Papers by Author: Boris S. Bokstein

Abstract: Three cross diffusion-stresses effects are considered: mobility-stress effect, flux-stress effect and vacancy-stress effect. The value of the migration volume for vacancies in Al is found from atomistic computer simulation. A cross vacancy-stress effect is applied to the process of the pores growth and dissolution in Ni-based superalloys.

Abstract: The data are presented for Ni selfdiffusion and Au heterodiffusion in nanocrystalline Ni. Volume diffusion coefficients are much greater than those for a coarse – grained polycrystals extrapolated from high temperatures. Interface diffusion parameters were calculated based on the assumption that B – kinetic regime is realized at temperature range more than 448 K, while C – kinetic regime is realised at temperatures less than 423 K. The consistency of obtained results with the proposed cluster diffusion model is discussed. Diffusion in Au – Cu thin films (from several tens to several hundreds nanometers) was studied with the use of the Rutherford Back Scattering, RBS, under the kinetic regime B (448 – 523 K). The RBS spectra were transformed in the concentration depth profiles for both volume and grain boundary (GB) diffusion. The triple products Pn = snδDn (sn is the enrichment coefficient, while δ is the nanograin boundary width) were calculated using Whipple model. As a result of this analysis the s – value for Cu – Au system was determined to be of the order of unity. The paper is focused on a difference between GB diffusion parameters in nano – and coarse grained materials.

Abstract: The effect of grain boundary segregation (GBS) on grain boundary diffusion (GBD) is analyzed in frame of the new model. This model supposes the formation of the specific “nuclei” in a grain boundary (GB) solid solution. Their composition is close to that of the nearest phase in grain in equilibrium with solid solution in grain. These GB “nuclei” form after the same solubility as in the bulk is reached. The size of these “nuclei” is close to an atomic size and consequently the new interfaces inside the two-dimensional GB are not formed. As the solute concentration in the bulk increases the solute concentration in GB increases as well, but only due to the increase of the “nuclei” fraction. At the same time the solute concentration in a disordered part of GB solid solution remains constant. The fraction of this part decreases. The retardation effect of GBS on GBD is connected with the ordering transition, the “nuclei” formation. A diffusion coefficient (D) in an ordered part of solid solution is close to the diffusion coefficient in a bulk phase. As a rule, it is less than a diffusion coefficient in a disordered part. The growth of an ordered part of GB solid solution (the fraction of the “nuclei”) leads also to the dependence of D on the solute concentration and to the additional curvature of the diffusion profiles.

Abstract: Thermodynamic model is developed of which it follows that saturation value of solute segregated in grain boundary, provided all available sites occupied, must be close to the solute concentration, in the nearest in composition phase in grain as it follows from phase diagram. Based on the analysis of experimental data for three binary systems (Cu –Sb, Fe – P and Fe – Sn) it is shown that the surplus concentration of the solute atoms in GB as compared with a concentration in grain is distributed equally between two fracture surfaces but takes up more than one atomic layer. The thickness of the segregation region (with surplus concentration in grain boundary) averages usually from 3 to 6 interatomic distances. As a rule, solute distribution is symmetrical on each side of the fracture surface. It is shown that maximum of grain boundary concentration falls on the first layer on the fracture surface. The method is developed of the calculation of the first layer concentration. It is shown that this concentration is close to the solute concentration in the nearest in composition phase in grain.