What is it about?
The high entropy paradigm for materials design hypothesizes that when multiple principal elements are present in a solid solution, such as in an alloy, the resulting properties are greater than the sum of their parts. This paradigm has been extended to ultra-high temperature ceramics (UHTCs), which are generally defined as refractory metal carbides, borides and nitrides with melting temperatures greater than 3000C, of which there were only a handful before 2015. With the advent of high entropy UHTCs, material possibilities for applications such as rocket nozzles and leading edges of hypersonic craft, have expanded significantly. This work is a systematic evaluation of the oxidation behavior of two promising high entropy UHTC compositions at flight-relevant temperatures and investigates the complex relationship between a high temperature oxidizing environment and a multi-principal element ceramic.
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Why is it important?
When we fly faster, we fly hotter. When we explore new worlds such as Venus, we push the boundaries of currently available materials. An understanding of how new material design paradigms impact material properties, such as oxidation resistance in this case, critically enables further advancement of new aerospace technologies. In this publication, we identify the most likely oxidation mechanisms operative between 1500-1800C, and the impact of complex compositions on these mechanisms.
Perspectives
This work found that the equimolar (HfZrTiTaNb) carbide and diboride materials performed the best at higher temperatures (~1800C), which is a promising finding for this class of materials that are expected to used at temperatures greater than 2000C. Future materials design efforts can then take these findings to engineer UHTCs with the desired oxide microstructures.
Lavina Backman
Naval Research Laboratory
Read the Original
This page is a summary of: Invited Article: The oxidation kinetics and mechanisms observed during ultra-high temperature oxidation of (HfZrTiTaNb)C and (HfZrTiTaNb)B2, Journal of Applied Physics, August 2024, American Institute of Physics,
DOI: 10.1063/5.0206227.
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Resources
Composition dependence of oxidation resistance in high entropy ultra-high temperature ceramics
High entropy ultra-high temperature ceramics (HE-UHTCs) have garnered intense research interest due to the potential for optimized oxidation and mechanical properties for extreme environment applications. HE-UHTCs are expected to oxidize according to the thermodynamic favorability of their respective oxidation reactions, which varies according to the periodic grouping. Based on this, the oxidation resistance of equimolar (metals-basis) group IV + V (HfZrTiTaNb), group IV + V + VI (HfZrTiTaMo) and group IV + VI (HfZrTiMoW) carbides and borides were evaluated at 1700°C in 1 mol% O2 for 5 min and compared. Group IV elements oxidized preferentially in all three compositions. Group V element-containing carbides exhibited the lowest oxidation resistance, attributed to the formation of intergranular liquid oxides. (HfZrTiMoW)C exhibited the best resistance among the carbides. The diborides exhibited similar material consumption, reinforcing the hypothesis that the oxidation behavior under these conditions is controlled by the presence of boria. These findings provide direction for HE-UHTC composition design for oxidation resistance.
Part I: Theoretical predictions of preferential oxidation in refractory high entropy materials
High entropy materials, which include high entropy alloys, carbides, and borides, are a topic of substantial research interest due to the possibility of a large number of new material compositions that could fill gaps in application needs. There is a current need for materials exhibiting high temperature stability, particularly oxidation resistance. A systematic understanding of the oxidation behavior in high entropy materials is therefore required. Prior work notes large differences in the thermodynamic favorability between oxides formed upon oxidation of high entropy materials. This work uses both analytical and computational thermodynamic approaches to investigate and quantify the effects of this large variation and the resulting potential for preferential component oxidation in refractory high entropy materials including group IV-, V- and VI-element based alloys and ceramics. Thermodynamic calculations show that a large tendency towards preferential oxidation is expected in these materials, even for elements whose oxides exhibit a small difference in thermodynamic favorability. The effect is reduced in carbides, compared to their alloy counterparts. Further, preferential oxidation in high entropy refractory materials could result in possible destabilization of the solid solution or formation of other, competing phases, with corresponding changes in bulk material properties.
Part II: Experimental verification of computationally predicted preferential oxidation of refractory high entropy ultra-high temperature ceramics
Refractory high entropy materials have garnered significant research interest due to their potential ability to fill a need in high temperature structural applications. However, challenges remain with respect to designing for oxidation resistance. A knowledge gap exists with respect to a rigorous understanding of the mechanisms driving oxidation processes unique to high entropy materials. This work provides an experimental complement to a companion publication, which outlines analytical and computational thermodynamic approaches that are envisioned to aid the design of refractory high entropy materials containing group IV (Hf, Zr, Ti) and group V (Ta, Nb) constituents. In this work, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2) carbide and diboride specimens were exposed at 1700°C in 1% O2 for 5 min. Experimental results show good agreement with the computational predictions for the same temperature, despite differences in the overall morphology of the oxidized regions. The carbide formed porous oxides, while the diboride formed a denser external scale. Oxidation products are dominated by group IV oxides, depleting the underlying materials, which were found to consist of primarily group V carbides and borides respectively. The results provide a first look at the oxidation of high entropy UHTCs at ultra-high temperatures and validate the preferential nature of high entropy material oxidation predicted by the computational approach developed for the study of this new class of materials.
Thermodynamic assessment of the group IV, V and VI oxides for the design of oxidation resistant multi-principal component materials
Multi-principal component materials (MPCMs) are currently being investigated for use in high and ultra-high temperature environments. The design of oxidation resistant multi-component materials requires as input the oxidation behavior of each of the components. FactSage free energy minimization software and databases were used to calculate the equilibrium oxide phases and free energies of formation for the oxides of the Group IV, V and VI refractory metals, and their carbides, nitrides and borides. The results are summarized in Ellingham diagrams. Periodic trends were noted; Group IV elements form the most stable oxides with the highest melting temperatures (Tm), Group V elements form oxides with low Tm, and Group VI elements form gaseous oxide species. Oxygen diffusion data from literature for some of these oxides were also reviewed and summarized. The results are utilized to identify strategies for optimizing oxidation resistance of MPCMs for service at temperatures above 1700°C.
Analysis of Test Specimen Temperature Gradients Incurred in Resistive Heating System Oxidation Studies of Ultra-High Temperature Ceramics
The need for advanced materials that can meet application requirements at ultra-high temperatures in oxidizing environments is an area of active research. One challenge facing the high temperature materials community is the ability to conduct controlled ultra-high temperature oxidation tests with minimal to no contamination or reaction with the chamber. A unique resistive heating system (RHS) capable of achieving ultra-high temperatures (> 1700 °C) to enable such experimentation is described. A concern of such a system is the potential presence of thermal gradients in directions not reflective of actual material applications, e.g., the hottest region being in the center of the sample. Experimental results from the oxidation of ZrB2 specimens at nominal temperatures of 1500°, 1700° and 1800 °C in low pO2 (0.1–1% O2 in Ar) environments are presented. Specimen thermal gradients generated during oxidation were evaluated using finite element analysis models. Thermal gradients on the order of the uncertainty in temperature measurements were calculated, confirming the RHS suitability for conducting ultra-high temperature oxidation exposures on ultra-high temperature ceramics.
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