Displacive Transformations in Non-Metallic Materials: Session 1
Program Organizers: Mohsen Asle Zaeem, Missouri University of Science and Technology

Monday 10:40 AM
July 10, 2017
Room: Wrigley
Location: Hyatt Regency Chicago

Session Chair: Mohsen Asle Zaeem, Missouri University of Science and Technology; Ning Zhang, Missouri University of Science and Technology

10:40 AM  Invited
Strain Phase/Domain Separation and Ferroelastic Domain Structure Formation: Long Qing Chen1; Fei Xue1; Yanzhou Ji1; 1Penn State University
    Phase decomposition is a well-known process leading to the formation of two-phase mixtures with different compositions. Here we show that a strain imposed on a ferroelastic crystal promotes the formation of mixed phases and domains, leading to a domain and phase de-strain process during a ferroelastic phase transformation, with local strains determined by the uniform stress condition that can be graphically represented by a common tangent construction on the free energy versus strain curves. It is demonstrated that a ferroelastic domain structure can be understood using the concepts of domain/phase rule, lever rule, coherent and incoherent de-strain, and strain spinodal within the de-strain model description, in complete analogy to phase decomposition. The de-strain model provides a simple thermodynamic tool to guide and design domain structures of ferroelastic systems or the microstructures of a crystal separating to a mixture of two phases with different densities or molar volumes.

11:20 AM  
Size Effects and Energy Landscape Engineering in Ferroelectric Pervoskites: Sam Reeve1; Karthik Guda Vishnu1; Alejandro Strachan1; 1Purdue University
    With recent demonstration of free energy landscape engineering using martensitic shape memory alloys (SMAs), we extend our methodology to ferroelectric materials. In both cases, the energy landscape features, namely two stable states (austenite and martensite for SMAs and polarized and non-polarized for ferroelectrics) separated by an unstable state. In our previous work on SMAs, landscape engineering by integrating a non-martensitic component produced tunable stiffness and ultra-low stiffness, while retaining the shape memory. We show here density functional theory (DFT) simulations to characterize the bulk phases observed in BaTiO3 and PbTiO3, their surface energies along various orientations, and how size affects the relative phase stabilities at the nanoscale. We further explore epitaxial integration of these ferroelectric pervoskites with non-ferroelectric materials in order to tune the polarized – non-polarized phase transformation landscape and to engineer ferroelectric materials with an increase in spontaneous polarization and transformation temperatures (Tc) closer to room temperatures.

11:35 AM  
In-situ Neutron Diffraction Investigation of the Stress-induced Martensitic Phase Transformation in Bulk Magnesia Partially Stabilized Zirconia under Compressive Load: Christiane Ullrich1; Stefan Martin1; Benedikt Reichel1; Ralf Eckner1; Markus Hölzel2; David Rafaja1; 1TU Bergakademie Freiberg; 2Technische Universitat Muenchen
    The stress-induced phase transformation of the metastable tetragonal phase to the monoclinic phase in MgO partially stabilized zirconia effectuates extraordinary mechanical properties like high fracture toughness. The microstructure development (stress and microstrain of the individual phases) and the martensitic phase transformation under load up to 1800 MPa was studied by in situ neutron diffraction in bulk samples. The phase fraction of the monoclinic ZrO2 as analyzed by Rietveld refinement started to increase at a stress of about 1600 MPa, thus indicating the onset of the martensitic transformation. After unloading, no reverse transformation was found. Ex-situ microstructure characterization using SEM/EBSD and TEM revealed that the transformation mainly occurred in straight bands whose direction is related to the crystallographic orientation of the zirconia grains. In these bands, local stress concentrations form and tetragonal lenses surrounded by a cubic matrix transform to the monoclinic phase.

11:50 AM  
Failure Mechanisms of Single Crystalline Yttria-Stabilized Tetragonal Zirconia Nanopillars: Effects of Orientation, Specimen Size, and Composition: Ning Zhang1; Mohsen Asle Zaeem1; 1Missouri University of Science and Technology
    This study investigates the failure mechanisms and the effects of specimen size and composition on the mechanical response of single crystalline yttria-stabilized tetragonal zirconia (YSTZ) nanopillars by using molecular dynamics (MD) simulations. The nanoscale plastic deformation behaviors of YSTZ are found to be strongly dependent on the crystallographic orientation of zirconia nanopillars. Competing mechanisms between dislocation motion and phase transformation are revealed. For the first time, experimental explored tetragonal to monoclinic martensitic phase transformation is reproduced by MD simulations. The commonly-known phenomenon “smaller is stronger” is observed in nanopillars with dislocation dominated deformation, while “larger is stronger” relation is observed in nanopillars with phase transformation mediated deformation. The strength of nanopillars is found to decrease when increasing the amount of Y2O3. This can be explained by creation of more oxygen vacancies, which act as week points to facilitate dislocation motion and the interior failure of transformed phase.