Boris S. Bokstein

Molecular dynamics study of self-diffusion in Zr

We employed a recently developed semi-empirical Zr potential to determine the diffusivities in hcp and bcc Zr via molecular dynamics simulation. The point defect concentration was determined directly from molecular dynamics (MD) simulation rather than from theoretical methods using T = 0 calculations. Our MD simulation indicates that the diffusion proceeds via the interstitial mechanism in hcp Zr, and both vacancy and interstitial mechanisms contribute to diffusivity in bcc Zr. The agreement with the experimental data is excellent for hcp Zr and rather good for bcc Zr at high temperatures, but there is considerable disagreement at low temperatures.

Recent Advances and Unsolved Problems of Grain Boundary Diffusion

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.

Grain boundary diffusion in thin films under stress fields

Some experimental data concerning stress effect on grain boundary (GB) diffusion and models describing the GB diffusion (GBD) in thin polycrystalline films for the B- and C-regimes under a stress field are reviewed. Numerical solutions for the conditions of reflecting free surface and rapid surface diffusion on the sink surface for the case of a constant source are obtained. The results of the model calculations are compared with the results of the investigation of Cu GBD in Ni thin films and with experimental data from the literature. The comparative influence of the GBD atomic mechanisms on the shape of GBD penetration plots is discussed.

Cross Diffusion-Stresses Effects

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.

Connection between Fe grain boundary segregation in Al and phase formation in the bulk

Abstract As it was done earlier for the systems Cu–Sb and Fe–P (metal–nonmetal), estimations show for the intermetallic Fe–Al system that the chemical composition of the grain boundary “embryos” is very close to the composition of the nearest chemical compound in the grain according to the phase diagram.

Concentration Profiles for Grain Boundary Diffusion in B-Regime with Regard to the Formation of Atomic Complexes in Grain Boundary

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.

Diffusion at Short Circuits: State of the Art

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.

Thermodynamics of grain boundary adsorption in binary systems with limited solubility

Abstract A thermodynamic analysis of grain boundary adsorption is presented. From the thermodynamic model developed it follows that in binary polycrystals with identical grain boundaries at some conditions three phases can coexist in equilibrium below the solidus temperature. The model predicts that two solid solutions in the grain and in the grain boundaries coexist below some critical value, X∗2, of solute concentration in the grain. At X∗2, a compositional phase transition occurs in the GB which is accompanied by a gap in the adsorption isotherm, a change of the slope of the GB tension isotherm, and the formation of a new 2D grain boundary phase. It is shown for three systems (Cu–Sb, Fe–P, and Sn–Bi) that the compositions of the GB phase and the phase in the grain are in close agreement with the predictions.

Grain Boundary Diffusion, Stresses and Segregation

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.

Computer Simulation of Diffusion in Dilute Al-Fe Alloys

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.