The (α + β) hydrogen miscibility gaps in hydrogenated palladium-rich Pd-Y(Gd)-Ag ternary alloys

Abstract

This work has been concerned with a study of palladium-rich Pd-Y(Gd)-Ag ternary alloys as possible diffusion membranes for hydrogen purification; these are expected to be stronger and more permeable to hydrogen than the commercially used membranes made from Pd-Ag alloys. As a preliminary to the hydrogen permeability study, the alloying behaviour of the ternary alloys and the extent of (α + β) hydrogen miscibility gaps in hydrogenated alloys were investigated. The metallographic examination and X-ray studies of annealed Pd 100 − x − y (Y(Gd)) x Ag y alloys with a silver content y(of 5–24 at.%) and x (at.%), the value of the content of yttrium and/or gadolinium, together satisfying the condition y + 3 x ⩽ 24 ( i.e. up to the valence electron concentration e a = 0.24 ) indicate that every one of these alloys forms as a single α-Pd phase; neither long-range order nor intermetallic compounds were observed. However, electron diffraction and electrical resistance measurements on Pd 95 − x Gd x Ag 5 with x = 6.3 gave faint indications of short-range order in Pd 7(Gd, Ag). It was found that the (α + β) hydrogen miscibility gaps in the hydrogenated alloys with any given silver content decrease gradually as the content of yttrium and/or gadolinium increases, and the critical compositions for the disappearance of the miscibility gaps are approximately isoelectronic with an electron-to-atom ratio e a of 0.24 for all the Pd 100 − x − y (Y(Gd)) x Ag y alloys.