Hydrogen-induced suppression of the peritectoid reaction of alpha -Pd+Pd3Gd (or H-Pd5RE (RE identical to Sm or Eu)) to Pd7RE (RE identical to Gd, Sm or Eu)

Abstract

The peritectoid reaction of alpha -Pd+Pd3Gd (or H-Pd5RE (RE identical to Sm or Eu)) to Pd7RE (RE identical to Gd, Sm or Eu) in Pd-Gd, Pd-Sm and Pd-Eu alloys is found to be suppressed after the slow cooling to room temperature of alloys in the presence of hydrogen (about 30 bar) which has been introduced above the peritectoid reaction temperature and then maintained during slow cooling. The suppression of the ordering to the Pd7Gd ordered structure in Pd-Gd alloys is believed to be due to the retardation of nucleation and growth of the Pd7Gd ordered domain during cooling as a consequence of a weakening of Pd-Gd bonding by the dissolved hydrogen in the alpha -Pd phase. In the cases of Pd-Sm and Pd-Eu alloys, the suppression is related to the transition of H-Pd5Sm(Eu) phases which initially coexist with alpha -Pd phases to L12-type Pd3Sm(Eu) phases under hydrogen exposure. The initially short-range-ordered Pd7Gd (Sm, Eu) phases coexist with the alpha -Pd, Pd3Gd or H-Pd5Sm(Eu) phases below the peritectoid reaction temperatures, but relatively at high temperatures they also have a tendency to disorder upon hydrogen exposure. Similarly, for the low-temperature hydrogen treatments, the initially coexisting H-Pd5Sm(Eu) phases with the Pd7Sm(Eu) phases also transform to the L12-type Pd3Sm(Eu) phases. The Pd3Sm(Eu) phases which have been transformed by hydrogen are reverse transformed into the alpha -Pd and H-Pd5Sm(Eu) phases by heating above the peritectoid reaction temperatures via the formation of Pd7Sm(Eu) phases.