Relaxation rate of conserved and nonconserved order parameters in replacive transitions

We apply the kinetic Bragg-Williams model, introduced recently, to coherent precipitation of ordered phases. We show that the relaxation rate of the long-range order parameter (nonconserved) and of the concentration (conserved order parameter) are coupled not only by the free energy as usually assumed, but also by nondiagonal terms in the linear operator that connects the relaxation rate of both order parameters to the driving forces, i.e., the partial derivatives of the free-energy functional with respect to the order parameters. Moreover, we show that the relaxation of the nonconserved order parameter may occur by two routes, a homogeneous one (on site relaxation) and a heterogeneous one (migration of domain boundary). All such effects are ignored by existing theories.