Study on atomic diffusion process promoted by hydrogen absorption measured by time domain analysis of nuclear resonant X-ray scattering

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One of the general properties of Metal-Hydrogen system is formation of superabundant vacancies and enhancement of atomic diffusion by many orders of magnitude in some cases. Such phenomena are extensively investigated for Pd, Cu, Ni. However, experimental investigation for Fe is very few. Main object of our research is to elucidate atom diffusion mechanisms under high hydrogen pressures by using nuclear resonant forward scattering(NRFS) of X-rays combined with high pressure techniques. In the experiment、atomic diffusion in 4μm thick 57Fe foil was investigated for temperatures between RT and 1000℃under hydrogen pressures up to 2. 8GPa.Detailed analysis of experimental data is still under way. The followings are the tentative conclusions of this research project.(1) Internal magnetic field of iron foil was measured by Zeeman splitting of hyperfine states. At hydrogen pressure of 2. 8GPa, the internal field disappeared above 640℃. It was concluded that it demonstrated the lowering ofα-γstructural phase transition temperature by hydrogen absorption.(2) It was observe at temperature 960℃that significant change in time spectrum of nuclear resonant forward X-ray scattering took place. We concluded that this abrupt change was due to the lowering of melting point of Fe foil with abundant hydrogen absorption.(3) No enhancement of atomic diffusion by hydrogen absorption was observed for the present experimental condition : Temperatures from RT and 1000℃and hydrogen pressure 2. 8GPa. Main reason for this absence of hydrogen effect is vacancy diffusion mechanism was surpassed by much faster diffusion process such as surface diffusion or subsurface diffusion. It illustrates the importance of surface phenomena in atomic transportation in polycrystalline materials.