Papers by Author: V.V. Bogdanov

Abstract: The kinetics of intermetallic growth in the bulk, along the surface and grain boundaries (GBs) was studied in the Cd-Ni, Cd-Cu and Cu-Sn systems. Bulk dynamic diffusion coefficients exceed by a few orders of magnitudes the tracer self-diffusion coefficients in homogeneous phases. The reasons for this difference are discussed in terms of departure of growing intermetallics from stoichiometry due to the simultaneous existence of two processes: interdiffusion through the growing phase layer and chemical reactions at interfaces. Accelerating contribution of GBs and free surfaces, as fast diffusion paths, into diffusion penetrability of growing intermetallics has been investigated. It was found that the rates of lateral phase spreading along free surfaces and GBs exceed several times the rate of phase growth in the bulk. Accelerating GB contribution depends on the grain size. Nano-dispersed thin films demonstrate maximal phase propagation rates, which several times exceed even the rates of lateral phase spreading along free surfaces in coarse-grained polycrystals.

Abstract: The kinetics of lateral Cu6Sn5 and Cu3Sn phase propagation induced by grain boundary (GB) interdiffuson in thin-film diffusion couples Cu-Sn were studied in a temperature range 160-180oC by optical microscopy, AFM, SEM, and energy-dispersive X-ray spectroscopy (EDS). Nano-grained Cu and Sn films were sequentially deposited on glass substrates with 5 – 20 µm overlap. To prevent surface diffusion and thus separate GB-diffusion contribution into kinetics of phase propagation, the surfaces of diffusion couples were covered by a thin (20 – 40 nm) carbon layer. It was found that the rates of lateral Cu6Sn5 and Cu3Sn phase spreading in thin-film couples exceed several times the spreading rates of the same phases over the surface of coarse-grained samples and 50 – 70 times exceed the rates in the bulk of massive samples. Kinetics of lateral phase spreading both in thin-film and in massive diffusion couples obeys parabolic law. Similarly to A and B regimes for GB tracer diffusion, A and B regimes of GB reactive diffusion were found in the spreading Cu3Sn phase. The kinetics of the phase propagation turned out independent of the film thickness (in the range 40 – 200 nm) if the films possessed similar grain size, whereas the kinetics was rather sensitive to the grain size and GB structure. Theoretical analysis of the phase propagation kinetics accelerated by GB diffusion has been done and the phase propagation rates have been calculated. By comparison experimentally measured phase propagation rates with the calculated ones we determined the GB diffusion coefficients of Sn in both growing phases.

Abstract: The kinetics of growth and lateral spreading of intermetallic layers during surface interdiffusion in Cu – Sn system has been studied in a temperature range 160 – 200oC by the methods of optical microscopy, SEM provided with X-ray microprobe, and AFM. Lateral phase spreading over the surface is characterized by competition between two phases: Cu6Sn5 and Cu3Sn. A steady state solution for concentration distribution on the surface of growing intermetallic phases, as well as kinetic equations of lateral spreading of growing phase layers have been obtained. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the dynamic surface diffusion coefficients have been calculated.

Abstract: Stress relaxation processes accompanying intermetallic growth during reactive diffusion between Cd and Ni have been studied by the methods of optical and scanning electron microscopy, provided with X-ray microanalysis. The experiments were carried out with the two-layer Cd-Ni samples at 250 and 280oC under hydrostatic pressures 350-900 MPa. The observed processes have been compared with those occurred at low pressures to demonstrate that the mechanisms of stress relaxation and thus the kinetics of intermetallic growth essentially depend on applied hydrostatic pressure. New mechanisms of stress relaxation were found, such as Cd extrusion and Cd whisker growth, which accompanied formation of Cd21Ni5 compound. It is shown, that the whisker growth is more probable at lower temperatures when the grain size is smaller, and the stress gradient, which support a driving force for whisker formation, is higher. An atomic mechanism for whisker growth, based on diffusion climbing of dislocation loops produced by Bardeen-Herring source for building new atomic layers, has been discussed.

Abstract: A method of the determining of the mutual diffusion coefficients in ternary intermediate phases is suggested. Methodology for determination of the interdiffusion coefficients of phases formed in a ternary diffusion couple in T-sample configuration is presented. The method requires only one annealing of the T-sample and makes it possible to calculate the matrix of the diffusion coefficients in the (CuNi)6Sn5 and Cu3Sn ternary intermediate phases under low temperature. The concentration profiles and electron micrograph of the diffusion zone was obtained in the ternary Cu-Ni-Sn system annealed for 64 hours under 200°C. Phase competition in this ternary system with special configuration of T-sample leads to lateral diffusion of Cu, to formation (CuNi)6Sn5 compound between Ni-Sn side and to suppression of other NixSny intermediate phases.

Abstract: Kinetics of phase growth in multiphase Cu-Sn and Ni-Sn systems and effect of third substitutive component (Ni or Cu respectively) were studied experimentally in temperature range 160-220oC and analyzed theoretically. Kinetic equations for calculation of diffusion coefficients in growing competitive phases were obtained. It was found that the substitutive components accelerated phase growth lowering the values of diffusion activation energy and pre-exponential factor. As a result the growth rates of competitive phases Cu6Sn5 and Cu3Sn in Cu-Sn system changed with substitutive Ni so, that only one (CuNi)6Sn5 phase survived whereas (CuNi)3Sn phase was suppressed. Obtained results are discussed in terms of higher disordering caused by substitutive components in ternary phases as compared with binary ones.

Abstract: The phenomenon of low-temperature homogenization (LTH) during interdiffusion is studied under condition a t Dv £ 2 / 1 ) ( (Dv is the bulk diffusion coefficient, a is the lattice parameter) using nano-objects of binary Cu-Ni and Cr-Ni systems compacted from nano-powders and produced by mechanical alloying. Two stages of LTH were detected: at the first stage (t £ 103 s) the volume fraction of solution rapidly grows; at the second stage (t > 103 s) the volume fraction of solutions grows slowly with practically constant average solution concentration. The first stage of LTH correlates with active grain growth caused by small size (l) of structural element and nonequilibrium structure of nano-objects. Obtained results are analyzed theoretically in terms of interdiffusion along migrating GBs due to grain growth at the first stage and DIGM mechanism at the second stage. It is shown that the GB concentration distribution during interdiffusion along migrating GBs and the kinetics of LTH depend on a parameter l/l where 2 / 1 ) / ( b b V sD d l= is the characteristic diffusion length. The mechanisms and criteria of LTH are proposed.