Self-diffusion was systematically investigated in well-compacted nanocrystalline (80 to 100nm) γ-Fe-40wt%Ni material at 600 to 1010K in all of the 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 diffusion flux 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 59Fe diffusion profiles with a Monte Carlo simulation of grain boundary diffusion. Due to the sample preparation process the grain boundary motion during the diffusion anneal was proved to be negligible. For the first time, it was shown that there existed an intermediate stage between the well-known kinetic regimes B and A if √Dvt ≃ d, where Dv was the bulk diffusivity and t was the time. The corresponding concentration profiles could be linearized in the coordinates of 1n c̄ versus y3/2 (c̄ was the layer tracer concentration and y was the penetration depth) and the equation to extract the grain boundary 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 grain boundary diffusion profiles in a nanocrystalline material could be done properly but was more sophisticated than in a coarse-grained material.

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. S.V.Divinski, F.Hisker, Y.S.Kang, J.S.Lee, C.Herzig: Zeitschrift fur Metallkunde, 2002, 93[4], 256-64