Ag diffusion and interface segregation in nanocrystalline γ-FeNi alloy with a two-scale microstructure

Solute diffusion of Ag in nanocrystalline gamma-Fe-40wt%Ni alloy was studied by means of the radiotracer technique in an extended temperature interval (489-1200 K). The powder metallurgical method was applied to produce nanomaterial which consisted of micrometer-large clusters (agglomerates) of nanometer sized grains. Two types of internal interfaces contributed as short-circuit paths for diffusion: the nanocrystalline grain boundaries (GB) and the inter-agglomerate interfaces (subscript a). Combining the recent results on Ag GB diffusion in coarse-grained gamma-Fe-40wt%Ni alloy and the present diffusion data in the nanocrystalline alloy the Ag segregation was determined as function of temperature. Ag segregates strongly at GBs in the gamma-Fe-40wt%Ni alloy with a segregation enthalpy of H-s = -47 kJ/mol. Knowing the segregation factor, the experimental data on Ag diffusion along both nanocrystalline and inter-agglomerate interfaces in the nanomaterial were systematically analyzed in dependence on the different kinetic regimes. The sensitive radiotracer experiments and the subsequent diffusion profile analysis resulted in a consistent set of diffusion data in the whole investigated temperature range with Arrhenius behavior for both the Ag nano-GB diffusion (D-0(gb) = 4.7 x 10(-4) m(2)/s, H-gb = 173 kJ/mol) as well as for the much faster inter-agglomerate interface diffusion (D-0(a) = 8.1 x 10(-5) m(2)/s, H-a = 91 kJ/mol). (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.