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Ultra-high temperature ceramics (UHTCs), most notably transition metal carbides and borides, exhibit melting temperatures exceeding 3000 °C, making them appropriate candidates to withstand the extreme temperatures (∼2000 °C) expected to occur at the leading edges of hypersonic vehicles. However, their propensity to react rapidly with oxygen limits their sustained application. The high entropy paradigm enables the exploration of novel UHTC compositions that may improve on the oxidation resistance of conventional refractory mono-carbides and -diborides. The oxidation kinetics of candidate high entropy group IV + V (HfZrTiTaNb)C and (HfZrTiTaNb)B2 materials were evaluated at 1500–1800 °C using Joule heating in one atmosphere 0.1%–1% oxygen/argon gas mixtures for times up to 15 min. Possible mechanisms based on the resulting complex time, temperature, and oxygen partial pressure dependencies are discussed. The carbides formed porous and intergranular oxides. Oxidation resistance was improved upon a continuous external scale formation. The diborides formed dense external scales and exhibited better oxidation resistance compared to the carbides. This improvement was attributed to the formation of liquid boria. Both compositions showed an unexpected reduction in material consumption at 1800 °C for all times tested, compared to results at lower temperatures. An in-depth analysis of the composition and morphology of the oxide scale and sub-surface regions for specimens tested at 1800 °C revealed that the formation of denser group IV-rich (Hf, Zr, Ti) oxides mitigated the formation of the otherwise detrimental liquid-forming group V (Ta, Nb) oxides, leading to the improved oxidation resistance. [ABSTRACT FROM AUTHOR]

ZrB 2 - based pasted, containing 5, 20, 45, 60 vol% short carbon fibers (Cf), were prepared in order to obtain complex shapes via direct ink writing, which were then sintered by hot pressing. The rheological behavior of the pastes indicated that at low Cf concentrations, the fibers act as a lubricant decreasing the paste viscosity. Above 20 vol% Cf, they act also as a binder creating an interconnected network that dominates over the lubrication effect, resulting in a viscosity increase. Complex shapes in the form of a hollow cylinder and a winglet with a functionally graded composition were then extruded and hot pressed, using a versatile powder bed method that enabled full densification with no need to use ad-hoc machined graphite inserts. Bars obtained from the direct ink writing process resulted in high alignment of the short fiber and displayed fracture toughness increase of 54% over that of the unreinforced matrix, while demonstrating a 42% increase with respect to conventionally hot pressed dry mixtures of ZrB 2 and short fibers. This work shows the great potential of manufacturing complex shapes in ultra-high temperature ceramics matrices reinforced with short fibers for a plethora of design possibilities, including functional grading with control over layer composition, thickness and even alignment of short fiber. [Display omitted] [ABSTRACT FROM AUTHOR]

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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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, (Hf 0.2 Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2) carbide and diboride specimens were exposed at 1700°C in 1% O 2 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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract 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 (T m), Group V elements form oxides with low T m , 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. [ABSTRACT FROM AUTHOR]

Boron nitride nanotubes (BNNTs) are gaining interest for high temperature applications due to their thermal stability at elevated temperatures. Although BNNT temperature limits are well studied, there remains a gap in quantifying the high temperature failure mechanisms and oxidation kinetics. This study characterizes the oxidation mechanisms and kinetics of BNNTs and Al 2 O 3 -coated BNNTs, via atomic layer deposition (ALD), at temperatures between 850 and 1000 °C. ALD Al 2 O 3 surface coatings improved BNNT oxidation resistance by over an order of magnitude for coating thicknesses between 7 and 56 nm. However, the improved oxidation resistance of Al 2 O 3 -coated BNNTs decayed after prolonged exposure to oxygen at elevated temperatures, gradually reaching the same extent of oxidation as uncoated BNNTs after approximately 200 min at 900 °C. This behavior is attributed to the crystallization of as-deposited amorphous Al 2 O 3 coatings at elevated temperatures leading to densification, cracking, and exposure of BNNTs to the oxidizing environment. While Al 2 O 3 coatings did not completely prevent oxidation, a significant improvement in oxidation resistance was observed, extending the thermal stability of BNNTs. [ABSTRACT FROM AUTHOR]

This work reports on a pump–probe laser-based heating and sensing metrology to study the failure mechanisms of materials during extreme heat fluxes localized near surfaces, the localization of which is controlled by the focus of the laser beam and sensed by the reflection of a secondary probe laser. We focus the demonstration of these power density at failure tests on the damage mechanisms of commercially pure titanium metal during and after high heat fluxes induced from the absorbed laser energy. Using this steady-state thermoreflectance pump–probe metrology, a localized region of the material was irradiated at a low modulated frequency, while the average change in the thermoreflectance signal was monitored. We observe surface and cross-sectional oxidation of the titanium, revealing correlations between microstructural evolution events and shifts in thermoreflectance trends as a function of absorbed power density. Furthermore, the damage morphology was shown to be heavily influenced by the size of the heater (dictated by the radius of the pump laser beam), which controlled the relative degree of thermomechanical, melting, and oxidative decohesion failure mechanisms in the samples. The analysis of the temperature distribution coupled with the observed microstructural damage gives rise to a high-throughput experimental technique to induce desired deformation modes through cyclic thermal testing. [ABSTRACT FROM AUTHOR]

Refractory high entropy alloys (RHEAs) hold the promise of superior mechanical properties at high temperatures but are plagued by a lack of oxidation resistance. In this work, the role of Al additions (4.8, and 13 at.%) to HfNbTaTiZr is explored as a means of improving protective oxide scale formation in RHEAs. Oxide formation was resolved using STEM-EDS/EELS, precession electron diffraction, and computational predictions of formation energy. Our results show that oxides with large negative formation energies (i.e., HfO 2 /ZrO 2) always formed near the metal/oxide interface while Al 2 O 3 and oxides with less negative formation energies formed near the surface. The addition of Al prevented pesting in both alloys and formed Al 2 O 3 with 13 at.% Al. While the Al 2 O 3 formed was not continuous, we report the lowest threshold yet to prevent pesting with only 4.8 at.% Al added. These findings provide guidance to future alloy development of Al 2 O 3 -forming alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) are common pump-probe techniques that are used to measure the thermal properties of materials. At elevated temperatures, transducers used in these techniques can become limited by melting or other phase transitions. In this work, time domain thermoreflectance is used to determine the viability of HfN thin film transducers grown on SiO2 through measurements of the SiO2 thermal conductivity up to approximately 1000 K. Further, the reliability of HfN as a transducer is determined by measuring the thermal conductivities of MgO, Al2O3, and diamond at room temperature. The thermoreflectance coefficient of HfN was found to be 1.4x10-4K-1 at 800 nm, one of the highest thermoreflectance coefficients measured at this standard TDTR probe wavelength. Additionally, the high absorption of HfN at 400 nm is shown to enable reliable laser heating to elevate the sample temperature during a measurement, relative to other transducers. [ABSTRACT FROM AUTHOR]

The oxidation resistance of Ni-based alloys is typically improved by additions of elements segregating to grain boundaries, contributing to reducing scale growth, and improving scale adherence. To develop a mechanistic understanding of doping, outward Al and inward O fluxes through the scale formed on β-NiAl with Ti, Y, and Ti+Y additions were characterized for oxidation at 1200 °C and times up to 100 h. Doped alloys exhibited varying reductions in oxidation rates and Al and O fluxes affecting scale grain morphologies. Ti+Y co-doping further reduced the outward Al and inward O transport compared to doping with Ti or Y alone. • Ti reduces inward oxygen transport and outward Al transport, and promotes preferential formation of new oxide along grain boundaries. • Y decreases outward Al transport and inward O transport and nearly suppresses the formation of the equiaxed region • Y and Ti co-doping reduces inward oxygen transport and outward aluminum transport compared to doping with Y only. • A continuous new oxide layer forming along grain boundaries is associated with grain boundary migration. [ABSTRACT FROM AUTHOR]