Phase Transformations in Ceramics: Science and Applications: On-Demand Oral Presentations
Sponsored by: ACerS Basic Science Division, ACerS Electronics Division, ACerS Engineering Ceramics Division
Program Organizers: Scott Mccormack, University Of California, Davis; Pankaj Sarin, Oklahoma State University; Sanjay V. Khare, University of Toledo; Waltraud Kriven, University of Illinois at Urbana-Champaign
Friday 8:00 AM
October 22, 2021
Room: On-Demand Room 4
Location: MS&T On Demand
Invited
New Insight into the Disordering Mechanism in Fluorite-related Ceramics: Maik Lang1; 1University of Tennessee
Structural disorder plays a critical role in technological applications to enhance specific functionalities, such as increasing conductivity via chemical doping in semiconductors. Recent results from neutron scattering experiments have shown that the atomic arrangements of many disordered crystalline materials are not random nor are they represented by the long-range structure observed from diffraction experiments. Structural heterogeneity at different length-scales appears to be a general characteristic of disordered materials. This presentation reviews neutron scattering experiments on ceramics that show complex disordering behavior across different length scales. Structural information from pair distribution functions with sensitivity to both cation and anion sublattices were utilized to investigate in detail the disordering process of A2B2O7 pyrochlore. The order-disorder transformation can be understood as a rearrangement of atomic-scale building blocks. The final atomic arrangement within the disordered phase involves a high level of order which can be predicted based on a set of fundamental chemical rules.
Invited
Phase Stability and Transformation in Borides Examined by X-ray Diffraction: James Cahill1; 1Lawrence Livermore National Laboratory
X-ray diffraction and transmission electron microscopy are used to study the phase stability of metal hexaborides and boron nitride at elevated temperatures. The short time scales of combustion synthesis are shown to produce nanodomains of multiple solid solutions within ternary hexaboride compounds as seen by peak broadening, splitting and shifting. The transformation of boron nitride from the cubic to hexagonal phase is investigated at high temperatures and ambient pressure in dry flowing helium, and transformation rates as measured by weight fraction of the hexagonal phase correspond to a zero-order transformation of cBN to hBN, which becomes inhibited as the particle size decreases. Variations in surface growth morphology between the particle sizes suggest growth inhibition due to a reduction in nucleation site availability.
Invited
Prediction of Diffusion-less Phase Transformations: Randall Hay1; Emmanuel Boakye1; Pavel Mogilevsky1; Thomas Key1; 1U.S. Air Force Research Laboratory
A general method to predict diffusion-less phase transformations and their associated orientation relationships and habit planes is developed and tested. The method assumes: 1. A superlattice common to the parent and daughter phases is a near-coincidence site lattice (NCSL) that also allows shear in an invariant plane or along an invariant line. 2. NCSLs are calculated for a maximum strain, a maximum shear, a maximum Miller index, and a maximum number of primitive unit cells in the superlattice. 3. Atom shuffles are calculated for motif translations of the daughter phase in fractional increments of NCSL lattice parameters. 4. A transformation is diffusion-less if all atom shuffles are <1/2 bond-length for any motif translation. Predictions are compared with the phenomenological theory for martensitic transformations. Limitations of the method, along with predictions for complex oxides such as rare earth disilicates, are discussed.
Order-disorder Relationships in Zirconium Carbides: Theresa Davey1; Ying Chen1; 1Tohoku University
Zirconium carbide is an ultra-high temperature ceramic with applications in nuclear and aerospace industries thanks to its maximum melting temperature around 3700K and high hardness at elevated temperatures. Zirconium carbide is a candidate for tuneable ceramics, as it has a wide range of stoichiometry, facilitated by carbon vacancies, with varying properties. However, carbon vacancies exhibit strong ordering that can persist to some degree at high temperatures. Furthermore, the ordering is affected by the presence of impurities e.g. oxygen. Fabrication requires cooling from higher temperature disordered states, resulting in trapping of metastable phases and complex partial ordering. The vacancy ordering significantly affects the thermodynamic and mechanical properties, and so fabricating zirconium carbide with specific properties can be a significant challenge. This work uses first-principles calculations to examine the properties as a function of temperature, composition, and degree of ordering, and combines theoretical and experimental data in a single, consistent CALPHAD model.
Computation of Fracture, Twinning, and Amorphization in Anisotropic Single and Polycrystalline Real-structured B4C Using Phase Field Approaches in the Finite Element Method: Benhour Amirian1; Bilen Abali2; Mali Moshtaghioun3; Jonathan Ligda4; Debjoy Mallick5; James Hogan1; 1University of Alberta; 2Technische Universität Berlin; 3Spanish Ministry of Science and Innovation; 4DEVCOM Army Research Laboratory; 5Amy Research Laboratory
In this work, a thermodynamically consistent physics-based phase field theory with detailed finite element procedure for various deformation mechanisms including fracture, twinning, and amorphization of realistically-structured boron carbide is formulated and solved at small and large strains with consideration of nonlinear anisotropic elastic behavior and anisotropic phase boundary energy. The capacity of such a model to reproduce specific experimental features of dynamically loaded single and polycrystalline B4C is investigated. The governing equations are solved using a monolithic scheme in a high-level python-based open-source platform, the FEniCS project. To demonstrate the performance of the proposed model, the results with three sets of representative examples are provided. Finally, the model is applied to an authentic microstructure of polycrystalline B4C, where the competition between the deformation mechanisms is accounted for. Altogether, the proposed model opens a number of interesting possibilities for simulating and controlling microstructure pattern development in materials experiencing extreme mechanical loading.