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

金属-氢系统的一般性质之一是形成过剩空位，在某些情况下原子扩散增强许多数量级。这种现象被广泛研究的Pd，Cu，Ni。然而，对Fe的实验研究很少。本论文的主要目的是利用X射线的核共振前向散射（NRFS）技术结合高压技术来研究氢原子在高压下的扩散机制。实验研究了室温至1000℃、氢压至2. 5 MPa、4μm厚的57 Fe箔中原子的扩散行为。实验数据的详细分析仍在进行中。以下是本研究项目的初步结论。(1)利用超精细态的塞曼分裂测量了铁箔的内磁场。在氢气压力为2. 8 GPa时，在640℃以上内场消失。结果表明，吸氢降低了α-γ结构相变温度。(2)在960℃时，观察到核共振前向散射时间谱发生了明显的变化。我们认为，这种突变是由于大量吸氢的铁箔熔点降低所致。(3)在室温和1000℃，氢压为2。8GPa。这种氢效应不存在的主要原因是空位扩散机制被更快的扩散过程如表面扩散或亚表面扩散所超越。说明了表面现象在多晶材料中原子输运中的重要性。

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.