THEORY OF INTERPHASE AND ANTIPHASE BOUNDARIES IN FCC ALLOYS

Equilibrium antiphase domain (APB) and interphase (IPB) boundaries in the CuAu system are examined theoretically using the cluster variation method with multiatom interactions whose magnitudes were previously obtained from a fit of the phase diagram. The IPB energy between equilibrium disordered f.c.c. and ordered Cu3Au phases is strongly temperature (and hence composition) dependent, being much higher for the copper-rich side of the congruent point Tc and having a maximum on that side at about 0.95 Tc. The IPB energies are only slightly anisotropic. The APB energies at constant chemical potential decrease monotonically with increasing temperature: at constant non-stoichiometric composition they increase at low temperature to a maximum well below the disordering temperature. Near the congruent point, the APB undergoes one or more second-order surface phase transitions in which an interfacial layer resembling the inhomogeneous CuAu (L10) phase develops within the boundary. The APB with (hk0) orientation (in our notation) should be perfectly wet by the disordered phase at the disordering temperature for that particular composition. © 1979.