Grain Boundary Diffusion and Segregation in Compacted and sintered Nanocrystalline Alloys

Abstract

An overview of self- (Fe, Ni) and solute (Ag) tracer diffusion in the well-compacted and sintered nanocrystalline (grain size d~100 nm) -Fe-40wt. % Ni alloy is presented. In the nanocrystalline material the individual nano-scaled grains turned out to be clustered in micrometersized agglomerates and two types of internal interfaces with different length scales and diffusion characteristics (the boundaries between the nano-grains and between the agglomerates of these nanocrystallites) act as different short-circuit diffusion paths. A systematics of grain boundary (GB) self- and solute diffusion in such bimodal structure is outlined. Well-known Harrison’s kinetic regimes of GB diffusion in a unimodal structure, i.e. the C, B, and A regimes are subdivided into the C.B, B.B, AB.B, and A.B regimes, which were observed experimentally. The theoretical framework to extract diffusivities of the nanocrystalline and inter-agglomerate boundaries from the multi-stage experimental profiles is presented. The combination of GB diffusion measurements of Ag solute diffusion in nano- and coarse-grained -FeNi alloys allowed to establish the segregation behavior of Ag. The absolute values and the Arrhenius parameters of Fe, Ni, and Ag diffusion along the nanocrystalline boundaries in the nano- -FeNi alloy are similar to the corresponding GB diffusivities in coarse-grained polycrystalline -FeNi. However, the activation enthalpy of diffusion along the inter-agglomerate boundaries turned out to be notably smaller and the absolute diffusivities larger by several orders of magnitude than the corresponding diffusion values via the nano-boundaries.