Ordered-disordered phase transitions and phase equilibria in face-centered cubic Cu-Pd alloys

Cu-Pd alloys are potential high-temperature brazing fillers due to their high melting points. To offer thermodynamic guidance for the development of brazing fillers, the present work experimentally investigated the ordered-disordered phase transitions and phase equilibria of face-centered cubic Cu-Pd alloys through conventional X-ray diffraction (XRD), electron probe microanalysis (EPMA), high-resolution transition electron microscopy (HRTEM) and selected area electron diffraction (SAED). The results reveal the presence of four distinct face-centered cubic (fcc) structures in the Cu-Pd alloys at 400 °С and 350 °С, consisting of the disordered α phase and three ordered phases: α´, α′′ and α′′′. The α′′′ phase is confirmed to be a two-dimensional long-period superstructure (2D-LPS), uniquely characterized by the non-conservative antiphase boundary parallel to the (100)_fcc plane and the double-layer superlattice reflection patterns in SAED. The lattice constants of the α′′′ phase are analogues to those of the well-established α′′ phase (1D-LPS), and their coherent growth makes them difficult to distinguish using conventional XRD analysis. The presently observed two-phase regions of α′ + α′′, α′′ + α′′′ and α′′′ + β deviate obviously from the previous reported data. Based on these findings, the Cu-rich side of the Cu-Pd phase diagram is refined accordingly.