Metastable copper-chromium alloy films

Due to the large positive heat of mixing associated with the Cu–Cr binary system, solid solutions exist only as nonequilibrium states. In this study, a series of metastable Cu–Cr alloys ranging in composition from 14.1 to 75.4% copper was fabricated by sputter deposition. Symmetric, asymmetric, and grazing incidence x-ray diffraction geometries were used to trace the phase transition from bcc to fcc crystal structures with increasing Cu fraction. It is shown that the transition takes place not by a two-phase region suggested by equilibrium thermodynamics, but rather through gradual disordering of the bcc lattice as copper atoms are substitutionally accommodated. At a critical saturation near 71% Cu, the bcc structure becomes unstable relative to the fcc and a phase transition occurs. The free energies of the kinetically constrained Cu–Cr system are modeled and the results are found to agree well with observed behavior.