Displacive Transformations in Non-Metallic Materials: Session 3
Program Organizers: Mohsen Asle Zaeem, Missouri University of Science and Technology
Wednesday 10:20 AM
July 12, 2017
Location: Hyatt Regency Chicago
Session Chair: Ning Zhang, Missouri University of Science and Technology
Effects of Texture, Specimen Size, and Composition on Mechanical Response of Yttria-Stabilized Tetragonal Zirconia Polycrystal Nanopillars: Mohsen Asle Zaeem1; Ning Zhang1; 1Missouri University of Science and Technology
Molecular dynamics (MD) is employed to study the nanoscale specimen size, grain size, grain orientation, and composition effects on deformation and mechanical response of yttria-stabilized tetragonal zirconia polycrystal (YTZP) nanopillars. The “larger is stronger” phenomenon is observed when increasing the cross-sectional area of YTZP nanopillars. The strength of zirconia nanopillars increases with a decrease in free-surface-to-volume ratio. For YTZP nanopillars with the same specimen size, a larger average grain diameter results in a stronger material, indicating an inverse Hall-Petch effect. As for orientation effect, YTZP nanopillars with mainly grain orientations that promote phase transformation show the highest strength, while those with grain orientations that promote dislocation motion show the lowest strength. The increase of Y2O3 involves creation of more oxygen vacancies, which accordingly accelerate phase transformation and dislocation emission, and eventually leads to the decrease of strength of the YTZP nanopillar. Formation of nanovoids/cracks is also observed during compression.
The Monoclinic to Tetragonal Phase Transformations in Hafnia and Zirconia: Ryan Haggerty1; Scott McCormack; Waltraud Kriven1; 1University of Illinois at Urbana-Champaign
The monoclinic to tetragonal phase transformations and back were studied in situ, in hafnia (HfO2) and zirconia (ZrO2) by synchrotron X-rays at temperatures to 1850 °C in air. The transformation temperatures, hystereses, lattice parameters, unit cell volumes and Bain strains were documented for both compounds. The temperature hysteresis was characterized and found to be 110±10°C for hafnia and 152±6°C for zirconia. The total volumetric strain of the tetragonal to monoclinic transformation was 2.99% for hafnia (1670°C) and 3.69% for zirconia (1044 °C). To explain the difference in transformation temperatures, a theory was investigated that there was a critical metal ion to oxygen bond size that cannot be exceeded in the monoclinic phase. In this study the average distance between these atoms was given as a function of temperature normalized by the transformation temperature. This was shown that the bond lengths in the monoclinic phases are similar at the transformation temperature.
Structural Diversity, Actuation and Motility in Confined Liquid Crystalline Polymers: M. Ravi Shankar1; 1University of Pittsburgh
Confinement of elasticas fabricated from twisted nematic liquid crystalline polymers is shown to engender a wide diversity of structural transformations and actuation modes in response to thermal and photo stimulus. This is found to range from the spontaneous creation of localized prismatic geometries, assembly of highly asymmetric and hierarchical architectures, as well as millisecond-scale mechanical instabilities. Such transformations are ultimately traceable to the balance of stretch and bending energies in conjunction with confinement imposed by the mechanical prebias. The mechanisms offer transduction of remote stimuli into linear and rotary actuation modes involving energy densities ~kJ/m3. Under certain conditions, spontaneous motility of samples is observed in response to an unstructured impulse on an arbitrary surface, which can enable new system design opportunities in soft robotics and manipulation.