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

Abstract

Solute diffusion of Ag in nanocrystalline γ-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 γ-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 γ-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=4.7×10 −4 m 2/s, H gb=173 kJ/mol) as well as for the much faster inter-agglomerate interface diffusion ( D 0 a=8.1×10 −5 m 2/s, H a=91 kJ/mol).