Grain-Boundary Diffusion and Segregation in the Sintered Nanocrystalline Materials with Hierarchical Structure

Abstract

Regularities of the diffusion of component atoms and atoms of highly segregated admixture in nanocrystalline material with a hierarchical structure are considered. Such a structure is characteristic for materials, fabricated from the milled powders of metal oxides by fritting, as a result of which the nanosized grains are grouped into the agglomerates of micrometre size. In such a type of the nanomaterials, there are two different-scale types of internal surfaces with different diffusion characteristics, namely, boundaries between nanograins and between their agglomerates, which are the elements of hierarchical material structure. The salient features of grain-boundary diffusion in such hierarchical structure are analysed. The kinetic regimes of diffusion are described, and presented equations allow determining the parameters of diffusion transfer over both types of separation surfaces on the basis of experimental data. The segregation behaviour of strongly-segregated admixture in the nanocrystalline γ-Fe–Ni alloy is studied. As shown, the rate of diffusion mass transfer along nanocrystalline boundaries in the material obtained by the sintering is practically similar to such one in a coarse-grained polycrystal of the same composition. Boundaries between agglomerates are nonequilibrium separation surfaces characterized by diffusion coefficients, which are high by orders of magnitudes the same for nanocrystalline boundaries.

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