Search

Phase field method is a powerful tool to simulate microstructure evolution and is widely used in nearly all fields of materials science. In this work, we apply the phase field approach coupled with thermodynamic models to simulate microstructural evolution and study the composition-microstructure-property relationship for high temperature coatings and ferroelectric materials at morphotropic phase boundary (MPB). The study on high temperature coatings in this work focuses on the fundamentals of interdiffusion microstructure maps as well as a special interdiffusion microstructure containing the so called type n boundaries. An inderdiffusion microstructure map (IMM) is a two dimensional diagram showing how interdiffusion microstructure varies when one end alloy composition (the base material) is fixed while the other (the coating material) is varied across a region of the phase diagram for dual alloys. It can thus predict the relationship between interdiffusion microstructure and initial alloy compositions and have importance to coating design. The fundamentals for constructing IMMs for dual-alloy systems are established based on the current phase field simulations as well as previous works, which includes the topology of IMM as well as three mechanisms of microstructure type change on an IMM. These fundamentals should be followed when constructing IMMs for a real alloy system. With regards to type n boundaries, which are defined as interface boundaries at which n phases changing on crossing them, the characterestics of type n boundaries and the condition for the formation of type n boundaries are explored in the current work. For n=3, type n boundaries are expected to be infrequent because the diffusion paths of them have to pass through a special feature which is defined as a feature that cannot be intersected by a random line. However, our simulations found that under the right conditions, such boundaries can occur and even if the initial alloy composition varies, it persists to occur. The simulation results are consistent with experimental observations. The study on ferroelectric MPB in this work focuses on the domain structure and the origin of good piezoelectric property at MPB. MPB of ferroelectric solid solutions is a composition phase boundary between tetragonal and rhombohedral phases and possesses the best piezoelectric property but the phase of the domains at MPB and the origin of good property at MPB are under serious debate. Through advanced phase field simulations, a hierarchical nanodomain structure at the MPB is predicted and agrees well with experimental observations. The phase of nanodomains at MPB is found to be monoclinic. The monoclinic phase is unstable according to the 6th order Landau free energy of the system but it was found that it is the long range elastic and electrostatic interaction energies in the multi-domain states that stabilize the monoclinic phase. Furthermore, the study on the piezoelectric property of the ferroelectric MPB systems discovered that polarization rotation through the monoclinic phase is responsible for their high d33. In conclusion, phase field simulation method is employed to understand the relations between composition and interdiffusion microstructure as well as relations among composition, ferroelectric domain structure and piezoelectric property of ferroelectric MPB materials and helps answering some important questions both in the field of interdiffusion microstrture and ferroelectric MPB.