MECHANICAL RELAXATION PROCESSES OF HYDROGEN IN DILUTE PALLADIUM RARE-EARTH ALLOYS

After charging with hydrogen, dilute Pd100-xREx alloys (RE=Y, Ce, Gd) (x=1...9) show three thermally activated relaxation maxima of the inte rnal friction between 10 and 300 K. Two of these maxima are identified as a Zener maximum of hydrogen pairs in the beta-phase and as a Snoek -Koster maximum, which are both also observed in H-charged pure Pd The third maximum appears only in the binary alloys. The activation param eters of this relaxation are tau(0)=3.10(-12+/-0.5) s for the pre-expo nential factor and the activation enthalpy lies between H-a=0.18 eV an d H-a=0.26 eV. H-a increases with increasing RE content whereas it dec reases with increasing hydrogen content. The relaxation strength is in dependent of the RE content and depends linearly on the hydrogen conce ntration. These features are characteristic for the formation and reor ientation of H-RE (RE=Y, Ce, Gd) complexes. Furthermore, the shape fac tor of the strain ellipsoid of the H-RE dipoles, which characterizes t he anisotropy and strength of the local atomic displacements, was dete rmined as \lambda(1)-lambda(2)\=0.036. (C) 1997 Elsevier Science S.A.