This grant is a combined experimental and theoretical examination of phase transformations in materials. Fast (millisecond ) synchrotron-based time resolved X-ray scattering is employed to study spinodal decomposition and ordering in alloys. Preliminary research has concentrated on the classic cases of spinodal decomposition in aluminum-zinc and ordering in copper-gold and showed that there is no significant timescale during which the Cahn-Hilliard-Cook linear theory or its extension, the Langer-Bar-On-Millier nonlinear theory are valid. Now the research is centered on alloys with smaller strain effects (iron-chromium and iron-cobalt), alloys with second-order ordering transitions (iron-cobalt and aluminum-iron), and alloys that undergo simultaneous ordering and phase separation (aluminum-iron). In some of these cases these fast experiments probe transformation kinetics in regions of the phase diagram in which neither phase is the true equilibrium structure. The simulations and theory part of the research pursues large scale studies of spinodal decomposition and continuous ordering in both Ising models and fluids, for short and long range interactions. Included are studies of crystallization in simple fluids. Limits of the present nucleation theories are studied by taking simulation data as the limit of miscibility is approached and seeing where the theory begins to fail. Geometric models are developed for nucleation and spinodal decomposition
