Hydrogen-induced phase separation in PdRh alloys

Abstract

The principal result of this research is the demonstration that phase separation takes place in RdRh alloys at moderate to elevated temperatures in the presence of H2: it occurs to a much lesser extent under similar conditions but in the absence of hydrogen. The main technique employed for determining whether or not lattice changes have occurred are ‘diagnostic’ isotherms measured before and after exposure to hydrogen. These isotherms are measured at a much lower temperature than those employed for the hydrogen heat treatment (HHT). This diagnostic technique has the advantage that it is more sensitive than other methods to lattice changes in alloys: however, it has the disadvantage that the nature of the changes must be inferred. Direct evidence for segregation following HHT in these alloys has been obtained from electron microprobe analysis via a wavelength-dispersive spectrometer. X-ray diffraction revealed that the original f.c.c. reflections of the XRh = 0.20 alloy were broadened after HHT so as to suggest the presence of two sets of f.c.c. reflections.It is suggested that the segregation corresponds to phase separation according to the PdRh binary phase diagram, at least in the elevated temperature range employed for HHT. 673 to 873 K, where the hydrogen solubility is small and where consequently the binary phase equilibrium should not be perturbed by the small amounts of dissolved hydrogen. The lattice changes were more marked in the presence of hydrogen than in its absence in all of the experiments which were carried out. This research demonstrates the potential utility of employing H-induced changes for phase diagram determination of Pd alloys and possibly for other alloy systems, e.g. Ni-based alloys.