispherical Ag-Cu alloy particles were grown by direct current (DC) magnetron sputtering. Phase separation of particles was found to be size- and composition-dependent. Particles smaller than 5 nm in diameter remained as a solid solution of the components for all tested compositions (15-80 at.% Ag). At 15 and 30 at.% Ag compositions phase separation was observed only for particles above 5 nm in diameter. Computer simulations by Stochastic Kinetic Mean Field model reproduced the size-dependence of the decomposition and the internal structure of two-phase particles. Theoretical explanation is given for the composition dependence of the phase separation tendency.
Authors: Vladimir V. Popov, Gerrit Reglitz, Evgeniy V. Shorohov, E.N. Popova, A.V. Stolbovsky, Sergiy V. Divinski, Gerhard Wilde
Abstract: Formation of microstructure in Ni under equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), its thermal stability and diffusion properties of grain boundaries are investigated. Grain boundary diffusion in the ultrafine-grained Ni is found to be significantly faster than in the coarse-grained Ni, which indicates a 'non-equilibrium' (deformation-modified) state of grain boundaries in the former. The effect of non-equilibrium state of grain boundaries on the level of internal stresses is analyzed.
Authors: Aleksei Itckovich, Mikhail Mendelev, Alexey Rodin, Boris Bokstein
Abstract: The peculiarities of grain boundary diffusion in Cu connected with the effect of atomic pairs formation in grain boundaries (GB) were studied using the molecular dynamics (MD) simulation. In present study Cu GB selfdiffusion was simulated with the use of semi-empirical potential. Besides, the ‘heterodiffusion’ simulation was performed with the artificially addеd energy of interaction (E) between identical atoms in arbitrary chosen pairs. To obtain reliable data on the mean square displacements (MSD) the simulation cell, consisted about three hundreds thousands atoms and two symmetrical GBs Σ5 (001)(012), was used. 70 pairs of identical Cu atoms in GBs, bonded into pairs, were chosen as initial state. Energy of interaction was varied between 0 and - 0.5eV/atomThe results obtained for selfdiffusion are in a good agreement with experimental results and other results of computer simulation. Two main effects for heterodiffusion are under discussion. The first is atomic exchange between GB zone and adjacent lattice zone, where the mobility of the atoms decreases significantly. As a result, the MSD decrease. Another effect is connected with attraction between the “marked” atoms, which leads to formation of relatively stable complexes and the MSD also decreases. The results obtained involve also dependence the number of the stable pairs on time and temperature and show the possibility of pairs to condense into ternary, quarterly and more numerous complexes.
Authors: Vadim Derkach, Amy Novick-Cohen
Abstract: In W.W. Mullins' classical 1957 paper on thermal grooving, motion by surface diffusion was proposed to describe the development of a thermal groove separating two grains in a simple semi-infinite planar geometry. After making a small slope approximation which is often realistic, Mullins' sought self-similar solutions, and obtained an explicit time series solution for the groove depth. In the years since, Mullins' grooving solution has become a standard tool; however it has yet to be rigorously demonstrated that self-similar solutions exist when the small slope approximation is not applicable. Here we demonstrate that reformulation of Mullins' nonlinear problem in arc-length variables yields a particularly simple fully nonlinear formulation, which is useful for verifying large slope grooving properties and which should aid in proving existence.
Authors: E.N. Popova, I.L. Deryagina, E.G. Valova-Zaharevskaya, A.V. Stolbovsky
Abstract: The structure and thermal stability of Cu-18Nb multicore composite fabricated by repeated cold-drawing of in situ melted mixture of Cu and Nb and subjected to high-pressure torsion (HPT) have been studied by SEM, TEM, X-ray analysis and microhardness measurements. In the cold-drawn state ribbon-like Nb filaments the thickness of 30-70 nm are located in Cu-matrix with sharp texture <110>Nb║<111>Cu║drawing axis. The Nb lattice is distorted, the interplanar spacing (110)Nb being extended along the drawing axis and compressed perpendicular to it, which testifies a semi-coherent character of Cu/Nb interfaces. At annealing these distorsions gradually vanish, and coagulation of Nb ribbons starts at 400С, actively develops at 600С and finishes at 800С with the formation of sausage-like filaments with round transverse sections, which is accompanied with about two-fold decreasing of microhardness. Under the HPT the composite structure is considerably refined, and almost equiaxed grains the sizes of 20-30 nm are formed, which gives rise to a dramatic increase of microhardness. The thermal stability of Cu-Nb composite after cold drawing and HPT is appreciably higher than that of pure Nb and Cu nanostructured by severe plastic deformation.
Authors: Efi Hadjixenophontos, Lukas Michalek, Andreas Weigel, Guido Schmitz
Abstract: The diffusion mechanism of H in metals and metal hydrides is studied particularly at high H2 pressures. Thin films of Mg and Ti offer a convenient tool to quantify the atomic transport. We show how different parameters of hydrogenation affect the kinetics. At 200°C, the Pd-Mg interface is predominant and a linear regime of hydrogenation is observed, whereas at 300°C a parabolic regime is detected. In Mg, the hydride forms from the surface to the substrate whereas in Ti growth of TiH2 starts from the substrate. A linear kinetics is seen during hydrogenation of Ti films, which is due to the oxide layer on top, measured to be about 10nm thick. In the studied high pressure regime, the hydrogenation is not pressure dependent any more. Quantitative calculation of the growth rate and the diffusion coefficient of H in the hydrides is presented.
Authors: Alon Kafri, Alexandra Makonovitsky, Roni Z. Shneck
Abstract: While studying activation sintering of tungsten, Evans [5] and Ito and Furusawa [6] revealed that W-Cr-Pd alloys exhibit unexpected oxidation resistance at elevated temperatures. The role of palladium in stimulating oxidation resistance in W-Cr alloys is the main aim of the present contribution. As previously observed, at 800 °C these alloys form a relatively dense protective scale that consists of an inner layer of Cr2O3, an intermediate layer of Cr2WO6 and an external layer of WO3. At 1200 °C only Cr2WO6 layer is found, since the Cr2O3 and WO3 evaporate. To determine the role of paladium, W and W-Pd alloys were coated with Cr layers and undergone diffusion experiments. An extraordinary affinity between the Cr and Pd was revealed, manifested by extremely fast inward diffusion of Cr along grain boundaries. In a second experiment the dissolution of Cr into W grains at 1300°C was followed and found to take place preferentially along grain boundaries. These observations assess that the Pd segregated at grain boundaries provides fast diffusion channels for Cr to the free surface and it imparts the significant improvement of the oxidation resistance of W alloys, as suggested by Lee and Simkovitz [10-12].

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