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

金属-氢体系的一般性质之一是形成超丰富的空位，在某些情况下原子扩散增强数个数量级。这种现象在Pd， Cu， Ni中得到了广泛的研究。然而，对铁的实验研究却很少。我们的主要研究目的是利用x射线的核共振前向散射（NRFS）与高压技术相结合来阐明高氢压力下原子的扩散机制。实验研究了4μm厚57Fe箔在温度为RT ~ 1000℃、氢气压力为2的条件下的原子扩散。8 gpa。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)采用超细态塞曼分裂法测量了铁箔的内部磁场。氢的压强是2。在640℃以上，内部场消失。结果表明，吸氢可以降低α-γ结构相变温度。(2)在960℃温度下，核共振正向x射线散射的时间谱发生了显著变化。我们认为这种突变是由于吸收氢丰富的铁箔熔点降低所致。(3)在本实验条件下，从室温到1000℃，氢气压力为2，没有观察到吸氢对原子扩散的增强作用。8 gpa。氢效应缺失的主要原因是空位扩散机制被表面扩散或亚表面扩散等更快的扩散过程所取代。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.