Defect and Diffusion Forum Vol. 354

Abstract: The effect of cold-rolling on mechanical and electrical properties, microstructure and recrystallization behaviour of the AlMn, AlScZr and AlMnScZr alloys was studied. The materials were investigated during isothermal annealing at 300, 400, 500 and 550°C and during step-by-step quasilinear annealing from 200°C up to 600°C with a heating rate of 100 K/h followed by subsequent isothermal annealing at 600°C/5 h. Precipitation reactions were studied by electrical resistometry and (micro) hardness measurements. The microstructure development was investigated by electron microscopy and electron backscatter diffraction examinations. The hardening effect is due to uniformly distributed Al₃Sc and/or Al₃(Sc,Zr) particles. The distinct changes in resistivity of the alloys above ∼ 300°C are mainly caused by precipitation of Mn-containing particles. It has a negligible effect on hardness. Phase transformations in the AlMn and AlMnScZr alloys are highly enhanced by cold rolling. The precipitation is dependent on the deformation degree – the more deformation the more intensive precipitation of the Mn-containing particles. The combination of Mn, Sc and Zr additions to Al substantially suppresses recrystallization at 550°C. A partial recrystallization was observed in the AlScZr alloy and AlMnScZr alloy after annealing 550°C/60 min and 550°C/760 min, respectively. The decomposition sequence of the supersaturated solid solution of the AlMnScZr alloy is compatible with the decomposition sequence of the AlScZr system accompanied and/or followed by the formation of Mn-containing particles.

Abstract: A combined mathematical model has been presented to evaluate the main physical processes taking place in the electro-discharge machining (EDM) gap. The model consists of electric field distribution evaluation by using the full Poisson equation for the environment of different electrical parameters as well as the electric charge distribution, and rends particles (grits) movement in the gap forced by diffusion as well as the electric field drag of charged grits. Both models are developed on the same rectangular grid which allows mutual influence of them.

Abstract: Various modes of severe plastic deformation (SPD), such as high-pressure torsion (HPT) at cryogenic temperature, equal channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), have been applied for nanostructuring of Ni, and the thermal stability of the structure obtained has been studied. The nanocrystalline structure with average grain sizes of 80 nm and the microhardness of 6200 MPa is produced by HPT in liquid nitrogen. DCAP and ECAP result in the submicrocrystalline structure of a mixed type, with ultra-fine grains separated by high-angle boundaries along with deformation bands and coarse cells with low-angle dislocation boundaries. The thermal stability of the structures obtained by ECAP and DCAP is approximately the same, and it is higher than after the HPT at cryogenic temperature.

Abstract: Grain boundary (GB) diffusion in an ensemble of three grain boundaries intersecting in the point of GB triple junction is described on the basis of quasi-steady Fisher’s model. Two versions of the configuration of the ensemble are considered, namely, with different number of GBs adjacent to the surface covered with a diffuser source and with different angle between GB and surface. Analytical expressions for the distribution of diffuser concentration along each GB of an ensemble are derived supposing that the GB diffusion fluxes are equal in the GB triple junction. The expressions for the diffuser concentration distribution along GBs in both ensembles include not only diffusion constants (like GB and bulk diffusion coefficients) but also structural characteristics of the ensemble of grain boundaries (i.e. the depth of the triple junction point under the surface and the angle between GBs in the triple junction point). The specific features of diffusion kinetics in the ensembles of different configuration with an angle of 120^o (the equilibrium angle in a polycrystal) were revealed by comparing the diffuser concentration distributions in the ensembles and in the single GB.

Abstract: We discuss the numerical modelling of unsaturated-saturated flow in porous media and contaminant transport with adsorption in 1D. We developed an efficient numerical approximation which is a good candidate for solving inverse problems involving the determination of model parameters (hydraulic and geochemical). Also an adsorption/desorption model for mass transfer through the boundary of porous media (contact solid-liquid) is discussed, too. Our numerical solution is based on the MOL method where space discretization leads to the corresponding system of ODE. To collect the measurement data for inverse problems we consider both the gravitational and centrifugal infiltration driven forces. In series of numerical experiments we demonstrate the effectiveness of our method.

Abstract: Gas carburizing is a widely used heat treatment process in which carbon is transferred into steel. The hardening reliability involves an active control of mass transfer during the process and this is why understanding diffusion in solids is so essential to model the process. The currently used models are often based on the simplest, one-dimensional form of the diffusion equation in which diffusivity depends on composition. The objective of this work is to develop a model of carbon diffusion in multicomponent alloy subjected to pulse carburizing. The model is based on the Darken method (bi-velocity method) in which the diffusion velocity depends on the diffusion potential gradient and is independent of the choice of the reference frame while the drift velocity is common for the carbon and steel components. Our model allows predicting the kinetics of carbon transfer at various treatment conditions and is applied to the pulse carburizing process at constant temperature. The process is carried out by repeating consecutively a carburization stage, when the carburizing gas is supplied into a carburizing chamber, and a diffusion stage at vacuum conditions, when the carburizing gas is exhausted and only the diffusion of carbon takes place. The numerical calculations are made for varying carburization and diffusion periods and are confirmed by the experimental results. On the basis of the series of computer experiments some findings that are important in designing the carburizing technology are formulated.

Abstract: The results of measurements of the bulk diffusion of tracer oxygen atoms in the oxides LnMnO_3+δ (Ln = La, Nd, Sm) in the temperature range 400 – 750°С are presented. The measurements were carried out on micro-and nanopowders. Nanoscale powders were prepared by mechanical activation. A method based on the study of the kinetics of oxygen isotope exchange between the powder and gaseous oxygen enriched with ¹⁸O isotope was used to obtain data on the diffusion coefficients. The average concentration of ¹⁸O isotope in the powders was measured using NRA technique. The obtained diffusion coefficients lay in the range of 10^-21 - 10^-24 m²/s, the diffusion activation energy for all the oxides have been close to 1 eV. These results suggest that the migration of tracer oxygen in oxides LаMnO_3+δ, NdMnO_3+δ and SmMnO_3+δ at low temperatures is realized via structural defects. As for the oxide LaMnO_3+δ, its oxygen diffusion coefficients at low temperatures have been lower than the values extrapolated from high temperatures. Such behavior of diffusion properties has not been previously observed in other metal oxides. In this regard, the vacancy formation energy in the rare earth manganites has been supposed to increase with decreasing temperature.

Abstract: Because of their low elasticity modulus, titanium alloys have excellent biocompatibility, and are largely used in orthopedic prostheses. Among the properties that are beneficial for use in orthopedic implants is the elasticity modulus, which is closely connected to the crystal structure of the material. Interstitial elements, such as oxygen, change the mechanical properties of the material. Anelastic spectroscopy measurements are a powerful tool for the study of the interaction of these elements with the metallic matrix and substitutional solutes, providing information on the diffusion and concentration of interstitial elements. In this study, the effect of oxygen on the anelastic properties of alloys in the Ti-15Mo-Zr system was analyzed using anelastic spectroscopy measurements. The diffusion coefficients, pre-exponential factors, and activation energies of these alloys were calculated for oxygen.

Abstract: This study investigated actively brazing Alumina-to-Alumina with Ag-Cu-Ti as the filler metal system and Alumina-to-Copper with Cu-Ti-Co as the filler-metal system. The research was conducted on four samples, two of which were alumina brazed to alumina (Samples 1 & 2), and the other two were alumina brazed to copper (Samples 3 & 4). The filler metal composition for each sample was as follows: Sample 1 consisted of Cu-96%, and Ti-4%; Sample 2 consisted of Ag-70%, Cu-26%, and Ti-4%; Sample 3 consisted of Cu-85%, Ti-10%, and Co-5%; and Sample 4 consisted of Cu-55%, Ti-40%, and Co-5%. The phase transformations between the filler and base metal of each brazed joint were studied using EDS, SEM, optical microscopy, and X-ray diffraction.

Abstract: The purpose of this study is to characterize the effects of heat damage on the electrical conductivity and static mechanical properties of aluminum alloys. The data resulting from the experiments of thermal exposure of several aluminum alloys are used to model the relations that describe the dependence of the electrical conductivity and hardness on the two main variables of these experiments: the temperature and the time of exposure. The dependence of yield strength and ultimate tensile strength on hardness values is characterized. For each case, different materials (alloys) exhibit similar general trends although there are different coefficients for each material to satisfy the general relation.