Defect and Diffusion Forum Vols. 237-240

Abstract: Aluminide diffusion coatings act as a remedy against the aggressive environments in which modern aero-gas turbines operate. Platinum addition to basic aluminide coatings significantly improves the oxidation resistance of these coatings. The increase in operating temperatures of industrial energy systems and gas turbines, has led to the extensive use of coatings capable of providing improved service life. Interdiffusion plays a critical role in understanding the integrity of such coatings. The Danielewski-Holly model of interdiffusion which allows for the description of a wide range of processes (including processes stimulated by reactions at interfaces) is employed for studying of interdiffusion in the Pt-modified β-NiAl coatings. Using the inverse method the intrinsic diffusivities of Ni, Al and Pt were calculated. Such obtained diffusivities were subsequently used for modelling of thermal stability of Pt-modified aluminide coatings in air and in argon atmosphere.

Abstract: Pulse loading of diffusion couples leads to the formation of the broad metastable solid solutions. Under higher temperatures, combined with high deformation rates, intermetallics also can form. Possible mechanisms of this phenomenon are discussed. Formation of nanostructure under uniaxial compression/decompression (observed in MD simulations) seems to be one of the possibilities.

Abstract: The precipitation of niobium carbides in industrial steels is commonly used to control the recrystallization process or the amount of interstitial atoms in solid solution. It is then important to understand the precipitation kinetics and especially the competition between homogeneous and heterogeneous precipitation, since both of them have been observed experimentally, depending on the alloy composition, microstructure and thermal treatments. We propose Monte Carlo simulations of NbC precipitation in α-iron, based on a simple atomic description of the main parameters which control the kinetic pathway : - realistic diffusion properties, with a rapid diffusion of C atoms by interstitial jumps and a slower diffusion of Fe and Nb atoms by vacancy jumps - a simple model of grain boundaries which reproduces the equilibrium segregation properties of Nb and C - a point defect source which drives the vacancy concentration towards its equilibrium value. Depending on the precipitation conditions, MC simulations predict different kinetic behaviours, including homogeneous and heterogeneous NbC precipitation, early segregation of C atoms and its importance as a first stage for NbC precipitation, wetting phenomena on grain boundaries and transient precipitation of metastable carbides.

Abstract: We propose a simple method for calculation of surface self-diffusion coefficients using kinetic data on the decay of thin films – void growth and transformation of the island shape to the equilibrium. Calculations are made taking into account equilibrium wetting angle of the film on a substrate. The kinetic data on the decay of Pd thin films on sapphire and silica substrates were obtained using Auger electron spectroscopy. By in situ monitoring the intensity of the Auger signal from the film, three different stages of the decay could be distinguished. The surface self-diffusion coefficients were calculated for the temperature range 583 – 823 K. The values of the surface diffusion coefficients and the activation energies are discussed compared to those obtained by other methods.

Abstract: Process of high-temperature (650°С) interaction of Cr(Si)-containing steel EP-823 with stagnant Pb melt containing oxygen (CО[Pb] » 10-5 - 10-6 mass.%) was theoretically and experimentally investigated. The structure and composition of oxide layer formed on the steel surface during exposure to Pb melt was examined. It is determined that thin (£ 1000 Å) Cr(Si) - rich oxide layer is formed on the steel surface in the early stages of oxidation. Oxide layer is being formed intensively over the grain boundaries followed by a lateral diffusion of Cr and spreading of Cr2O3 over alloy surface. Iron diffuses through the Cr(Si) - rich continuous oxide layer in the course of time. The formed oxide layer protects the steel against liquid metal penetration. Kinetics of iron diffusion dissolution in the liquid Pb is analytically described taking into account the chemical interaction between iron and oxygen. It is assumed that oxygen ions serves as a “traps” for iron ions and eliminates them from the diffusion flux. Fe−O complexes are considered as separate slowmoving components of the melt. In order to formulate the diffusion problem equations with additional parameter describing the volume reaction between Fe and O in melt and boundary conditions involved the time dependence of oxygen concentration at the interface of both melt and solid metal sides were used. Result is obtained in analytical form using the Laplace transformation. Analysis of obtained relations allow to assert that in the case of dissolution of iron in the lead melt containing “oxygen traps” the diffusion zone are less than that in the conditions of dissolution (without “traps”). However, the total concentration of iron both on surface of oxide layer and in the contact zone of melt is increased.

Abstract: Structure and phase composition of binary Al–Zn, Al–Mg and ternary Al–Zn–Mg alloys were studied before and after high pressure torsion (HPT) with shear strain 300. The size of (Al) grains and crystals of reinforcing second phases decreases drastically after HPT reaching nanometer range. As a result of HPT, the Zn-rich (Al) supersaturated solid solution decomposes completely and reaches the equilibrium state corresponding to room temperature. The decomposition is less pronounced for Al–Mg and Al–Zn–Mg alloys. We conclude that the severe plastic deformation of supersaturated solid solutions can be considered as a balance between deformation-induced disordering and deformation-accelerated diffusion towards the equilibrium state.

Abstract: Mechanical properties of interphase boundaries (IB), stability of defects and microstructure in heavily deformed binary eutectics (Al-Sn, Zn-Sn, Pb-Sn, Cd-Sn, Bi-Sn) have been investigated at room temperature. Experiments were carried out on atomically clean surfaces of alloys and on bimetallic joints with clean interface. It has been shown that after severe deformation the phases are strengthened and relaxation processes occur mainly on the boundaries in all eutectics. For superplastic eutectics with low interphase boundary energy the intensive development of the diffusion – controlled processes of self- healing, sintering, segregation and enveloping were observed. These diffusion processes, directed to restoration of a contact, are the reason of softening of interphase boundaries and superplastic viscous flow. It has been shown that for the eutectics with high IB energy (Al−Sn, Zn−Sn) the interphase boundary sliding leads to the formation of narrow IB cracks with sharp angles. Diffusion healing of micropores and cracks on these IB does not develop at room temperature and deformation defects are stable. Such deformed structure of IB defines low temperature brittleness of Zn-Sn, Al-Sn eutectics and hydrogen brittleness of Al-Sn.

Abstract: The kinetics of liquid Ga penetration along grain boundaries in pure Al, Al-Mn (50 ppm Mn) and Al-Ga alloys (with concentration 0.7, 1 and 3% mass. Ga) was studied. It was shown that crack-like channels filled by liquid Ga were formed along all grain boundaries. Their width was about 1-3 µm. Investigation of quenched state showed that in all samples, channels had grown with linear kinetics, and that the propagation rate depended on impurity concentration in alloys: 35µm/s for Al-Mn alloy, 14 µm/s for pure Al and 9±1 µm/s for Al with (0.7-3) % Ga alloys. The difference in penetration rates of pure Al and Al based alloys are discussed in terms of internal stress and mechanical properties.