| Despite the engineering importance of utilizing coherent structural phase transformations to synthesize and control microstructures in crystalline solids and, hence, to manufacture metal and ceramic alloys with desirable properties, there exists little work on theoretical characterization and prediction of the dynamic evolution of nonequilibrium coherent structural patterns which is dominated by accommodation of the transformation-induced elastic strain. It is the purpose of the present thesis to advance a kinetic theory for coherent phase transformations and to develop a set of numerical methods which would have a potential for future routine utilization in the microstructure control during processing and service performance of advanced multiphase materials. By advancing and combining the major recent development in theories of microelasticity, thermodynamics and kinetics of phase transformations, a coherent stochastic field kinetic theory and corresponding simulation techniques have been developed. The unique feature of the developed approach is its ability to deal with arbitrary coherent microstructures produced by diffusional and/or displacive transformations with arbitrary transformation strain. It is also able to describe, within the same physical and mathematical formalism, the processes of nucleation, growth and coarsening as well as homogeneous transformations such as spontaneous ordering and spinodal decomposition. Its applications to various types of phase transformations which are typical for advanced engineering materials ranging from high-temperature superalloys to transformation-toughened ceramics have led to understanding of complicated morphological patterns such as the basket-weave structure, precipitate macrolattice, concave particle shape, "split" pattern, discontinuous rafting structure and GP-zones in cubic systems and alternating bands structure, tweed and twin patterns in cubic + tetragonal systems. A variety of new and interesting kinetic phenomena underlying the development of these morphological patterns are discovered. They include the correlated nucleation, reverse coarsening, particle translational motion, shape transition and splitting. Some of them could be of significant value for alloy design. Despite the simplifications of the models, the simulation predictions are in excellent agreement with experimental observations, indicating that this new technique can be efficiently utilized in modeling microstructural evolution in real materials with a potential for applications to intelligent design and processing of materials with phase transformations. |
