Theory of anisotropic growth rates in the ordering of an FCC alloy

A multiple-order-parameter theory of ordering on a binary face-centrsd-cubic (f.c.c.) crystal lattice is used to model diffuse interphase boundaries and provide expressions for the anisotropy of the kinetic coefficient that characterizes the speed of the order-disorder boundary. The anisotropy is varied parametrically with the ratio of two gradient energy coefficients. In contrast to the results from single-order-parameter theories, the orientation dependence of the kinetic coefficient differs significantly from that of the surface energy. Although the interfacial free energy anisotropy from this model is nor strong enough ro eliminate ally orientations in the (three-dimensional) equilibrium shapes, the kinetic coefficient is sufficiently anisotropic to eliminate some orientations during growth. The long-time kinetic growth shapes show the development of edges and corners in a definite sequence as the anisotropy increases. (C) 1997 Acta Metallurgica Inc.