Strain and texture evolution (domain switching) of polycrystalline, ferroelectric BaTiO3 was investigated in four‐point bending geometry. Lattice strains were measured by in situ synchrotron X‐ray diffraction to address problems related to modeling the constitutive behavior of highly asymmetric ferroelectrics. The hkl‐dependent strain measured by X‐ray diffraction was found to be smaller relative to both bulk strain measured by conventional, contact‐based techniques and elastically computed strain, and reasons for this inconsistency are discussed. A self‐consistent model with capabilities of quantifying domain switching and estimating hkl‐dependent strain is applied to allow a direct comparison with diffraction data.

The evolution of ferroelectric domain structures inside a single grain embedded in a polycrystalline BaTiO3 ceramic was investigated under temperature and electric field using the three-dimensional X-ray diffraction (3D-XRD) method. The orientation of domains within the grain was studied during the phase transformation from the cubic to tetragonal crystal structure. The peak widths broadened from 0.10 ± 0.01∘ to 0.29±0.08∘ along the azimuthal direction during cooling. Four individual tetragonal domain structures were developed from the cubic grain. A twinning model based on {101} habit planes is discussed. While the twinning model predicts 89.47∘ misorientation between 90∘ domains and 1.049∘ misorientation between domain variants, the measured misorientations neither support the twinning model nor are the domain structures mutually orthogonal. The average misorientation of the domain structures at room temperature with respect to the cubic grain was about 0.3∘. Upon application of an electric field, the volume fractions of the domain structures changed systematically favoring growth of domain structures with small polarization angle with respect to applied field direction. No rotation of domain structures was observed upon application of an electric field which is consistent with domain boundary migration.

The evolution of ferroelectric domains inside a single grain of a polycrystalline BaTiO3 ceramic was investigated under quasistatic heating by using polychromatic scanning x-ray microdiffraction. Four domain orientations were observed, three of which exhibited a classic of ∼90° ferroelastic relationship. The fourth domain orientation was found to be crystallographically related with one of the other orientations by a rotation of either 180.47° or 0.47°. While heating the polycrystalline BaTiO3 from room temperature to above the Curie temperature (125 °C), all four ferroelectric domain orientations rotated toward a paraelectric cubic orientation which was found to be at an intermediate orientation relative to the four domain orientations. The crystallographic relationships of the domains with respect to paraelectric phase were explained using a domain structure model by Nepochatenko.

The main goal of this study is the in-situ investigation of the ferroelectric domain structure inside polycrystalline BaTiO 3 under thermo-electro-mechanical loading conditions . The outcome is two-fold: (i) the characterization t echniques were improved to study the polycrystalline ferroelectrics in the mesoscale; an d (ii) the texture, lattice strain and volume fraction of domains were tracked under applied elec tric field and mechanical stress. Two novel synchrotron-based characterization techni ques, three-dimensional X-ray diffraction (3-D XRD) and Scanning X-ray Microdiffr action (μSXRD) were used in this study. The methodology and standards in both techni ques differ from each other and the present study provides a framework to bridge these t chniques. Although these methods have been developed earlier, their application and adaptation to ferroelectrics required some care. For instance, diffraction spots often overlap ped and made it difficult to identify individual domains and/or grains. In order to elimi nate the spot overlap, the polycrystalline BaTiO3 sample was heated above the Curie temperature wher e t (tetragonal) domains disappear and attain the orientation of the grain. Next, the sample was cooled slowly to the room temperature and the evolution of the ferroelec tric domains was studied at temperature and under electric field. The orientation relation ships, volume fractions and lattice strain evolution of 8 domain systems were studied. Whereas the orientation of the domains remained unc hanged under electric field, the fraction of the energetically favorable domain vari ants increased. Due to local constraints, complete switching from one domain variant to anoth er was not observed. The misorientation angles between domain variants sligh tly deviated from the theoretical value (=89.4°) by 0.2-0.3°. The deviation angle can be e xplained with the phase-matching angle developed during the cubic-tetragonal phase transfo rmation to maintain strain compatibility of neighboring domains. The multiscale strain evol uti n of ferroelectric domains in a polycrystal was investigated quantitatively for the first time. Under electric field, lattice strains of up to 0.1% were measured along the appli ed field direction. The present study offers a framework to characteriz e the polycrystalline materials with complex twin structures. By using the methodo l gy described in this study, 3D-XRD

Non-180° domain switching leads to fracture toughness enhancement in ferroelastic materials. Using a high-energy synchrotron X-ray source and a two-dimensional detector in transmission geometry, non-180° domain switching and crystallographic lattice strains were measured in situ around a crack tip in a soft tetragonal lead zirconate titanate ceramic. At KI=0.71MPam1/2 and below the initiation toughness, the process zone size, spatial distribution of preferred domain orientations, and lattice strains near the crack tip are a strong function of direction within the plane of the compact tension specimen. Deviatoric stresses and strains calculated using a finite element model and projected to the same directions measured in diffraction correlate with the measured spatial distributions and directional dependencies. Some preferred orientations remain in the crack wake after the crack has propagated; within the crack wake, the tetragonal 001 axis has a preferred orientation both perpendicular to the crack face and toward the crack front.