Abstract :
We present a study of the temperature- and concentration-dependent hydrogen self-diffusion constant D in a Zr69.5Cu12Ni11Al7.5 metallic glass using the technique of nuclear magnetic resonance diffusion in a static fringe field of a superconducting magnet. In the investigated temperature interval between room temperature and 420 K, D is in the range 10−8–10−9 cm2/s and obeys Arrhenius form, D = D 0 exp ( − E a / k B T ) , indicating classical over-barrier-hopping hydrogen diffusion with an activation energy Ea. The concentration dependence of Ea and D was studied for the hydrogen-to-metal ratio H/M between 0 and 1.9. At low hydrogen content, H/M<0.2, Ea shows no noticeable concentration dependence, whereas D exhibits a weakly pronounced maximum at H/M=0.1 In the intermediate concentration regime, 0.2<H/M<0.9, Ea increases due to lattice expansion, whereas site blocking effect (increased occupancy in hydrogen sites) decreases the diffusion prefactor D0, so that D decreases in a linear-like manner from H/M=0.2–0.9 by a factor 2.7. In the high concentration range, 0.9<H/M<1.9, D and Ea do not change any more with H/M. We propose that this H/M-independence originates from hydrogenation-induced microstructural changes (formation of crystalline hydrid phases) of the material at high hydrogen content, opening new paths for hydrogen diffusion that are extrinsic to the Zr69.5Cu12Ni11Al7.5 bulk metallic glass state.
