Papers by Author: Sergiy V. Divinski

Abstract: Tracer diffusion is one of most reliable techniques for providing basic kinetic data in solids. In the present review, selected direct methods, in particular the radiotracer measurements as a superior technique due to its high sensitivity, Secondary-Ion-Mass-Spectroscopy (SIMS) profiling, X-Ray Diffraction measurements and Rutherford Backscattering Spectrometry are presented and discussed. Special attention is put on the radiotracer technique describing the currently used sectioning techniques in detail with a focus on the experimental applications and complications. The relevant experimental results are exemplary shown. Furthermore, the most recent developments and advances related to the combined tracer/inter-diffusion measurements are highlighted. It is shown that this approach offers possibilities to provide the concentration-dependent tracer diffusion coefficients of the constituting elements in multi-component alloys in high-throughput experiments. Possibilities of estimating the tracer diffusion coefficients following different types of diffusion couple methods in binary and multicomponent systems are briefly introduced. Finally, specificity of SIMS analysis of diffusion in fine-grained materials are carefully analyzed. If applicable, a direct comparison of the results obtained by different techniques is given.

Abstract: The impact of a pulsed magnetic field on Sn diffusion in α-Fe is investigated within the magnetic field intensity range of 79.6–398.0 kА/m and the frequency range of 1–21 Hz at 730, 740, and 750 °С. The solute diffusion coefficients were measured by X-ray diffraction analysis. The pulsed magnetic field was found to influence significantly the diffusion coefficients of Sn in α-Fe. A resonant maximum of the bulk diffusion coefficient is observed at the frequency of 13 Hz and the magnetic field intensity of 398.0 kА/m. Possible mechanisms of the established magneto-diffusion coupling are discussed.

Abstract: Impact of a constant magnetic field on decomposition of supersaturated solid solution is investigated for the system Cu-Be-Ni. A technical bronze Cu-1.9Be-0.3Ni (in wt.%) was water-quenched after holding at 800°C (0.5 h) and subsequently heat treated at 325°C, 350°C and 400°C for 1 hour without and with application of a constant magnetic field of 0.7 T. The annealing in magnetic field is found to influence significantly the precipitation characteristics in diamagnetic Cu-based alloy, especially at 325°C. The nucleation barriers for discontinuous precipitation at grain boundaries are decreased, while the growth rates seem to be decreased, too, in magnetic field. A possible mechanism of the magnetic effect on discontinuous precipitation in the Cu-based is discussed.

Abstract: Diffusion couple technique is an efficient tool for the estimating the chemical diffusion coefficients. Typical experimental uncertainties of the composition profile measurements complicate a correct determination of the interdiffusion coefficients via the standard Boltzmann-Matano, Sauer-Freise or the den Broeder methods, especially for systems with a strong compositional dependence of the interdiffusion coefficient. A new approach for reliable fitting of the experimental profiles with an improved behavior at both ends of the diffusion couple is proposed and tested against the experimental data on chemical diffusion in the system Fe-Ga. An extension of the approach for reliable description of the up-hill diffusion phenomenon in multicomponent systems is presented.

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.

Abstract: The radiotracer technique was used to measure the grain boundary diffusion of ⁴⁴Ti and ⁶³Ni in slightly Ni-rich polycrystalline NiTi compound in the temperature range of 673 - 923 K. The temperature dependence of the grain boundary triple product P (P = sδD_gb, s is the segregation coefficient, δ is the grain boundary width, and D_gb is the grain boundary diffusion coefficient) for Ti and Ni was determined. The triple products of both Ti and Ni grain boundary diffusion in NiTi reveal a unique behavior with significant deviations from an Arrhenius-type dependence. Probable evolution of the grain boundary structure with temperature was used to interpret this phenomenon.

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: Disks of copper samples were produced by High Pressure Torsion (HPT). Specimens for tensile creep experiments were cut from the disks and subjected to creep deformation at 348 K to obtain elongations greater than 30%. Electron backscatter diffraction (EBSD) was used to analyze the texture after HPT deformation and after additional tensile elongation.

Abstract: Copper of different purity levels (4N, 5N) produced by High Pressure Torsion (HPT) with varying processing parameters is investigated utilizing the radiotracer technique. While the degree of deformation is constant, the effect of the applied quasi-hydrostatic pressure and of the impurity concentration on the as deformed samples is analysed. By applying the radio tracer method micro structural aspects are revealed that are not easily accessible by conventional methods. The measurements indicate the formation of a percolating porosity during the HPT process as a function of the applied pressure and (although less pronounced) of the impurity concentration.

Abstract: Some unresolved problems of grain boundary diffusion – restrictions of Fisher-Gibbs model, refinement of the conditions for B- and C-regimes, relation between segregation (s) and enrichment (b) coefficients, grain boundary width, non-linear segregation effects on grain boundary diffusion – are discussed.