High Temperature Oxidation of Metals and Ceramics: Oxidation of Ceramics and HEA/Refractory Alloys
Sponsored by: TMS Corrosion and Environmental Effects Committee
Program Organizers: Kenneth Kane, Oak Ridge National Laboratory; Elizabeth Sooby, University Of Texas At San Antonio; Patrick Brennan, General Electric Research; Lavina Backman, U.S. Naval Research Laboratory; Kinga Unocic, Oak Ridge National Laboratory; Richard Oleksak, National Energy Technology Laboratory; David Shifler, Office of Naval Research; Raul Rebak, GE Global Research

Wednesday 2:00 PM
October 12, 2022
Room: 335
Location: David L. Lawrence Convention Center

Session Chair: Lavina Backman, Naval Research Laboratory

2:00 PM  Invited
Alumina Forming MAX Phases: Current Status and Future Perspectives: Miladin Radovic¹; Yexaio Chen²; DongGi Ha¹; James Smialek³; ¹Texas A&M University; ²ASM; ³NASA Glenn
    MAX phases are a family of 80+ ternary carbides and nitrides, and even larger number of their solid solutions, that share common unique naonolayered structure and chemical formula Mn+1AXn, where n = 1, 2 or 3, M is and early transition metal, A is an A-group element and X is either C or N. The main reason for growing interest in MAX phases lies in their unusual, and sometimes unique combination of mechanical and thermal properties that makes them a good candidates for structural applications at high temperatures. Most importantly for their applications at high temperature, some of them – most notably Ti2AlC and Cr2AlC – shows exceptional oxidation resistance due to formation of the protective, self-healing and adherent alumina protective oxide scale. In this presentation, our current understanding of what makes some of the MAX phases exceptionally oxidation resistant will be reviewed and discussed in more details.

2:30 PM
Design of Ultra-high Temperature Ceramics for Oxidation Resistance : Niquana Smith¹; Elizabeth Opila¹; ¹University of Virginia
    Ultra-high temperature ceramics (UHTCs) are capable of withstanding extreme temperatures. Specifically, hafnium carbide (HfC) and tantalum carbide (TaC) have the highest melting points amongst UHTCs, however, rapid oxidation of these materials is a persistent problem. In this work, HfC, TaC, and nominally single phase (Hf,Ta)C mixtures were densified using spark plasma sintering. Oxidation exposures were conducted in a resistive heating system at 1400°C in 1% oxygen/argon for times up to 10 minutes. Scanning electron microscopy and energy dispersive spectroscopy were used to characterize the oxide morphology, composition, and to determine the oxide thickness. Oxide thickness results demonstrated that pure TaC oxidizes more rapidly than pure HfC. The resulting oxide of the mixed (Hf,Ta) carbides was denser and thinner compared to the end members. X-Ray diffraction analysis showed that the complex oxide Hf6Ta2O17 was the predominant oxide phase, contributing to the change in oxide morphology and reduced oxide thickness.

2:50 PM
High Temperature Oxidation Behavior of Ta vs TaC: Connor Stephens¹; Elizabeth Opila¹; ¹University of Virginia
    Ultra-high temperature materials are needed for leading-edge components of hypersonic vehicles which can withstand large stresses and high temperatures in highly oxidizing environments. Select transition metals and metal carbides have high strength and form stable oxides at “ultra-high” temperatures (Tm > 1700°C). However, they have notably poor oxidation resistance and the oxidation mechanisms above 1300°C are not well understood. Additionally, a direct comparison of the high-temperature oxidation behavior of metals and carbides has not been previously reported. In this work, the oxidation kinetics of Ta and TaC were studied by resistively heating samples to 1300-1500°C in a 1% O2 (bal. Ar) environment for 2-10 minutes. The resulting oxides were characterized via SEM (plan view and cross section) and XRD. The carbide exhibited a more rapid oxidation rate than the metal. Further comparison of the high temperature oxidation behavior of these materials will be discussed.

3:10 PM
Evaluating the Oxidation Behavior of 1300C Capable Nb-Si-based Alloys: Patrick Brennan¹; Rebecca Casey¹; Chen Shen¹; Scott Oppenheimer¹; Bernard Bewlay¹; Akane Suzuki¹; ¹General Electric Research
    This presentation will cover the efforts to study the oxidation behavior of novel Nb-Si-based alloys strengthened with silicide phases that are being developed for the application of 1300C capable stage 1 turbine blades for land-based power generation gas turbines. The design challenge for developing such a Nb-based alloy is achieving a balance between room temperature fracture toughness, high temperature tensile strength and creep resistance, and resistance to high temperature oxidation. The primary focus of this presentation will be to report how the composition and microstructure of the candidate alloys affects the oxidation mechanisms and the extent of degradation observed in Nb-Si alloys during exposure in air at 800 °C and 1300 °C as well as the trade-offs required to balance environmental stability with desirable mechanical properties in Nb-Si systems.

3:30 PM Break

3:50 PM
High Temperature Oxidation Behavior of Equimolar NbTiZr: Charlie Brandenburg¹; David Beaudry²; Michael Waters³; Lauren Walters³; Elaf Anber²; Jean-Philippe Couzinie⁴; Loic Perriere⁴; Mitra Taheri²; James Rondinelli³; Elizabeth Opila¹; ¹University of Virginia; ²Johns Hopkins University; ³Northwestern University; ⁴Institut de Chimie et des Matériaux Paris-Est
    Refractory Multi-Principal Element Alloys (RMPEAs) are of interest for their favorable mechanical properties at high temperatures, however their oxidation mechanisms are not well understood. The oxidation kinetics of equimolar NbTiZr were studied by thermogravimetric analysis in the temperature range of 900-1250°C in 1% oxygen (balance argon). The oxidation kinetics were non-parabolic at all temperatures but slowed with time. A minimum oxidation rate was observed at 1050°C. Microscopy and X-ray diffraction analysis revealed that the outer oxide scale consisted of TiO2, and complex oxides TiNb2O7 and Zr6Nb2O17. Sample cross sections revealed several distinct layers of a multi-phase oxide microstructure. Spinodal decomposition of the alloy into high oxygen content Zr-rich and low oxygen content Nb-rich regions was observed at the oxide-alloy interface which acted as a template for the inner oxide layers. The observed decomposition is consistent with Monte Carlo simulations.

4:10 PM
Microstructure, High Temperature Oxidation and Mechanical Properties of Fe-Cr-Ni-Al Medium Entropy Alloy: Yu-Jin Hwang¹; Kyu-Sik Kim²; Young-Sang Na³; Ka-Ram Lim³; Kee-Ahn Lee¹; ¹Inha University; ²Agency for Defense Development; ³Korea Institute of Materials Science
    Fe-based medium entropy alloy is a material that has recently attracted interests because it can lower alloying cost and has excellent mechanical properties compared to common high entropy alloys. In this study, non-equiatomic Fe-Cr-Ni-Al medium entropy alloy (MEA) was proposed, and its microstructure, high temperature oxidation and mechanical properties were investigated. Fe-Cr-Ni-Al MEA consisted of BCC Fe-Cr-rich matrix and B2 Ni-Al-rich precipitates about 150nm size. Isothermal oxidation tests were performed at 800, 900, 1000, 1100℃ in air environment for 24 hrs. As a result, outstanding oxidation resistances could be confirmed in this MEA alloy. After isothermal oxidation, α-Al2O3 layer was uniformly distributed at specimen surface regardless of oxidation temperature. Room temperature and 550℃ compression test results showed that the compressive yield strengths were 1150MPa and 757MPa, respectively. Based on the above results, the effect on the microstructural characteristic on the oxidation mechanism and deformation behavior were also discussed.

4:30 PM
Thermochemical Stability of High Entropy Rare Earth Oxide (HERO) Coatings for Refractory Alloys: Kristyn Ardrey¹; Elizabeth Opila¹; Patrick Hopkins¹; Bicheng Zhou¹; Prasanna Balachandran¹; ¹University of Virginia
    High entropy rare earth oxide (HERO) coatings are under investigation for application on Nb-base alloys to increase operating temperatures in aircraft turbine engines. Nb-base alloys are the most formable of the refractory alloys, but they lack the ability to form a protective oxide layer, making an environmental barrier coating essential. Potential coating candidates for use in a combustion environment are rare earth oxides. Rare earth oxides have a similar thermal expansion with Nb-base alloys and provide thermochemical stability in a combustion environment, giving them the ability to act as an environmental barrier coating (EBC). Furthermore, with the high entropy approach, thermal conductivity may be reduced by mixing multiple rare earth oxides into a single HERO coating, providing dual thermal/environmental barrier coating capability. This presentation will focus on the effect of HERO coating composition on thermochemical stability in turbine environments and interactions with the underlying Nb-base alloy during high temperature exposures.

4:50 PM
High-temperature Oxidation-resistant Mechanism of Refractory High Entropy Alloy: Kun Wang¹; Yonggang Yan¹; ¹Alfred University
    Oxidation-resistant refractory high entropy alloys (RHEAs) which consist of refractory metal elements and oxidation-resistant metal elements, are prospective materials to sustain high-temperature oxidation up to 1000 °C or above for a long duration. In this work, the refractory elements are selected from W, Mo, Ta, V, Nb, Cr, Ti etc., while the oxidation-resistant elements are selected from Ti, Al, Cr etc., to gain different composition combinations. The single-phase equimolar oxidation-resistant RHEAs are predicted by machine learning. Then multiple single-phase bulk RHEAs are prepared by the powder metallurgy method. The oxidation experiments of RHEAs are conducted at 1000 °C for various oxidation times. The RHEAs with superior oxidation resistance are selected for the subsequent nanoscale microstructural analysis. A diffusion-controlled three-layers oxide scale model was proposed to explain the oxidation kinetics and uncover the oxidation-resistance mechanism, which assists to guide the design of oxidation-resistant RHEAs.

5:10 PM
A Computational Investigation of the Early Stages of Degradation of High Entropy Alloy Surfaces: Adib Samin¹; Tyler Dolezal¹; ¹Air Force Institute of Technology
    The early stages of oxidation on the surface of Al₁₀Nb₁₅Ta₅Ti₃₀Zr₄₀ were studied using Density Functional Theory and thermodynamic modeling. Surface slabs were generated from a bulk configuration sampled from equilibrium using a multicell Monte Carlo method for phase prediction. The oxygen adsorbed with a preference towards sites with Ti and Zr. The surface was highly reactive to oxygen, yielding a dominating oxygen coverage of two monolayers over the temperature range of 100 to 2600 K and oxygen pressure range of 10-30 to 105 bar. Inward oxygen diffusion at low coverage was preferred in regions rich with Zr but slowed with the addition of Ti and Al. Diffusion rates drastically reduced at 1 ML, especially in the region rich with Ti and Zr, where strong metal-oxygen bonds were reported. Our results indicated that a high content of Ti and Zr increased the reactivity of the HEA surface to oxygen.