Self-Diffusion Behavior of Nanocrystalline γ-FeNi

Abstract

59Fe and 63Ni radiotracer diffusion was measured in well-compacted and sintered nanocrystalline γ-Fe-40 wt.% Ni alloy by the serial sectioning technique in an extended temperature range from about 600 to 1000K covering Harrison's A, B, and C diffusion regimes. Because the microstructure of the nano-material under investigation revealed two different levels of organization - the individual nano-scale grains were organized in micrometer-size agglomerates - two types of internal interface exist, namely boundaries between the nano-grains and between the agglomerates of these nanocrystallites. A systematics of grain boundary (GB) diffusion regimes in this two-scale structure was elaborated. The absolute values and the Arrhenius parameters of Fe and Ni diffusivity in coarse-grained polycrystalline γ-Fe. The activation enthalpy of diffusion along the inter-agglomerate boundaries is remarkably smaller and the absolute diffusivities lager by two to three orders of magnitude than the corresponding nano-GB diffusion values. The existence of both equilibrium and non-equilibrium internal interfaces is a common feature of such nanocrystalline materials and it has to be taken into account when comparing diffusion data of different studies.