Alexey O. Rodin

A new model of grain-boundary segregation with the formation of atomic complexes in a grain boundary

The process of grain-boundary segregation (GBS) has been considered under an assumption that the formation of associates (atomic complexes with a composition that corresponds to the nearest chemical composition in the phase diagram) is possible in a grain boundary (GB). The grain boundary is considered as a two-component mixture of A and B atoms, which can exist both in free and bound states (bound in a complex). The formation of complexes with an arbitrary composition and complexes of an AB type have been considered. It has been shown that, even in the absence of segregation (b = 1), the interactions of atoms in a GB that induce the formation of complexes leads to an enrichment of GBs in impurity atoms. It has been demonstrated that under certain conditions, a GBS isotherm can exhibit saturation corresponding to the chemical composition of the complex.

Interdiffusion in binary Ni-Re alloys

Processes of interdiffusion have been studied in the Ni-Re system. The method of electron-microprobe analysis was used to measure the interdiffusion coefficients D̃ in Ni4.4Re/Ni, Ni7Re/Ni Ni9Re/Ni, and Ni9Re/Ni4.4Re (wt %) diffusion pairs in the temperature range of 1050–1350°C. The averaged values of the preexponential factor and the activation energy for interdiffusion were found to be D0 = 1.16 × 10−4 m2/s and Q = 317 kJ/mol, respectively. The value D0 ∼ 10−4 m2/s is typical of substitutional alloying elements in fcc metals; the value of Q is determined to be close to that for W and is significantly higher than those that correspond to other alloying elements in nickel superalloys. No clearly pronounced concentration dependence of the coefficients of interdiffusion has been revealed at a rhenium concentration of 4.4–9 wt %.

Diffusion of copper along the grain boundaries in aluminum

The grain boundary diffusion (GBD) of copper in aluminum is investigated in the range t = 300−400°C. Investigations were performed on a scanning electron microscope equipped with an attachment for electron probe X-ray microanalysis. The triple product sδDgb (where s is the segregation coefficient, δ is the width of the grain boundary, and Dgb is the GBD coefficient) was calculated by the Fisher criterion using two methods (namely, the copper concentration in the grain boundary, depending on the penetration depth, was determined and the angles in the vertex of the concentration profile was measured using an optical microscope). In the first case, sδDgb was 5.1 × 10−11 exp(−102/(RT)) m3/s; in the second case it was 1.4 × 10−11 exp(−94/(RT)) m3/s. The obtained results are compared with innumerous literature data.

Diffusion processes in multicomponent nickel-base superalloy-nickel system

Optical and scanning electron microscopy, as well as electron microprobe analysis and electron backscatter diffraction, have been used to study diffusion processes that occur in a diffusion pair that consistsof a single-crystal CMSX-10 nickel-base superalloy and polycrystalline nickel, at temperatures of 1050–1250°C. It has been found that, in this system, the distributions of γ-stabilizing elements (Cr, Co, W, and Re) are described by the Boltzmann solution for diffusion between two semiinfinite plates of a binary alloy. The processing of these distributions has shown that the diffusion coefficients of Cr, Co, W, and Re in the multicomponent system are close to those in binary alloys of these elements with Ni. The diffusion redistribution of the elements leads to the dissolution of the γ′ phase in the nickel-base superalloy, growth of nickel grains toward the superalloy constituent of the diffusion pair, and the formation of porosity on both sides of the migrating interface, which is determined from a crystal misorientation of the alloy single crystal and nickel grains.

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.

Mössbauer investigation of Fe segregation in Al

Abstract Investigations of Fe segregation and competitive Fe–Cu segregation at Al grain boundaries was performed using Mossbauer spectroscopy. An additional component of the Mossbauer spectra was discovered. It was identified as corresponding to Fe atoms segregated at the grain boundaries (GB) of Al. It was shown that Cu atoms drive out Fe atoms from GB if the Cu concentration is several times more than Fe one.

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.

Pseudopartial Grain Boundary Wetting: Key to the Thin Intergranular Layers

The thin layers of a second phase (also called complexions) in grain boundaries (GB) and triple junctions (TJs) are more and more frequently observed in polycrystals. The prewetting (or premelting) phase transitions were the first phenomena proposed to explain their existence. The deficit of the wetting phase in case of complete wetting can also lead to the formation of thin GB and TJ phases. However, only the phenomenon of pseudopartial (or pseudoincomplete, or constrained complete) wetting permitted to explain, how the thin GB film can exist in the equilibrium with GB lenses of a second phase with non-zero contact angle.

About Fe Diffusion in Cu

The temperature dependence of the bulk diffusion coefficient of Fe in Cu is determined by EDX in the temperature range from 923 to 1273 K, , m2/s. These results are different from that obtained earlier by radiotracer technique: activation energy is less by 30 kJ/mol and pre-exponential factor is 50 times smaller. Deviations from ideality of investigated solutions do not explain the differences; consequently, the thermodynamical factor would not responsible for such an effect. Fast grain boundary diffusion of Fe in Cu was not observed in the temperature range from 823 to 1073 K.

Liquid Ga Penetration along Al Grain Boundaries: Effect of External Stress and Ga Undercooling

The effects of compressive stress and undercooling of Ga to liquid gallium penetration along grain boundaries (GBs) of aluminum were investigated. It was shown that the penetration rate does not change with the temperature if gallium is in liquid state. The effect of compressive stress applied to Al samples was demonstrated. The time to wetting of all aluminum GBs increased several orders of magnitude if the compressive stress was between 0.1 and 6 MPa. It was proved that solid-liquid transformation does not take place during penetration process.