DIFFUSION OF HYDROGEN IN NANOCRYSTALLINE TRANSITION-METAL ALLOYS

Nanocrystalline Fe(90)Z(10), Fe60Co30Zr10, and Co90Zr10 alloys with av erage grain sizes of 15 nm to 200 nm were produced by crystallization of melt-sp un amorphous ribbons. After hydrogen charging, the diffusio n of hydrogen was studied by magnetic after-effect measurements and by thermal desorption experiments.Nanocrystalline Fe90Zr10 and Fe60Co30Z r10 alloys show a strong concentration dependence of the short-range a nd long-range diffusion of hydrogen, which is characteristic for a sta tistical distribution of interstitial site and saddle-point enthalpies . The origin of these statistical diffusion potentials can be assigned to the interfacial regions between the grains possessing a high hydro gen solubility in these alloys. In contrast, nanocrystalline Co90Zr10 alloys show no concentration dependence of hydrogen diffusion. The des orption kinetics observed over a wide range of grain sizes indicate th at hydrogen is mainly dissolved within the grains and diffuses very fa st in the interfacial regions. Therefore the degassing rate depends on the grain size and follows Einstein's diffusion law.