Grain-boundary diffusion of Fe in nanocluster-strengthened ferritic steel (Fe–14Cr–3W–0.4Ti–0.25wt%Y2O3) was investigated. The steel was produced by mechanical alloying followed by hot extrusion. The final grain size was about 200nm. The diffusivity of Fe was measured at 423 to 820K. The grain boundary penetration at lower temperatures revealed a specific time dependence, which indicates a residual interconnected porosity in the ferritic steel. In order to quantify the percolating porosity, conventional radiotracer (59Fe) diffusion measurements were combined with a study of room temperature penetration of liquid 110mAg solution to distinguish between solid-state diffusion along boundaries and penetration along the surface of interconnected cavities. The presence of porosity affected the diffusion process, introducing a hierarchy of internal interfaces. The grain boundary diffusion coefficient and the diffusivity along internal surfaces were determined in the so-called type C–C, C–B and B–B kinetic regimes of interface diffusion in a hierarchical microstructure. Using the residual activity method and a 65Zn tracer, the volume fraction of the percolating porosity was estimated to be 0.6%.

Grain Boundary Diffusion of Fe in Ultrafine-Grained Nanocluster-Strengthened Ferritic Steel. R.Singh, J.H.Schneibel, S.Divinski, G.Wilde: Acta Materialia, 2011, 59[4], 1346-53