Ultra-high Temperature Applications Research Articles

Mechanical properties of pristine and nanocrystalline graphene up to ultra-high temperatures

Despite being the basic building block of most carbon materials used in high and ultra-high temperature applications, the mechanical properties of graphene at such temperatures remain totally unknown. Here we compute these properties using tensile molecular dynamics simulations at temperatures ranging from room temperature to 5000 K (i. e. about the melting temperature) for pristine and nanocrystalline graphene. Mechanical simulations at such temperatures are made possible via the use of the screened environment dependent reactive empirical bond order potential [Perriot et al. Phys. Rev. B 88, 064101 (2013)]. We show that even at the highest temperature, both graphene and nanocrystalline graphene retain high stiffness, strength and fracture strain. Analyses of tensile curves and trajectories (including an analysis of in-plane non-affine atomic displacements) show that both pristine and nanocrystalline graphene show brittle rupture, whatever the temperature, even though some early sign of ductility can be detected for the nanocrystalline graphene at 5000 K. This rules out the possibility of a ductile fracture in graphene at any temperature below the melting point. Finally, the simulations also highlight the importance of bond rotation (Stone-Wales) based defects in the fracture initiation mechanism for pristine graphene. Especially at intermediate temperature (3000 K) crack nucleation in pristine graphene is shown to take place exactly at the moment and place of the first Stone-Wales defect formation.

Grain boundary segregation induced strong UHTCs at elevated temperatures: A universal mechanism from conventional UHTCs to high entropy UHTCs

Ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties, including ultra-high melting point, excellent chemical stability, and good oxidation resistance, which make them promising candidates for aerospace and nuclear applications. However, the degradation of high-temperature strength is one of the main limitations for their ultra-high temperature applications. Thus, searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed. To achieve this goal, grain boundary segregation of a series of carbides, including conventional, medium entropy, and high entropy transition metal carbides, i.e., Zr0.95W0.05C, TiZrHfC3, ZrHfNbTaC4, TiZrHfNbTaC5, were studied by atomistic simulations with a fitted Deep Potential (DP), and the effects of segregation on grain boundary strength were emphasized. For all the studied carbides, grain boundary segregations are realized, which are dominated by the atomic size effect. In addition, tensile simulations indicate that grain boundaries (GBs) will usually be strengthened due to segregation. Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs, since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.

Effect of W and C addition on the microstructure and phase composition of W–ZrC composites prepared by using Zr2Cu alloy and variant reactant compositions

W–ZrC composites were successfully fabricated by reacting amounts of Zr2Cu alloy with powder mixtures of varying composition including stoichiometric WC, WC with additional/W (WC/W), and WC with additional/C (WC/C). The use of WC/C powders to reactively form W–ZrC composites with increased ZrC phase content is a new approach that shows potential for ultra-high temperature applications where higher ZrC content, relative to W, is preferred such as land and aeroengines, aerospace thermal protection, and nuclear. The focus of this work was to stabilize diffusion between W and ZrC during formation, limit interfacial phases, and increase the ZrC content. This was done by varying reactant powder compositions while understanding the final phase composition and microstructure. The composites were prepared by heating the powder mixtures to 1400 °C for 8 h in a furnace with 96% Ar - 4% H2 gas atmosphere. The W–ZrC sample prepared using WC/C powders gave the W–ZrC composite with the highest ZrC phase compared to W and showed promise for phase stabilization. Additionally, there was no W2C, or residual WC identified in the final composite indicating complete conversion from WC to ZrC when adding additional carbon to the WC powder. WC/C sample had the highest hardness due to the increased ZrC phase content. This research addresses the critical comparison of carbon addition on the W–ZrC composites and produced composite with high ZrC content, limited interfacial phases and interlocking microstructure.

Investigation of the phase formation in magnetron sputtered hard multicomponent (HfNbTiVZr)C coatings

Multicomponent carbides have gained interest especially for ultra-high temperature applications, due to their ceramic hardness, good oxidation resistance and enhanced strength. In this study the phase formation, stability and mechanical properties of (HfNbTiVZr)C multicomponent carbide coatings were investigated. Phase stability was predicted by the CALPHAD (CALculation of PHAse Diagrams) methods. This revealed that the multicomponent solid solution phase is only stable at elevated temperatures, namely above 2400 °C. At lower temperatures a phase mixture was predicted, with a particular tendency for V to segregate. Magnetron sputtered thin films deposited at 300 °C exhibited a single NaCl-type multicomponent carbide phase, which attributes to the kinetic stabilisation of simple structures during thin film growth. Films deposited at 700 °C, or exposed to UHV annealing at 1000 °C, however, revealed the decomposition of the single-phase multicomponent carbide by partial elemental segregation and formation of additional phases. Thus, confirming the CALPHAD predictions. These results underscore the importance of explicitly considering temperature when discussing the stability of multicomponent carbide materials, as well as the applicability of CALPHAD methods for predicting phase formation and driving forces in these materials. The latter being crucial for designing materials, such as carbides, that are used in applications at elevated temperatures.

Laser powder bed fusion of MoSiBTiC alloy powders produced by freeze-dry pulsated orifice ejection method

A novel MoSiBTiC alloy was fabricated via laser powder bed fusion (L-PBF) combined with the freeze-dry pulsated orifice ejection method (FD-POEM). Spherical MoSiBTiC powders with uniform size distribution and good dispersion of component elemental particles were prepared by FD-POEM, without a melting process. Due to their distinctive mesh-like structure, the laser absorptivity of the FD-POEM particles was as high as 87.3%, allowing the elemental powders to be sufficiently melted under laser irradiation. The L-PBF build was primarily composed of a unique supersaturated solid solution Mo (Mosss) dendrites enriched with Si, Ti and B elements, Mo + Mo5SiB2 nanostructures, and TiC nanoparticles. The Mosss phase exhibited much higher hardness than Mo phase of the as-cast alloy attributing to the grain refinement and the solid strengthening effect. These results of this study provided an approach of fabricating high-performance Mo-based materials in ultrahigh-temperature applications.

Ablation behaviour of Cf–ZrC-SiC with and without rare earth metal oxide dopants

Continuous carbon fibre/carbon composites are candidates for ultra-high temperature applications due to their ability to retain their strength at elevated temperatures, however they easily oxidize and ablate in high temperature oxygen-based environments, a fact that seriously limits their applications in the aerospace industry. The introduction of ultra-high temperature ceramics (UHTCs) as a matrix in carbon fibre preforms effectively improves their oxyablation resistance at ultra-high temperatures in air atmospheres. In the present study, zirconium carbide-based ultra-high temperature ceramic matrix composites (UHTCMCs) simultaneously doped with silicon carbide and rare earth (RE) metal oxides were prepared by a slurry impregnation and pyrolysis method using phenolic resin as the bonding agent. The combination of injection and vacuum impregnation achieved a maximum density of up to 60% of theoretical. These composites were subsequently evaluated in terms of their ablation properties using an oxyacetylene (OAT) torch at temperatures of ∼2500 °C for 60 s. No significant surface damage was observed on any of the ablated samples. When ceria was used as the RE metal oxide dopant, the evidence suggests that cerium silicate was formed by the ceria dissolving in the molten silica; though this didn't appear to offer very much additional oxyablative protection. In contrast, when yttria was used, it improved the performance via stabilization of the cubic zirconia that formed on oxidation; this provided the greatest protection of all the samples tested. The combination of both ceria and yttria yielded the combined mechanism.

Durable crosslinked films based on poly (arylene ether nitrile) materials for ultrahigh temperature applications over 300°C

Searching for outstanding films with high temperature resistance has sparked fierce interests in the electronics industry. In this study, a novel high-temperature-resistance phthalonitrile end-capped polyarylene ether nitrile (HTR-PEN-Ph) film was fabricated via cross-linking reaction, applying two different curing programs as contrast. The fabricated HTR-PEN-Ph films were verified through FTIR, gel content test to be confirmed the cross-linking reaction. Then thermal results elucidated that PEN-Ph films treated with two-stage curing program possessed a superior glass transition temperature ( T g) in comparison with untreated one, increasing by 165–270°C. Besides, an evident increment of 5 wt.% decomposition temperature ( T 5%) was seen from the HTR-PEN-Ph film, which was 27–43°C higher than the untreated one. Furthermore, the HTR-PEN-Ph films exhibited notable dielectric stability over 300°C and mechanical properties after the two-stage curing program. Based on these satisfactory results, this study is of great potential to be applied in the field of industrial manufacture to fabricate a range of high-performance films.

Effect of Mo on high-temperature strength of refractory complex concentrated alloys: A perspective of electronegativity difference

Refractory complex concentrated alloys, RCCAs, show great potential for ultrahigh-temperature applications. High-temperature strength is one of the key requirements for RCCAs to qualify for that purpose. Some RCCAs already show superior high-temperature strength than that of commercial Ni-based superalloys, but many other RCCAs do not. It is thus important to identify the key factors that control the high-temperature strength of RCCAs. In this work, based on a statistic analysis of the yield strength at 1000 °C (σy1000) for 55 reported RCCAs, interestingly, it is revealed that Mo-containing RCCAs have in general a higher σy1000 than those RCCAs not containing Mo. The effect of Mo is attributed to its larger electronegativity and hence the larger electronegativity difference to other alloying elements, rather than to its higher melting point. The previously established proposal that a large electronegativity difference favoring the charge transfer and creating an atomic-level pressure that contributes to the strengthening of RCCAs at room temperature, seems effective even at a high temperature of 1000 °C. In addition, the large electronegativity difference also favors the room-temperature strength of single-bcc-phase RCCAs. The findings from this work, further verified experimentally in six new RCCAs, shed light on new research directions to develop RCCAs with decent strength at both high temperatures and room temperature, using a simple descriptor on the electronegativity difference.

On phase stability of Mo-Nb-Ta-W refractory high entropy alloys

Refractory high entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. Accurate phase stability is the prerequisite for design of high-performance RHEAs, but still missing in the literature due to the big challenge in experiments. In this paper, the reliable 3rd-generation Gibbs energy expressions for pure Mo, Nb, Ta and W were firstly evaluated by integrating the physical based Segmented Regression model with thermal vacancy description from 0 K to high temperatures. The thermodynamic database of the Mo-Nb-Ta-W quaternary system was then developed by combining the established 3rd-generation Gibbs energies with the phase equilibrium and thermodynamic data by using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated phase equilibria, thermodynamic properties as well as the A2/B2 ordering transition behaviors show good agreement with those from experimental determination and theoretical calculation. The phase constitutes and elemental distributions in an equiatomic MoNbTaW as-cast alloy were experimentally investigated and compared with the non-equilibrium solidification simulations to further verify the database reliability. The high-throughput mapping of phase stability within the composition-temperature space in various Mo-Nb-Ta-W alloys was then constructed by applying thermodynamic calculations. The alloying effects on the formation of B2 phase were systematically analyzed. The Mo-Nb-Ta-W alloys have a wide temperature region with a single A2 phase, while the A2 + B2 phase regions exist at very low temperatures. It is highly anticipated that the present accurate 3rd-generation thermodynamic database provides an important basis for efficient design and development of novel Mo-Nb-Ta-W RHEAs.

Revealing Microstructural Features and Mechanical Properties of Si-B Alloys – Materials for Novel Functional and Structural Ultra-High Temperature Applications

Revealing Microstructural Features and Mechanical Properties of Si-B Alloys – Materials for Novel Functional and Structural Ultra-High Temperature Applications

Laser additive manufacturing of pure molybdenum using freeze-dry pulsated orifice ejection method-produced powders

Preparing appropriate initial powders is a key factor in using laser powder bed fusion (L-PBF) for high-performance refractory materials. In this study, we developed a novel technology, the freeze-dry pulsated orifice ejection method (FD-POEM), for producing spherical molybdenum powders with controllable particle size and distribution and good flowability without melting. The Mo powders, which had unique mesh-structures, exhibited higher laser absorptivity than the dense ones. Pure Mo builds were fabricated using L-PBF. Fine MoO2 particles were formed and uniformly distributed along the grain boundaries and inside the equiaxed grains. High-resolution transmission electron microscopy revealed an interfacial orientation correlation between Mo [1 -1 -1]//MoO2 [0–1 0] and Mo (1 1 0)//MoO2 (0–1 2). The Vickers hardness of the Mo build increased compared to that of pure Mo owing to grain refinement and the oxide-dispersion-strengthening effect of in-situ MoO2. This study provides the basis for powder fabrication using FD-POEM and the processability of Mo parts via L-PBF in ultrahigh-temperature applications.

Enhanced ferroelectric, piezoelectric and dielectric properties of (1-x)CaBi2Nb2O9-xBaZr0.2Ti0.8O3 high-temperature piezoelectric composite ceramics

Among Aurivillius layer-structured materials, CaBi2Nb2O9 is a best potential candidate for ultrahigh-temperature applications because of its highest Curie temperature of about 940 °C. In this paper, (1-x)CaBi2Nb2O9-xBaZr0.2Ti0.8O3 composite ceramics were prepared by conventional solid-state sintering method. The dielectric results show that the introduction of BaZr0.2Ti0.8O3 not only increases the permittivity of the material, but also reduces its dielectric loss. The optimum electrical properties were obtained in the x = 0.01 sample with piezoelectric coefficient (d33) of 15.1 pC/N and high ferroelectric remnant polarization (Pr) of 9.9 μC/cm2. Furthermore, the composite samples show good thermal depoling performance, the d33 of the x = 0.01 sample is 13.8 pC/N, which is about 91% of the initial value after depoling at 800 °C. Therefore, (1-x)CaBi2Nb2O9-xBaZr0.2Ti0.8O3 is one of the candidates for high temperature piezoelectric materials.

Microstructure and Oxidation Behavior of Nb-Si-Based Alloys for Ultrahigh Temperature Applications: A Comprehensive Review

Nb-Si-based superalloys are considered as the most promising high-temperature structural material to replace the Ni-based superalloys. Unfortunately, the poor oxidation resistance is still a major obstacle to the application of Nb-Si-based alloys. Alloying is a promising method to overcome this problem. In this work, the effects of Hf, Cr, Zr, B, and V on the oxidation resistance of Nb-Si-based superalloys were discussed. Furthermore, the microstructure, phase composition, and oxidation characteristics of Nb-Si series alloys were analyzed. The oxidation reaction and failure mechanism of Nb-Si-based alloys were summarized. The significance of this work is to provide some references for further research on high-temperature niobium alloys.

Development of platinum-group-metals-containing high entropy alloys with outstanding thermal capability and hot hardness

Four equi- or near equi-molar platinum-group-metals-containing high entropy alloys (PGM-HEAs) have been developed with the aim to identify promising materials for ultra-high temperature applications. Experimental results indicate that IrNiPtRh and IrNiPtRhCo HEAs can form single phase fcc solid-solution, while secondary phases are also formed within the fcc matrix in IrNiPtRhAg0.5 and IrNiPtRhAl0.5 alloys, which are identified as (Ag, Pt)-rich fcc2 and (Pt, Ni)3Al-L12 phases, respectively. For mechanical performance, the room to high temperature hardness of PGM-HEAs is outstanding, especially for IrNiPtRhAl0.5, which shows significantly improved hot hardness than those of Ni-based superalloys, such as IN718 and Udimet 720Li, at temperatures above 800 °C, while it also exhibits much higher melting tolerance. It is related to the contributions from the advantages of platinum-group-metals, strong solution hardening, and L12 precipitation strengthening, thus providing superior resistance against thermal softening. It is believed that the current findings will shed light toward a promising research direction for ultrahigh-temperature applications, which is hard to achieve for the traditional platinum-group-alloys. In addition, the design space of compositionally complex alloys for future investigations would be significantly broadened.

Challenges of Developing Ductile and Lighter Refractory High Entropy Alloys

In the recent past refractory high-entropy alloys (RHEAs) have attracted a lot of attention due to their interesting mechanical properties. These are simple solid solution alloys either with face-centered cubic (FCC) or body-centered cubic (BCC) crystal structures. BCC alloys exhibit relatively higher strengths at high temperatures and therefore, can be considered as structural materials for ultrahigh temperature applications (≥ 1300 ºC). However, the main challenge in developing BCC structured RHEAs for structural applications is balancing their high temperature (HT) strength with their room temperature (RT) tensile ductility and density. Therefore, searching for new and lighter RHEAs with optimized strength and tensile ductility is still a challenge. Improvement of oxidation resistance RHEAs is also an important issue as refractory metals are prone to oxidation in air at high temperatures. This paper describes various theoretical and experimental research works to develop stronger, ductile and lighter RHEAs alloys with novel compositions. The paper also describes various attempts in the development of RHEAs with high creep strength, RT ductility, fracture toughness, lower density, thermo-mechanical fatigue resistance, and host of other physical properties. It will also describe the advance manufacturing techniques in fabricating the components out of these compositions.

Ablation resistant ZrC coating modified by polymer-derived SiC/TiC nanocomposites for ultra-high temperature application

To improve the ablation resistance of ZrC coating on SiC-coated carbon/carbon composites above 2000 °C, SiC/TiC nanocomposites (SiC/TiC-NCs) powders derived from single-source precursor were incorporated into ZrC coating, denoted as ZrC-SiC/TiC-NCs, via supersonic atmospheric plasma spraying (SAPS). After SAPS, the incorporated SiC/TiC-NCs evolved into TiC/(SiC and ZrxTiyC) embedded in amorphous SiC. The ablation resistance of the ZrC-SiC/TiC-NCs coating was evaluated by oxyacetylene flames with a heat flux of 4.18 MW/m2. For comparison, the ZrC-SiC-NCs coating without Ti modification was seriously damaged due to rapid gas denudation. The good ablation resistance of ZrC-SiC/TiC-NCs coating is mainly attributed to the distinctive “capsule-like” multi-crystalline microstructure of SiC/TiC-NCs. During ablation, TiO2 and ZrxTiyO2, due to the oxidation of TiC and ZrxTiyC, contributed to the formation of Zr-Ti-Si-O glass with high viscosity and low evaporation pressure, improving the ablation resistance.

In-Situ Reduction of Mo-Based Composite Particles during Laser Powder Bed Fusion

Raw powders are processed in water during the freeze-dry pulsated orifice ejection method (FD-POEM), leading to the inclusion of oxygen impurities. This study proposes a strategy for removing the oxygen content and enhancing the mechanical performance of laser powder bed fusion (L-PBF) builds from powders using carbon nanotubes (CNTs) and H2 reduction. Spherical 1.5 wt.% CNT/Mo composite powders with uniform dispersion were fabricated via FD-POEM. The quantity of MoO2 decreased significantly, and a hexagonally structured Mo2C phase was simultaneously formed in the L-PBF build. The Mo2C with network structure was distributed along the boundaries of equiaxed Mo grains, leading to an increased Vickers hardness of the matrix. This study demonstrates the feasibility of fabricating oxygen-free and high-strength refractory parts during L-PBF for ultrahigh-temperature applications.

Oxidation Resistance, Creep Strength and Room-Temperature Fracture Toughness of Mo–28Ti–14Si–6C–6B Alloy

A novel Mo–28Ti–14Si–6C–6B (atom %) alloy was designed for ultrahigh-temperature applications. Arc-melting and appropriate heat-treatment at 1600°C results in the targeted four-phase microstructure comprising Moss, Mo5SiB2 (T2), Ti5Si3 and TiC. Previously developed three-phase alloys, comprising Moss–T2–Ti5Si3 and Moss–T2–TiC, are outperformed by this novel alloy as an adequate balance of oxidation resistance, creep strength and fracture toughness is attained. The oxidation behavior is characterized by substantial oxide scale formation at 1200°C being accompanied by a mass loss of around -50 mg/cm2 after 100 h of cyclic oxidation. The oxide scale is found to be composed of a top TiO2 scale and an underlying duplex SiO2/TiO2 scale. Minimum creep rate of the alloy was in the order of 10–7 s–1 at 1200°C and 300 MPa and its room-temperature fracture toughness was (12.8 ± 1.2) MPa(m)1/2.

Microstructure evolution and mechanical properties of a novel γ′ phase-strengthened Ir-W-Al-Th superalloy

The present work introduces a novel γ′ phase-strengthened Ir-W-Al-Th superalloy for ultrahigh-temperature applications. First, the as-cast microstructure and phase transformation of Ir-13W-6Al-0.15Th (at%) alloy during solid solution and aging were investigated. Phase transformation was observed during heat treatment. The primary γ′ phase disappeared via the redissolution of γ′ → γ at 1800 °C. The recrystallization took place at 1450 °C and very fine equiaxed γ′/γ grains formed after 240 h. The cold-rolled microstructure indicated that the room-temperature γ phase was brittle and cracks mainly emerged along grain boundaries. Ir-13W-6Al-0.15Th alloy exhibited a higher nanohardness than other Ir-based superalloys and pure Ir, which can be attributed to the solid solution and precipitation (γ′) strengthening. In addition, the as-cast Ir-13W-6Al-0.15Th alloy shows a medium room-temperature compressive yield strength and good ductility.

High-entropy carbide-nitrides with enhanced toughness and sinterability

High-entropy ceramics (HECs) have attracted much attention due to their huge composition space, unique microstructure, and desirable properties. In contrast to previous studies, which have primarily focused on HECs with one anion, herein, we report a new family of ceramics with both multi-cationic and -anionic structures, i.e., high-entropy carbide-nitrides (Ti0.33Zr0.33Hf0.33)(C0.5N0.5), (Ti0.25Zr0.25Hf0.25Nb0.25)(C0.5N0.5) and (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)(C0.5N0.5). These as-synthesized HECs are mainly comprised of a face-centered cubic solid-solution phase accompanied by minor inevitable oxide phases. The formation mechanism of the solid-solution phase is discussed in terms of the lattice size difference and thermodynamic competition between configurational entropy and mixing enthalpy. It is found that the increment in the configurational entropy can effectively lower the sintering temperature and increase the fracture toughness. Particularly, the newly developed (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)(C0.5N0.5) exhibits a decent fracture toughness of 8.4 MPa m1/2 and a low sintering temperature of 1750°C, making it promising for ultra-high temperature applications. Our work not only enriches knowledge regarding the HECs categories, but also opens a new pathway for developing HECs with multi-cationic and -anionic structures.