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

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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.