Hydrogen-induced rearrangements in Pd-rich alloys

Homogeneous fee Pd alloys have been found to undergo lattice rearrangements such as phase separation in the presence of dissolved H. For Pd-Rh alloys the phase-separated form is the thermodynamic stable state but for others which have been investigated, for example, Pd-Ni, Pd-Pt, the binary homogeneous substitutional solutions are the thermodynamically stable state. Phase separation for the latter alloys is a consequence of a ternary (Pd+M+H) equilibrium. The rearranged lattices are metastable after evacuation of the dissolved H at a temperature where the recovery back to its homogeneous state does not take place at a measurable rate. These H-induced changes differ from the lattice rearrangements which accompany hydride formation in metals and alloys where the hydride phases are very stable and which have different structures from the parent alloy, e.g. the formation of dihydrides in Zr or Zr-based alloys such as Zr-Nb (5 at.%). These very stable hydride phases will return to their initial states after removal of H at the required elevated temperatures. By comparison, H can be removed at moderate temperatures from the fee Pd-based alloys because H is not very strongly held in the lattices. The metastable, H-free phase-separated Pd alloys return towards their homogeneous state after annealing in vacuo at, for example, 673 K, demonstrating that the phase-separated binary alloy is metastable. Results of II-induced changes in Pd-rich alloys are discussed for different ranges of temperature and hydrogen pressure for several Pd alloy systems. A ternary-phase diagram is calculated using known thermodynamic parameters for hydrogen solution in the homogeneous binary alloys and the thermodynamics of mixing of the metals. Using the (Pd+Pt) system as an example, a ternary (Pd+Pt+H) equilibrium is shown to cause phase separation. (C) 1999 Elsevier Science S.A. All rights reserved.