Modelling Phase Ordering Kinetics in Alloys

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9520923 Chakraborty Kinetics of ordering in alloys will be investigated using models based on descriptions of the electronic structure of metallic alloys. By predicting microstructures and growth laws, these studies will provide the opportunity of making quantitative comparisons between theory and experiment. The type of modeling will also bridge the gap between experiments and theoretical results obtained using kinetic Ising models and their continuum version, the time-dependent Ginzburg-Landau model. The microscopic models of alloys have two features which make the study of growth kinetics relevant and interesting. The models include a description of the coupling between elastic strains and chemical order and they include typical metallic interactions such as Osize-effectO terms and long- range interactions arising from Fermi surface effects. Two types of models will be considered in the current work. One is a particular version of the embedded atom approach which provides a very good description of non-transition metal alloys, and the other is a model based on the direct pair- correlation function of alloys which can be calculated from the coherent potential approximation to disordered alloys. The latter approach is known to provide an excellent description of transition metal alloys and particularly of any fermi-surface realted effects. The study of kinetics can be divided into three separate parts: (1) direct Monte Carlo simulations of embedded atom Hamiltonians including chemical and displacement effects, (2) simulations of Langevin equations based on Ginzburg- Landau functionals constructed from microscopic models,and (3) studies of a generalized Ising model which provides the simplest model of ordering accompanied by a structural transition. The generalized Ising model has a wider range of applicability than alloys and will provide a description of kinetics in any system where there is a coupling between a structural distortion and some orderi ng field. %%% Theoretical research will be conducted on the properties of metallic alloys. In particular, studies will be made on how the components of a binary alloy mix together to form various phases and structures. Understanding how these phases evolve will contribute to predicting the properties of these alloys and how various processing paths affect these properties. ***