Stabilization of the ordered structure of thin condensed foil of the Pd-Cu solid solution in a hydrogen environment

Structural transformations of thin (≈4 µm) Pd-Cu alloy foil fabricated by magnetic sputtering of a target with the composition corresponding to the highest temperature of the existence of the ordered structure (β phase, a CsCl lattice) have been studied in the course of heating and cooling in vacuum or in a hydro� gen atmosphere. Initial foil has an ordered structure (β phase), a gradient grain substructure, and domi� nant 〈211〉 texture. In vacuum, phase disordering (the β → α transi� tion) occurs within the range 773-873 K, i.e., within the boundaries of the equilibrium phase diagram of the system. This is accompanied by collective recrystalli� zation that eliminates the gradient of the grain sub� structure and by appearance of 〈110〉 texture of the β phase. Annealing in a hydrogen environment led to an increase of the existence range of a singlephase structure (β phase) by nearly 200 K: at 1123 K, the intensity of the 111α reflections was ≈60% of the 110β intensity. In this paper, we report for that first time that annealing of the Pd-Cu alloy in a hydrogen environ� ment extends the temperature range of the existence of the ordered structure with a CsCltype lattice. The Pd-Cu solid solutions are among alloys rec� ommended for fabrication of selective membranes for deep purification of hydrogen (1, 2). In this aspect, the possibility of solid solution ordering to form the β phase (CsCl lattice), shown by the phase diagram (3), is of special interest since the activation energy of hydrogen diffusion in the ordered Pd-Cu alloy (0.035 eV) is almost one order of magnitude lower than in the disordered solid solution (0.325 eV) and in palladium (0.23 eV) (4). As follows from the studies of kinetics of phase transformations that accompany the formation of the ordered structure of the Cu-49 at % Pd alloy (5-7), the degree of ordering essentially depends on the preliminary deformation of the samples and their subsequent heat treatment. Longterm heat treatment after intensive plastic deformation afforded a nearly singlephase ordered structure (7). It has been demonstrated that singlephase ( β phase) condensed foil can be fabricated by magnetic sputtering of a Pd- Cu alloy target of the corresponding composition (8, 9). Studying the hydrogen permeability of Pd-Cu foil has confirmed the expected result of ordering: specific hydrogen permeability of ordered alloy membranes is ten or more times higher than that of membranes made of the alloy with a disordered structure (10). In this context, it is of interest to study the effect of the major component of the membrane environment (hydrogen gas) on the temperature range in which the ordered structure of a thin Pd-Cu alloy foil persists.