Microstructural phase coexistence kinetics near the polymorphic phase boundary

By implementing a novel multiphase field model for ferroelectric systems, the phase coexistence of the tetragonal (T) and rhombohedral (R) phases in Pb-free BZT-40BCT was analyzed. Metastable coexistence of the T and R phases is predicted between a thermodynamic upper limit at T-CR = 49.90 degrees C and a kinetic lower limit determined by the time-temperature-transformation behaviour. Predicted domain microstructures exhibit faceted domain walls and curved T-R phase interfaces that are consistent with recent TEM studies in the vicinity of the polymorphic phase boundary (PPB). Further, miniaturization of the domain structure near the PPB is a result of the relatively low interfacial energies and a pinning effect caused by the large metastable phase fraction that originates from the vanishing macroscopic driving force for phase transformation. Particularly, the vanishing of rhombohedral domain wall energies as T -> T-CR enables a phase transformation-induced polarization rotation mechanism and predicts a hierarchical domain morphology for the R phase. These results are in agreement with the higher piezoelectric response reported near the maximum temperature for R+T coexistence and the observations of a miniaturized nanodomains structure within micron-sized, wedge-shaped domains in the R phase for the BZT-xBCT system. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

A novel multiphase field model was used to analyze the phase coexistence of tetragonal and rhombohedral phases in Pb-free BZT-40BCT. The study predicted metastable coexistence between the two phases, with domain microstructures exhibiting faceted domain walls and curved T-R phase interfaces near the polymorphic phase boundary. The miniaturization of domain structures near the boundary is due to low interfacial energies and a pinning effect from a large metastable phase fraction.