59Fe grain boundary diffusion in nanostructured γ-Fe-Ni -: Part I:: Radiotracer experiments and Monte-Carlo simulation in the type-A and B kinetic regimes

For the first time, self-diffusion was systematically investigated in well-compacted nanocrystalline (grain size d approximate to 80-100 nm) gamma-Fe-40 wt.% Ni material in a wide temperature range (600-1010 K) in all Harrison-type kinetic regimes. Samples were prepared by sintering the nanocrystalline Fe-Ni powder mixture produced by ball milling of the component oxides after reduction in hydrogen atmosphere. The samples revealed a frequently observed bimodal microstructure consisting of nano-scaled grains and micrometer-scaled agglomerates of the nano-grains. Two different types of short-circuit paths were found to control the diffusionflux in such material. Owing to the applied sensitive radiotracer technique Fe diffusion in both types of interface boundaries could be successfully characterized by combining the evaluation of the experimentally determined Fe-59 diffusion profiles with a Monte-Carlo simulation of grain boundary (GB) diffusion. Part I presents the results obtained at elevated temperatures in the type-B and A regimes. Due to the sample preparation process the GB motion during the diffusion anneal was proven to be negligible. For the first time, it was shown that there exists an intermediate stage between the well-known kinetic regimes B and A if rootD(v)t similar or equal to d, where D-v is the bulk diffusivity and t is the time. The corresponding concentration profiles could be linearized in the coordinates of ln (c) over bar vs. y(3/2) ((c) over bar is the layer tracer concentration and y is the penetration depth) and the equation to extract the GB diffusion coefficient from these data was derived. The limits of the new AB-type stage were established. It was demonstrated that the processing of the nonconventional experimental GB diffusion profiles in a nanocrystalline material can be done properly but is more sophisticated than in a coarse-grained material.