Search

The stress-induced reorientation of O(N)–H(D) pairs has been investigated at MHz frequencies in Ta by propagating longitudinal ultrasonic pulses along a crystal lattice direction. The resulting anelastic relaxation is found to be composed of two processes which are attributed to two distinct configurations of the pair. A comparison of the transition rates of free and trapped H(D) atoms in Nb and Ta suggests that both processes observed in Ta involve transitions of H(D) atoms between equivalent nn sites, which are probably slightly displaced from normal tetrahedral sites (dis-1T↔dis-1T transitions). In Nb bound H(D) atoms should be in a state delocalized over two equivalent sites, probably slightly displaced from normal tetrahedral sites (dis-2T state), and the reorientation in this metal would take place via (dis-2T↔dis-2T) transitions.

Internal friction measurements have been carried out in a Pd 95 Ce 5 alloy containing different amounts of D (0.003 n D =D/(Pd+Ce) 1 , P 2 and P 3 have been observed at around 160, 115 and 103 K, respectively. The highest temperature peak P 1 is due to D-dislocation interactions (Snoek-Koester type of relaxation), the peak P 2 to stress-induced reorientation of Ce–D pairs and the peak P 3 is due to stress-induced changes in the short-range order of D within the sublattice of the octahedral interstitial sites (Zener type of relaxation). The relaxation parameters of peak P 2 are: τ 0 −1 =7×10 12 s −1 and W =0.22 eV, while an estimate of the anisotropy factor for the Ce–D pairs gives a value of 0.038.

Changes with increasing hydrogen content (H n ) in the form of the temperature T dependences of elastic energy dissipation (internal friction) over the range of 90–300 K at about 3 kHz have revealed two well-defined Q −1 peaks in the Q −1 −T plots that have been associated with stress-assisted rearrangements of hydrogen interstitials in the bulk lattices of palladium and palladium-silver alloys. Combined with information concerning hydrogen content dependences of other physical properties of the Pd-Ag-H system, the data obtained from the studies of internal friction have shown an indication of providing valuable guidance concerning structural correlations with the phase relationships and hydrogen content dependences of physical and thermodynamic parameters of these and other metal-hydrogen systems.

The amplitude dependence of the dynamic Young modulus has been investigated during temperature cycling in the vicinity of the martensitic transformation. The Young modulus E is smaller at higher vibration amplitudes. On cooling, the difference ΔE between the value of E at the lowest strain e 0 and at the current strain e passes through a pronounced structured maximum in the transition region. On heating, ΔE shows a hardly noticeable peak, followed by a steep decrease over the reverse transition region. Non-linear effects also occur far above the martensite start temperature M s and are the result of precursor effects.

Studies of anelastic phenomena resulting from the introduction of hydrogen into palladium—silver alloys have been extended to include Pd 73 Ag 27 and Pd 68 Ag 32 alloy compositions. Results of measurements over 90–300 K at 3 kHz have been compared with features observed in earlier experiments with other PdAg alloys. Changes with alterations of silver and hydrogen compositions in two anelastic relaxations associated with stress-assisted rearrangements of hydrogen interstitials have been discussed.