HYDROGEN-INDUCED SUPPRESSION OF THE PERITECTOID REACTION OF ALPHA-PD-PD5RE (RE-EQUIVALENT-TO-SM OR EU)) TO PD7RE (RE-EQUIVALENT-TO-GD, SM OR EU)(PD3GD (OR H)

The peritectoid reaction of alpha-Pd + Pd3Gd (or H-Pd5RE (RE = Sm or E u)) to Pd7RE (RE = Gd, Sm or Eu) in Pd-Gd, Pd-Sm and Pd-Eu alloys is f ound to be suppressed after the slow cooling to room temperature of al loys in the presence of hydrogen (about 30 bar) which has been introdu ced above the peritectoid reaction temperature and then maintained dur ing 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 su ppression is related to the transition of H-Pd5Sm(Eu) phases which ini tially coexist with alpha-Pd phases to Ll2-type Pd3Sm(Eu) Phases under hydrogen exposure. The initially short-range-ordered Pd7Gd (Sm, Eu) p hases coexist with the alpha-Pd, Pd3Gd or H-Pd5Sm(Eu) phases below the peritectoid reaction temperatures, but relatively at high temperature s they also have a tendency to disorder upon hydrogen exposure. Simila rly, for the low-temperature hydrogen treatments, the initially coexis ting H-Pd5Sm(Eu) phases with the Pd7Sm(Eu) phases also transform to th e Ll2-type Pd3Sm(Eu) phases. The Pd3Sm(Eu) phases which have been tran sformed by hydrogen are reverse transformed into the alpha-Pd and H-Pd 5Sm(Eu) phases by heating above the peritectoid reaction temperatures via the formation of Pd7Sm(Eu) phases.