SELF-DIFFUSION IN HIGH-DENSITY NANOCRYSTALLINE Fe

In order to gain insight into the characteristic behavior of grain boundaries in

nanocrystalline (n-) materials, high-density n-Fe specimens are prepared by compaction of gas-condensed nanocrystallites at elevated temperatures and the self-diffusion coefficients are measured by radiotracer techniques. The self-diffusion coefficients of n-Fe (relative density higher than 91 %) determined by assuming type-C kinetics are similar to those extrapolated from high temperature data of conventional grain boundaries, suggesting that the grain boundaries in the high-density n-Fe are similar to those in conventional polycrystalline Fe.

Nanocrystalline (n-) materials are expected to exhibit properties modified with respect to conventional polycrystals owing to the much increased volume fraction of grain boundary (GB) regions (1). Especially, the enhanced atomic diffusion in n-materials due to the increased GBs is a key feature not only for understanding the macroscopic properties such as the plasticity and magnetism but also for the study of the interfacial structure. A recent Fe diffusion study in high-density n-Pd shows that the interface diffusivity is similar to that expected for GB diffusion of conventional polycrystalline (poly-) metals from extrapolation of high-temperature

data to lower temperatures (2). To gain insight into the diffusion phenomena as well as the interface structure in n-metals with cubic body-centered structure, a tracer-diffusion study is performed on n-Fe. Since porosity may greatly affect the apparent diffusion phenomena (3), we prepared high-density n-Fe specimens with density higher than 91 % of the poly-Fe value.