Kinetics of Phase Transformations

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9633596 Ludwig While much progress has been made in understanding equilibrium phase diagrams, transformation kinetics has been more difficult to calculate or simulate with quantitative accuracy. This is particularly true when there are complicating factors such as long-period superlattice phases and coupling between strain and ordering. By combining synchrotron-based-time-resolved x-ray scattering capabilities with recently-developed binding energy calculations and supercomputer technology significant advances can be made. State-of-the-art energy calculations in Monte-Carlo simulations are used to understand, and predict, complex ordering kinetics as observed by time- resolved x-ray diffraction. The calculations allow for local relaxation of atoms off their ideal lattice sites as well as global lattice relaxation and symmetry change. The focus is on the classic CuAu alloy, which possesses ordered and modulated phases as well as a lattice symmetry change upon ordering. Other important issues to be investigated include the presence of separate ordering spinodals for modulated and ordered phases, formation kinetics of the ordered phase from the modulated phase, and the coupling between strain and order. In addition to CuAu alloys, other systems which exhibit long-period superlattice phases will be investigated, in particular Ti-Al alloys which exhibit a devil's staircase of modulated phases. %%% Ordering kinetics in many alloys is complicated by the presence of long-range superlattice phases and/or changes in lattice constant and symmetry. Understanding the changes in such systems is a major challenge to materials scientists. ***