Mechanical relaxation processes of hydrogen in dilute palladium-rare earth alloys

Abstract

After charging with hydrogen, dilute Pd 100 − x RE x alloys (RE=Y, Ce, Gd) ( x = 1 … 9) show three thermally activated relaxation maxima of the internal friction between 10 and 300 K. Two of these maxima are identified as a Zener maximum of hydrogen pairs in the β-phase and as a Snoek-Köster maximum, which are both also observed in H-charged pure Pd. The third maximum appears only in the binary alloys. The activation parameters of this relaxation are τ 0 = 3·10 −12±0.5 s for me pre-exponential factor and the activation enthalpy lies between H a = 0.18 eV and H a = 0.26 eV. H a increases with increasing RE content whereas it decreases with increasing hydrogen content. The relaxation strength is independent of the RE content and depends linearly on the hydrogen concentration. These features are characteristic for the formation and reorientation of HRE (RE=Y, Ce, Gd) complexes. Furthermore, the shape factor of the strain ellipsoid of the HRE dipoles, which characterizes the anistropy and strength of the local atomic displacements, was determined as | λ 1– λ 2|=0.036.