Phase-field simulations of coherent precipitate morphologies and coarsening kinetics

The primary aim of this research is to enhance the fundamental understanding of coherent precipitation reactions in advanced metallic alloys. The emphasis is on a particular class of precipitation reactions which result in ordered intermetallic precipitates embedded in a disordered matrix. These precipitation reactions underlie the development of high-temperature Ni-base superalloys and ultra-light aluminum alloys. Phase-field approach, which has emerged as the method of choice for modeling microstructure evolution, is employed for this research with the focus on factors that control the precipitate morphologies and coarsening kinetics, such as precipitate volume fractions and lattice mismatch between precipitates and matrix.; Two types of alloy systems are considered. The first involves L1 ₂ ordered precipitates in a disordered cubic matrix, in an attempt to model the γ′ precipitates in Ni-base superalloys and δ′ precipitates in Al-Li alloys. The effect of volume fraction on coarsening kinetics of γ′ precipitates was investigated using two-dimensional (2D) computer simulations. With increase in volume fraction, larger fractions of precipitates were found to have smaller aspect ratios in the late stages of coarsening, and the precipitate size distributions became wider and more positively skewed. The most interesting result was associated with the effect of volume fraction on the coarsening rate constant. Coarsening rate constant as a function of volume fraction extracted from the cubic growth law of average half-edge length was found to exhibit three distinct regimes: anomalous behavior or decreasing rate constant with volume fraction at small volume fractions ($\lesssim$20%), volume fraction independent or constant behavior for intermediate volume fractions (∼20–50%), and the normal behavior or increasing rate constant with volume fraction for large volume fractions ($\gtrsim$50%).; The second alloy system considered was Al-Cu with the focus on understanding precipitation of metastable tetragonal &thetas;′-Al ₂Cu in a cubic Al solid solution matrix. In collaboration with Chris Wolverton at Ford Motor Company, a multiscale model, which involves a novel combination of first-principles atomistic calculations with a mesoscale phase-field microstructure model, was developed. Reliable energetics in the form of bulk free energy, interfacial energy and parameters for calculating the elastic energy were obtained using accurate first-principles calculations. (Abstract shortened by UMI.)...