Keith J. Bowman

Phase coexistence and ferroelastic texture in high strain (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 piezoceramics

Dielectric permittivity and x-ray diffraction measurements were used to identify a region of phase coexistence between the rhombohedral and tetragonal phases near the morphotropic phase boundary in (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-BCT). This phase coexistence prevails over a considerable composition and temperature range and is bounded by single rhombohedral or tetragonal phases. The maximum piezoelectric response measured in terms of maximum strain divided by maximum electric field, Smax/Emax, is extraordinarily high, with the largest value of 1310 pm/V for x = 0.45. Electrical poling induces ferroelastic domain textures in both the rhombohedral and tetragonal phases simultaneously, which increases the piezoelectric performance significantly. The stability of that ferroelastic texture is limited by the phase transition at the morphotropic phase boundary, suggesting coupling between both coexisting phases and limiting potential applications. The results were confirmed using in situ temperature...

Thermal expansion behavior and macrostrain of Al2O3/Al composites with interpenetrating networks

Abstract The aim of the study is to investigate the thermal expansion behavior and internal residual strains in metal reinforced ceramic matrix composites (CMCs). A variety of Al2O3/Al CMCs with an interpenetrating network structure were produced using a pressure infiltration technique. The samples contained two variations in the average ligament diameter of the metal phase of 0.15 μm and 1 μm and metal contents ranging from 13 to 40 vol.%. Coefficients of thermal expansion (CTEs) were found to vary significantly with temperature. It is proposed that this indicates an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs of metal/ceramic composites. Overall strain increases with temperature scaled proportionally with metal content. Comparisons were made with uninfiltrated porous ceramic preforms and a pure metal sample. Hysteresis was observed between the heating and cooling of the composite samples at constant rate.

Structure and phase transitions in 0.5(Ba0.7Ca0.3TiO3)-0.5(BaZr0.2Ti0.8O3) from −100 °C to 150 °C

The solid solution of (x)Ba0.7Ca0.3TiO3-(1-x)BaZr0.2Ti0.8O3 is known to exhibit high piezoelectric constants. Discrepancies in the reported phase transitions and structure around room temperature, however, have complicated the understanding of the enhanced properties. Rietveld refinement of high-resolution X-ray diffraction is employed here to establish and refine the crystallographic structure at temperatures from −100 °C to 150 °C for x = 0.5. A combination of rhombohedral R3m and tetragonal P4mm is found to coexist at temperatures of 20 °C and −25 °C, bordered by single phase rhombohedral and tetragonal regions at lower (i.e., −100 °C) and higher (i.e., 70 °C) temperatures, respectively. The diffractograms also show signs of strain and domain wall scattering that are linked to the sample history.

Thermal residual stresses in co-continuous composites

The thermal residual stresses in two types of co-continuous composites copper/aluminum oxide (Cu/Al2O3) and aluminum/aluminum oxide (Al/Al2O3) were measured by neutron diffraction experiments. These stresses were generated during the cooling after high processing temperature. The coefficient of thermal expansion (CTE) mismatch of metal and ceramic phases led to significant amount of thermal stresses. In both the composites, the metallic phase was found to be under tension and aluminum-oxide phase under compression. Even though the magnitude of compressive stress in both the composites was similar; the two metal-phases had very different magnitude of tensile stresses. The difference in volume fraction, CTE, elastic stiffness and plastic flow properties led to this difference. The hydrostatic stresses were found to be predominant in both the phases. Finite element simulations were used to predict the stress distributions inside each phase and at the interfaces. A representative unit cell approach was considered to represent the composite. Concept of effective ΔT was utilized to simulate the thermal stress distribution inside the two phases in the unit cell. This model utilized the neutron diffraction measurements to predict the stress distribution inside each phase and at the interface. The simulations showed that significant amount of tensile stresses develop at the metal–ceramic interfaces.

Critical evaluation of the Lotgering degree of orientation texture indicator

Preferred orientation in textured ceramics is often assessed by comparing the relative intensities of x-ray diffraction reflections to those of a randomly oriented ceramic using the Lotgering degree of orientation ( f ). However, this paper provides evidence that indiscriminate assessments of f can be misleading. Using measured intensities of a modestly textured tape cast bismuth titanate (Na 0.5 Bi 4.5 Ti 4 O 15 ) ceramic, calculated f values vary from 7.4 to 73.2% depending on the reflections included in the calculation. The texture is also quantified by calculating the orientation distribution function (ODF) using measured pole figures. A model is then presented that demonstrates f is nonlinear with the multiple of preferred (00 l )-orientations, the standard unit of the 00 l pole figure.

Brittle intergranular failure in 2D microstructures: Experiments and computer simulations

Brittle intergranular fracture (BIF) is a common mode of failure for monolithic ceramics and intermetallics, as well as for some refractory metals and metals exposed to environmental corrosion, stress corrosion cracking or high temperature creep. As interest in applications for these materials grows, research programs have been developed to characterize and predict their fracture behavior. In order to experimentally quantify the effects of microstructure on local BIF, systems which have a minimum number of variables which influence fracture must be used. Evaluation of materials with two dimensional (2D) microstructures can considerably reduce the complexity of the system. In addition, providing a biaxial stress state in the 2D microstructure ensures that all boundaries experience exclusively Mode I loading prior to failure. Biaxial elastic loading of this simplified microstructure allows the calculation of (a) local stress and strain fields (and their concentrations) prior to failure, as well as (b) prediction of grain boundary strength criteria, and (c) prediction of intergranular crack paths. This can be achieved by conducting computer simulations of the experimentally observed fracture phenomena in polycrystalline specimens having a given texture and microgeometry. These simulations use high resolution finite-difference grids below the crystal scale, and involve the derivation of a spring-network model for arbitrary in-plane crystal anisotropy. Since the grain boundary strength criterion is easily controllable in such simulations, it can be inferred by a comparison with actual experimental results. The latter is complemented by results on fracture of materials with very weak grain boundaries, thus providing a clear perspective on evolution of the failure process for varying degrees of embrittlement.

R-curve behavior in alumina-zirconia composites with repeating graded layers

Abstract The single-edge-V-notched-beam testing geometry was used to measure the crack growth resistance (R-curve) behavior of multilayer graded alumina–zirconia composites for crack extensions parallel to the graded direction. Fracture mechanics weight function analysis was applied to explain the R-curve behavior of a compositional and grain-size graded microstructure. The results were then used to differentiate the influence of residual stress from other closure stresses, attributed to crack bridging, on the measured R-curve behavior.

Domain wall motion and electromechanical strain in lead-free piezoelectrics: Insight from the model system (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 using in situ high-energy X-ray diffraction during application of electric fields

The piezoelectric compositions (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) span a model lead-free morphotropic phase boundary (MPB) between room temperature rhombohedral and tetragonal phases at approximately x = 0.5. In the present work, in situ X-ray diffraction measurements during electric field application are used to elucidate the origin of electromechanical strain in several compositions spanning the tetragonal compositional range 0.6 ≤ x ≤ 0.9. As BCT concentration decreases towards the MPB, the tetragonal distortion (given by c/a-1) decreases concomitantly with an increase in 90° domain wall motion. The increase in observed macroscopic strain is predominantly attributed to the increased contribution from 90° domain wall motion. The results demonstrate that domain wall motion is a significant factor in achieving high strain and piezoelectric coefficients in lead-free polycrystalline piezoelectrics.

Fracture resistance curve behavior of multilayered alumina–zirconia composites produced by centrifugation

Abstract The single-edge-V-notched-beam (SEVNB) testing method was used to measure the crack growth resistance (R-curve) behavior of multilayered alumina–zirconia composites. Crack initiation and extension from the V-notch tip were observed via in situ optical microscopy. The resulting R-curves were compared with an R-curve measured from a monolithic composite having a similar composition and a homogeneous microstructure, where the influence of layer–layer interfaces, gradient microstructures, and the direction of crack propagation on the resulting R-curves were observed. Additionally, the stress intensity factor for crack initiation from the V-notch tip was ∼0.2 MPa·m1/2 higher than the stress intensity factor to further extend this crack.

Reduction of the piezoelectric performance in lead-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 piezoceramics under uniaxial compressive stress

The effect of a uniaxial compressive stress on the properties of BZT-BCT samples across the morphotropic phase boundary (MPB) is investigated using direct piezoelectric coefficient measurements. In contrast to many lead zirconate titanate compositions, the piezoelectric coefficient decreases monotonically with increasing stress and does not show an initial increase or plateau. Electrically softer rhombohedral and MPB compositions are found to be more susceptible to a decrease in piezoelectric coefficient under an increasing pre-stress than tetragonal compositions. Depoling due to ferroelastic domain switching alone, as observed by x-ray diffraction, does not explain this reduction, but instead a decreasing domain wall density is proposed to be responsible for reduced piezoelectric coefficients under increasing compressive stress. The relaxation of the piezoelectric response after complete unloading supports this proposed mechanism.