  The earlier-described cluster methods are used to generalize the
Ginzburg-Landau gradient expansion for the free energy of an inhomogeneous
alloy to the case of not small values of order parameters and variations of
composition. The results obtained reveale a number of important differences
with the expressions used in the phenomenological phase-field approach.
Differential equations relating the local values of concentration and order
parameters within the antiphase or interphase boundary (APB or IPB) are
derived. These equations are applied to study the segregation at APBs near the
phase transition lines; the structure of
  APBs and IPBs near tricritical points; pre-wetting and wetting APBs in phases
with single and several order parameters; and also some effects of anisotropy
of APBs under L1$_0$ and L1$_2$-type orderings.
