| Understanding of the complicated interdiffusion microstructures developed between two contact alloys at elevated temperatures is critical to the design of many materials, e.g. coated turbine blades. In the present thesis, a novel computational approach based on phase field method has been developed to investigate for the first time the interdiffusion microstructures for ternary and two-phase systems. This approach possesses many advantages over the previous simulations which were limited to one-dimensional (1D) diffusion in a common matrix phase while second-phase particles were treated as point sources or sinks of solute atoms. The proposed approach can simultaneously account for the diffusion in different phases with arbitrary volume fractions and morphologies. The elastic interaction between particles as well as the stress effect on the interdiffusion can also be considered.; This approach has been first applied for a fundamental research to investigate the Kirkendall effect in ternary; two-phase diffusion couples. Model alpha + alpha' alloys were designed so that changes in the microstructure could be attributed to either capillarity or the Kirkendall effect. It has been found that the Kirkendall effect, which was introduced by setting atomic mobilities to different values, changed the diffusion path slope and led to the formation of "horns" on the two-phase diffusion path. Both precipitates and the so-called Type 0 boundary migrate as a result of the Kirkendall effect. Also the initial slope of the diffusion path differs significantly from the earlier work and the path is time dependent due to temporal changes in the microstructure. In addition, the phase field method provides a detailed picture of Kirkendall marker movement in a two-phase microstructure. The marker plane bends around precipitates and individual markers move along curved paths.; The practical application of the approach has then been carried out for Ni-Al-Cr ternary system, the most important system in the coating design. CALPHAD technique has been incorporated to provide chemical free energy and kinetic data. The simulated gamma + beta/gamma and gamma + beta/gamma + beta diffusion couples present microstructure features which show reasonable agreement with experimental observations. New features such as curved diffusion paths in two-phase region and formation of single-phase layer have also been observed. |
