Phase Transformations in Ceramics: Science and Applications: Session I
Program Organizers: Pankaj Sarin, Oklahoma State University; Scott Mccormack, University Of California, Davis; Waltraud Kriven, University of Illinois at Urbana-Champaign; Theresa Davey, Bangor University; Sanjay V. Khare, University of Toledo

Wednesday 8:00 AM
October 12, 2022
Room: 408
Location: David L. Lawrence Convention Center

Session Chair: Pankaj Sarin, Oklahoma State University; Waltraud Kriven, University of Illinois at Urbana-Champaign

8:00 AM
A Thermodynamic Database for Ultra-high Temperature Ceramics: Weiwei Zhang¹; Paul Mason¹; Adam Hope¹; ¹Thermo-Calc Software Inc.
    Hypersonic aircraft and space vehicles need ultra-high temperature ceramics that can work under extreme environments. These are typically non-oxides with melting/decomposition temperatures higher than 3000°C, including borides, nitrides, and carbides of Group IV-V metals. There is a great potential for engineers to understand the phase stability, thermodynamic properties and melting temperature etc. for various compositions using Integrated Computational Materials Engineering (ICME) tools to predict the materials behavior and to design materials that can work under different operating conditions. The basis of most ICME frameworks derive from an understanding of the underlying thermodynamics and phase equilibria that drive phase transformations. A thermodynamic database has been developed using the CALPHAD approach for the Hf-Si-Ta-Zr-B-C-N system (including, but not limited to ZrB2-HfB2-ZrC-HfC-TaC-HfN-SiC) that fulfills those needs. All the binaries and 26 ternary systems have been assessed. In total, 35 phases are included. Validations have been made using multicomponent experimental data from the literature.

8:30 AM
Simulation of ZrO2 Phase Transformation for Superelasticity Understanding: Deepak Dhariwal¹; Kathy Lu¹; ¹Virginia Tech
    Martensitic transformation in ZrO2 holds great promise in energy applications such as micro actuation and high energy damping. ZrO2-based superelastic ceramics exhibit phase change under stress, which is extremely sensitive to crystallite size, grain boundary orientation, and particle geometry. Using phase-field approach to describe the kinetics of phase transformation and finite-element analysis for crystallite/oligacrystal deformation, we explore the impact of various material parameters (e.g., single crystal size, oligacrystal grain orientation), geometrical constraints (e.g., shape, size), and loading conditions (e.g., axial, hydrostatic, or dynamic). Physics of the model is highlighted, and optimal material performance is discussed.

8:50 AM
Crystallization Kinetics of Yttria-doped Ytterbium Disilicate Environmental Barrier Coatings: Dawson Smith¹; Molly O'Connor²; Robert Golden³; Marshall Sweet⁴; Rodney Trice¹; Michael Titus¹; ¹Purdue University; ²Praxair Surface Technologies; ³Rolls Royce; ⁴
    Yttria-Doped Ytterbium Disilicate (Y/YbDS) environmental barrier coatings protect Ceramic Matrix Composites (CMCs) in gas turbine engines from reaction with the atmosphere. These coatings are partially amorphous due to high cooling rates in atmospheric plasma spray deposition - thus, they must be crystallized with a heat treatment before use. Crystallization and thermal expansion can crack the coating if the heat treatment is not properly designed. No current study has facilitated design of crack-free heat treatments using crystallization kinetics and strains of Y/YbDS. To do this, we used Differential Scanning Calorimetry (DSC), employing the Ozawa-Flynn-Wall kinetic analysis and Avrami-Erofeev reaction model to calculate transient activation energy, pre-exponential factor, and mechanism of crystallization. These models indicated that activation energy and pre-exponential factor increased during crystallization, while the local Avrami exponent indicated that crystal growth is three dimensional and diffusion-controlled, and that nucleation rate increases with heating rate. Crystallization strains were confirmed with dilatometry.

9:10 AM
Phase Stability of Co-substituted Rare Earth Disilicate Systems for Environmental Barrier Coatings: Christine Brockman¹; V. V. Rohit Bukka¹; Clinton Switzer¹; Amjad Almansour²; Pankaj Sarin¹; ¹Oklahoma State University; ²NASA Glenn Research Center
    Environmental barrier coatings (EBCs) enable the use of ceramic matrix composites (CMCs) in turbine engine components by protecting the CMC from material oxidation and recession in extreme environments. Rare earth (RE) disilicates have been used as an EBC top coat layer to slow the degradation of the underlying EBC layers. Y₂Si₂O₇ is a promising material for EBCs, but the presence of polymorphs at high temperatures limits its usage. A method to stabilize Y₂Si₂O₇ has been investigated, through the co-substitution with another RE material, scandium. In this study, results from our investigations on the effect of the addition of Sc₂Si₂O₇ from 0-40 mol% on the thermal properties of Y₂Si₂O₇ will be presented. Powder samples of Y₂Si₂O₇ and Sc₂Si₂O₇ were synthesized using the steric entrapment method. X-ray diffraction was used to study the change in lattice parameters and phase transformation properties as a function of composition and temperature.

9:30 AM
In-situ High Temperature Coefficient of Thermal Expansion of Metal-diborides Through X-ray Diffraction: Fox Thorpe¹; Elizabeth Sobalvarro Converse²; Jesus Rivera²; Harry Charalambous²; Gabriella King²; Wyatt Du Frane²; Joshua Kuntz²; Scott McCormack¹; ¹University of California, Davis; ²Lawrence Livermore National Laboratory
    The coefficient of thermal expansion was determined for a series of metal-diborides (TiB2, ZrB2, HfB2, NbB2, TaB2) in-situ up to 3000°C. In these experiments, solid spherical samples were suspended and rotated by a gas stream through a conical nozzle levitator, heated by a 400 W CO2 laser at beamline 6-ID-D of the Advanced Photon Source at Argonne National Laboratory. The experiments were conducted in ultra-high purity Argon and an Argon-3% Hydrogen mixture to mitigate oxidation at high temperature. X-ray diffraction patterns suitable for Rietveld refinement were collected at 100 ˚C temperature intervals. An analysis of the degree of anisotropy of the thermal expansion was conducted for each of the diborides. A further study on thermal gradients within the samples using in-situ XRD was conducted on TiB2.

9:50 AM Break

10:10 AM
Phase Control of Polytypic Ba0.5Sr0.5MnO3 Films on Polycrystalline Substrates: Catherine Zhou¹; Gregory Rohrer¹; Paul Salvador¹; ¹Carnegie Mellon University
    We investigate the role of oxygen partial pressure, strain, and substrate orientation on the phase stability of polytypic Ba0.5Sr0.5MnO3 (BSMO) thin films using combinatorial substrate epitaxy (CSE), which is a high throughput film growth method on polycrystalline substrates. The lowest energy structure for BSMO is the hexagonal (4H) perovskite, which differs from the cubic (3C) structure in the stacking sequence of nearly close-packed planes. The challenge is to stabilize 3C-BSMO using pulsed laser deposition and to evaluate the feasibility of stabilizing 3C films with greater Ba-content. Films are characterized from electron backscatter diffraction data using a dictionary indexing approach to identify the local phase and orientation relationship. We find that (100) 3C-BSMO is stabilized at 60 nm on near-(100) substrate orientations, which is thicker than previous studies on single crystals and suggests that the role of strain is not as significant on phase stability as oxygen activity during growth.

10:40 AM
Martensitic Transformation in Shape Memory Ceramic Composites: Donnie Erb¹; Hang Yu¹; ¹Virginia Polytechnic Institute and State University
    Zirconia-based shape memory ceramics represent a distinct family of shape memory materials due to their ability to reversibly transform between tetragonal and monoclinic phases. The shape memory and superelastic effects of zirconia have been demonstrated in the form of micro-pillars and micro-particles. However, owing to the intrinsic brittleness and the large volume change upon martensitic transformation, scaling shape memory ceramics up for bulk applications has been extremely challenging. Here, we explore the scaling-up of shape memory ceramics in the form of metal matrix composites, which are produced using additive friction stir deposition with uniform particle distribution and good interface quality. Stress-induced martensitic transformation is observed during compression of the composites, showing significant energy dissipation in the absence of surface cracks. Thermally-induced reverse martensitic transformation is confirmed in the post-compression samples, wherein no conspicuous peaks are observed in the heat flow measurement.

11:00 AM
HfW₂O₈ and Hf_1-xTi_xW₂O₈ Negative Thermal Expansion and Phase Transformation: Benjamin Hulbert¹; Dylan Blake¹; Kuo-Pin Tseng¹; Waltraud Kriven¹; ¹University of Illinois at Urbana Champaign
    HfW₂O₈ has been the focus of many thermal expansion studies due to its isotropic negative thermal expansion shown previously at temperatures below 700 ℃ as well as the P2₁3 to Pa¯3 phase transformation at 190 ℃. This presents the first thermal expansion measurement of HfW₂O₈ at its equilibrium temperature range of 1100 ℃ to 1275 ℃. The mechanism behind this negative thermal expansion is discussed. The Hf_1-xTi_xW₂O₈ solid solution from x = 0.01 to 0.07 was studied to determine how Ti⁴⁺ substitution affects the phase transformation, thermal expansion, and WO₄ tetrahedra disorder. Powdered samples were synthesized via the organic-inorganic steric entrapment method. Crystallized powdered samples mounted in fused-silica capillaries underwent in-situ, high temperature characterization at the National Synchrotron Light Source II and the Advanced Photon Source in a quadrupole lamp optical furnace to achieve elevated temperatures from 25 ℃ to 1400 ℃ in air.