Experimental determination of the α/β phase equilibria in the Pd–Cu system using a composition-gradient method

Accurate knowledge of phase equilibria in Pd–Cu alloys is essential for the design of hydrogen permeation membranes, where the high-permeability β (B2) phase plays a critical role. However, previous reports based on resistivity or lattice-parameter extrapolation have yielded inconsistent boundaries. In this study, the phase equilibria between the α (A1) and β phases above 400 °C in the Pd–Cu system were determined using composition-gradient diffusion couples combined with severe deformation and long-term annealing. This approach enabled reliable FE-EPMA/WDS measurements of equilibrium compositions within the narrow two-phase region from 400 °C to 600 °C. The results reveal that the β single-phase region extends towards the Pd-rich side compared with previous reports, with the maximum stability temperature estimated between 610 °C and 620 °C at ~41 at.% Pd. Transmission electron microscopy further confirmed that the β phase exhibits the B2 structure in the nonstoichiometric compositions of 41 at.% Pd. The revised phase diagram provides an accurate basis for evaluating the thermal stability of Pd–Cu alloys and offers critical guidance for the optimization of practical hydrogen permeation membranes. • Determined α/β equilibria in Pd–Cu alloys using diffusion couples. • Found β-phase stable up to 610–620 °C and shifted to Pd-rich compositions. • Confirmed B2 ordering of β-phase at off-stoichiometric compositions. • Clarified β-phase stability explaining Pd–40 wt.% Cu membrane behavior.