We report ultrahigh dielectric and piezoelectric properties in BaTiO3-xBaSnO3 ceramics at its quasi-quadruple point, a point where four phases (Cubic-Tetragonal-Orthorhombic-Rhombohedral) nearly coexist together in the temperature-composition phase diagram. At this point, dielectric permittivity reaches ∼ 75000, a 6-7-fold increase compared with that of pure BaTiO3 at its Curie point; the piezoelectric coefficient d33 reaches 697 pC/N, 5 times higher than that of pure BaTiO3. Also, a quasi-quadruple point system exhibits double morphotropic phase boundaries, which can be used to reduce the temperature and composition sensitivity of its high piezoelectric properties. A Landau-Devonshire model shows that four-phase coexisting leading to minimizing energy barriers for both polarization rotation and extension might be the origin of giant dielectric and piezoelectric properties around this point.
For more than half a century, the morphotropic phase boundary (MPB) has drawn constant interest in developing piezoelectric materials, as the phase instability at the region significantly enhances piezoelectricity. However, the local structure/symmetry at the MPB region is still under controversy. The investigation on morphology and origin of the local structure at MPB is of considerable importance to provide a microstructure basis for high piezoelectricity. In the present study, we thus use high resolution transmission electron microscopy to investigate the microstructure feature of MPB at PMN-PT ceramics. The local structure is shown to be the coexistence of nano-scaled {110}-type rhombohedral (R) twin and {110}-type tetragonal (T) twin. Such nano-scaled coexistence can be due to a nearly vanishing polarization anisotropy and low domain wall energy at MPB, which thus facilitates polarization rotation between 〈001〉T and 〈111〉R states and leads to high properties of MPB compositions.
As one of the core effects on the high-temperature structural stability, the so-called ''sluggish diffusion effect'' in high-entropy alloy (HEA) has attracted much attention. Experimental investigations on the diffusion kinetics have been carried out in a few HEA systems, such as Al-Co-Cr-Fe-Ni and Co-Cr-Fe-Mn-Ni. However, the mechanisms behind this effect remain unclear. To better understand the diffusion kinetics of the HEAs, a combined computational/experimental approach is employed in the current study. In the present work, a self-consistent atomic mobility database is developed for the face-centered cubic (fcc) phase of the Co-Cr-Fe-Mn-Ni quinary system. The simulated diffusion coefficients and concentration profiles using this database can well describe the experimental data both from this work and the literatures. The validated mobility database is then used to calculate the tracer diffusion coefficients of Ni in the subsystems of the Co-Cr-FeMn-Ni system with equiatomic ratios. The comparisons of these calculated diffusion coefficients reveal that the diffusion of Ni is not inevitably more sluggish with increasing number of components in the subsystem even with homologous temperature. Taking advantage of computational thermodynamics, the diffusivities of alloying elements with composition and/or temperature are also calculated. These calculations provide us an overall picture of the diffusion kinetics within the Co-Cr-Fe-Mn-Ni system.Keywords atomic mobility database Á CALPHAD Á computational thermodynamics Á high entropy alloy Á sluggish diffusivity Á solid solution alloy
Nanoscale composite precipitates of Alloy 718 have been investigated with both high-resolution scanning transmission electron microscopy and phase field modeling. Chemical analysis via energy-dispersive x-ray spectroscopy allowed for the differentiation of γ′ and γ″ particles, which is not otherwise possible through traditional Z-contrast methods. Phase field modeling was applied to determine the stress distribution and elastic interaction around and between the particles, respectively, and it was determined that a composite particle (of both γ′ and γ″) has an elastic energy that is significantly lower than, for example, single γ′ and γ″ precipitates which are non-interacting.
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