We describe a general method to model multicomponent ordered crystals using
the phase-field crystal (PFC) formalism. As a test case, a generic B2 compound
is investigated. We are able to produce a line of either first-order or
second-order order-disorder phase transitions, features that have not been
incorporated in existing PFC approaches. Further, it is found that the only
elastic constant for B2 that depends on ordering is $C_{11}$. This B2 model was
then used to study antiphase boundaries (APBs). The APBs were shown to
reproduce classical mean field results. Dynamical simulations of ordering
across small-angle grain boundaries predict that dislocation cores pin the
evolution of APBs.