Papers by Author: Boris S. Bokstein

Abstract: A short review of the most important reliable, as well as doubtful, in authors opinion points in a wide field of grain boundary diffusion, stresses and segregation.

Abstract: Grain boundary liquid grooving process takes place during the contact of solid metal phase with the metal melt. The liquid bismuth network formation along grain boundaries (GB) and triple junctions (TJ) was investigated in copper polycrystalline samples. The experimental observation in situ technique of Bi penetration through the Cu plate was used. Microscopic images of the liquid channels network were obtained. The temperature dependencies of GB and TJ effective penetration depths were determined. The effect of the GB and TJ diffusion on the liquid channels growth mechanism was discussed.

Abstract: The interaction between liquid and solid metals where the liquid-solid interface contains three grain boundary lines which meet in triple junction point is considered. The assumption that the liquid grooves may be formed not only along grain boundaries but along triple junctions is presented. The variation of Gibbs energy during the formation of triangle pyramidal groove along triple junction is determined. The dependence of Gibbs energy variation from groove dimensions shows that the wetting of triple junctions occurs by lower temperatures than the wetting of grain boundaries. This result allows to take into account the existence of grain size effect on the liquid phase penetration depth into the polycrystalline sample. The proposed mechanism of wetting in polycrystalline metal contains two stages: the outstrip melt penetration along triple junctions and the liquid grooving on grain boundaries forming the triple junctions. One of the processes – triple junction diffusion or liquid diffusion – may control the wetting in the polycrystalline sample.

Abstract: We performed molecular dynamics simulation of diffusion along symmetric <100> 5 and <111> 7 and one non-symmetric <100> 5 tilt grain boundaries in Al in the presence of Fe impurities. The simulation results are in reasonable agreement with available experimental data. The addition of Fe considerably decreases both Al and Fe diffusivities and increases the activation energy for diffusion. The simulation data indicate that the mechanism of diffusion is different in different grain boundaries. The diffusion along <100> 5 grain boundaries reminds that in liquid alloys.

Abstract: Thermodynamics of the atomic complexes formation in a grain boundary (GB) is discussed applying to AB complex. It is shown that the AB complex formation leads to the nonlinearity of segregation isothermes. Consequently, the concentration profiles for GB diffusion are nonlinear. The total (free and bound into complexes) concentration of diffusant B decreases at the given depth as the molar fraction of the complexes increases.

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.

Abstract: Evidence for solid-state diffusion (the second half of the 19th century). The first measurements of solid state diffusion (W. Roberts-Austen, 1896–1922). The first tracer experiments to determine the solid-state diffusion (G. von Hevesy, 1913–1923). The first evidence of accelerated diffusion in polycrystalline materials (1924–1935). Autoradiographic studies of grain boundary diffusion (50s of 20th century). The first quantitative experimental and theoretical studies of the “short circuiting” diffusion (beginning from 1949, D. Turnbull and R. Hoffman – General Electric Research Lab.): radiotracer serial sectioning method, the Fisher model (1951) for grain boundary diffusion, exact solutions and developments of the Fisher model (1954–1963). The progress in the experimental methods for determination of grain boundary diffusion data and results of measurements for different metallic systems (up to date). The measurements of grain boundary diffusion parameters in the B and C regimes. Grain boundary diffusion and grain boundary segregation. Nonlinear segregation effects. Structural effects of grain boundary diffusion. Diffusion in bicrystals. Diffusion in nanocrystals. Computer simulation of grain boundary diffusion. Mechanisms of grain boundary diffusion.

Abstract: During the solidification and the homogenization of single-crystal nickel-base superalloys, pores form which lead to the degradation of the mechanical properties, especially the fatigue lifetime. Hot Isostatic Pressing (HIP) is used for the pores dissolution. In this paper, analytical models for the formation of the homogenization pores and their dissolution during the HIP are developed. The results are compared with experimental data.

Abstract: We present results of the molecular dynamics study of Al selfdiffusion, Al and Fe diffusion in Al-Fe dilute alloys. We found that addition of Fe does not change the vacancy formation energy but considerably slows down Al diffusion. We also found that Al and Fe migration energies, i.e. energies of vacancy exchange with Al and Fe atoms, differ very strongly. Both activation energies for Al and Fe diffusion are in satisfactory agreement with available experimental data.

Abstract: Thermodynamic and kinetic models are developed for grain boundary segregation and diffusion with regards to the possible complexes formation in grain boundary. The equilibrium state for ideal solutions can be described by the equilibrium constants b and K. The first corresponds to the pure segregation, i.e. the exchange A and B atoms between grain boundary and the bulk. The second represents the equilibrium of the reaction of the complexes formation in grain boundary. The segregation isotherm and diffusion profiles are calculated. It is shown that both b and K equilibrium constants define completely dependence of the total grain boundary concentration of B atoms on the bulk concentration and distribution of B atoms between two states: free (pure exchange) and tied (in the complexes). Segregation in both forms (free B atoms and tied B atoms) decreases grain boundary diffusivity in comparison with the absence of segregation.