  We develop and analyze a two-mode phase-field-crystal model to describe fcc
ordering. The model is formulated by coupling two different sets of crystal
density waves corresponding to <111> and <200> reciprocal lattice vectors,
which are chosen to form triads so as to produce a simple free- energy
landscape with coexistence of crystal and liquid phases. The feasibility of the
approach is demonstrated with numerical examples of polycrystalline and (111)
twin growth. We use a two-mode amplitude expansion to characterize analytically
the free-energy landscape of the model, identifying parameter ranges where fcc
is stable or metastable with respect to bcc. In addition, we derive analytical
expressions for the elastic constants for both fcc and bcc. Those expressions
show that a non-vanishing amplitude of [200] density waves is essential to
obtain mechanically stable fcc crystals with a non-vanishing tetragonal shear
modulus (C11 - C12)/2. We determine the model parameters for specific materials
by fitting the peak liquid structure factor properties and solid density wave
amplitudes following the approach developed for bcc [K.-A. Wu and A. Karma,
Phys. Rev. B 76, 184107 (2007)]. This procedure yields reasonable predictions
of elastic constants for both bcc Fe and fcc Ni using input parameters from
molecular dynamics simulations. The application of the model to two-dimensional
square lattices is also briefly examined.
