59Fe grain boundary diffusion in nanostructured γ-Fe-Ni -: Part II:: Effect of bimodal microstructure on diffusion behavior in type-C kinetic regime

For the first time, the grain boundary diffusion was investigated in a nanocrystalline material in the whole spectrum of possible kinetic regimes: types-C, B, AB, and A. The microstructure of the nanocrystalline gamma-Fe-40 wt.% Ni alloy revealed two different levels of organization: the individual nano-scaled grains build up micrometer-large clusters or agglomerates of the grains. The existence of the two types of internal interfaces in the material - the boundaries between the nano-grains and between the agglomerates of these nanocrystallites (the inter- and intraagglomerate boundaries) - affected notably the C-regime of Fe-59 radiotracer diffusion and resulted in two-stage diffusion penetration profiles. The careful profile processing according to formal criteria of the given kinetic regimes allowed, for the first time, to elaborate a consistent picture of the diffusion behavior in this nanocrystalline material with a complex bimodal distribution of equilibrium and nonequilibrium internal interfaces in the same material. The absolute values of the measured diffusion coefficients and the Arrhenius parameters of Fe diffusion along the nanocrystalline boundaries in nano-gamma-Fe-40 wt.% Ni (D-0 = 4.2 x 10(-3) m(2)s(-1), Q = 187 kJ mol(-1)) are very similar to the Fe grain boundary diffusivity in coarse-grained gamma-Fe. The activation enthalpy of diffusion along the inter-agglomerate boundaries (D-0 = 3.4 x 10(-3) m(2)s(-1), Q = 148 kJ mol(-1)) is notably smaller and the absolute diffusivity is larger by two to three orders of magnitude than the corresponding values of Fe diffusion via the nanoboundaries. The observed features are explained by a quasi-equilibrium structure of the nanocrystalline GBs, related to the grain growth during sample preparation, whereas the residual interagglomerate boundaries remained in nonequilibrium state. The potential contribution of the numerous triple junctions in the nano-material to short-circuit diffusion was investigated in detail and was found to be negligible under the present experimental conditions.