Effect of the crystal structure of the trapping effect of hydrogen in metals

为了阐明氢捕获效应对晶体结构的影响，采用内摩擦技术和氢扩散率和溶解度的电化学测量方法研究了Nb-Ti （bcc）、Pd-Y和Pd-Cu-Ti （fcc和B2）体系。观察到的实验结果总结如下：1)氢原子在Nb-Ti合金（bcc）中被困在两种隧道态中。2) Y或Ti的加入增加了Pd （fcc）中氢的扩散系数。3)在B2结构的Pd-Cu合金中加入Ti降低了氢扩散系数，而在fcc Pd-Cu合金中加入Ti对氢扩散系数几乎没有影响。上述结果表明，bcc合金和fcc合金之间的氢取代相互作用存在本质差异。氢在bcc或B2合金中的扩散受量子力学过程（隧穿效应）的支配。在不含合金元素的纯bcc金属中，这种量子力学氢扩散发生在整个晶格上。然而，在bcc合金中，合金元素周围形成了其他隧道态，整个晶格上不发生均匀扩散。这是隧穿态和自由态之间的量子力学跃迁概率减小的结果，因为这些态之间存在能量差。另一方面，对于fcc结构，初始扩散过程以经典方式进行，氢在激发后跃迁到氢势鞍点附近的状态，因此合金元素的加入对扩散机制影响不大。作为一种复苏，允许并不会阻碍fcc结构中的扩散，相反，某些合金元素会由于晶格的膨胀而促进扩散。

For the purpose of the clarifying the dependence of the hydrogen trapping effect on the crystal structure, the Nb-Ti (bcc), Pd-Y and Pd-Cu-Ti (fcc and B2) systems were investigated with techniques of the internal friction and the electrochemical measurements of hydrogen diffusivity and solubility. The observed experimental results are summarized as follows :1) Hydrogen atoms are trapped within two types of tunneling states in the Nb-Ti alloys (bcc).2) The addition of Y or Ti increases the diffusivity of hydrogen in Pd (fcc).3) The addition of Ti to the Pd-Cu alloys with B2 structure decreases the hydrogen diffusivity while it has almost no effect on the diffusivity in the fcc Pd-Cu alloys.The above results suggest an essential difference in the hydrogen-substitutional interaction between the bcc and fcc alloys. The hydrogen diffusion in bcc or B2 alloys is governed by the quantum mechanical process (tunneling effect). In pure bcc metals without alloying elements, such a quantum mechanical hydrogen diffusion occurs over whole lattice. However in the bcc alloys, other tunneling states are formed around the alloying elements and the homogeneous diffusion over the whole lattice dose not occur. This is a result of the reduction of the probability for the quantum mechanical transition between the tunneling states and the free states because of the energy difference between these ststes. On the other hand for the fcc structure, the original diffusion process occurs in a classical manner where the hydrogen jumps after excited to the ststes near saddle point of the hydrogen potential and thus the addition of the alloying elements dose not much affect the diffusion mechanism. As a resuslt, the diffusion in fcc structure is not retarded by the allowing and on the contrary some alloying elements enhance the diffusion due to the expansion of the lattice.