Ultrahigh-temperature Materials Research Articles

Microstructure and Properties of Complex Concentrated C14–MCr2 Laves, A15–M3X and D8m M5Si3 Intermetallics in a Refractory Complex Concentrated Alloy

Abstract: The refractory complex concentrated alloy (RCCA) 5Al–5Cr–5Ge–1Hf–6Mo–33Nb–19Si–20Ti–5Sn–1W (at.%) was studied in the as-cast and heat-treated conditions. The partitioning of solutes in the as-cast and heat-treated microstructures and relationships between solutes, between solutes and the parameters VEC and Δχ, and between these parameters, most of which are reported for the first time for metallic UHTMs, were shown to be important for the properties of the stable phases A15–Nb3X and the D8m βNb5Si3. The nano-hardness and Young’s modulus of the A15–Nb3X and the D8m βNb5Si3 of the heat-treated alloy were measured using nanoindentation and changes in these properties per solute addition were discussed. The aforementioned relationships, the VEC versus Δχ maps and the VEC, Δχ, time, or VEC, Δχ, Young’s modulus or VEC, Δχ, nano-hardness diagrams of the phases in the as-cast and heat-treated alloy, and the properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. The significance of the new data and the synergy and entanglement of solutes and phases for the design of metallic ultra-high temperature materials were discussed.

Development of a highly loaded zirconium carbide paste for material extrusion additive manufacturing

A highly loaded, aqueous-based zirconium carbide (ZrC) paste was developed for ceramic on-demand extrusion (CODE). Commercial ZrC powder was ball milled to reduce the particle size to ∼1 μm. Paste composition was determined by rheological characterization and observation of printing behavior; the final paste consisted of 46–47 vol% ZrC, 43.5 vol% distilled water, 8.1 vol% dispersant, and 1.6 vol% binder. Rheological characterization of the paste showed a yield stress of ∼8 Pa and shear thinning behavior (ΔG′/Δσ* = −55). The paste was printed using a 610 μm nozzle; a test print showed shape fidelity using the developed paste. A pressureless sintering study was performed; a sintering temperature of 2000°C with a 2-hour hold produced samples of 90 % relative density with 3.8 μm grains, which was consistent with literature. The developed paste, in combination with the pressureless densification results, adds a new, ultra-high temperature material to ceramic extrusion additive manufacturing.

Synthesis and thermal insulation properties of (Ti1/5V1/5Mo1/5Nb1/5Ta1/5)(C0.55N0.45) high-entropy carbonitride porous ceramic

Porous high-entropy transition metal compounds, an important type of ultra-high temperature material, have attracted increasing attention in the applications of extreme environments due to their low density, good thermal insulation and stable chemical properties. A robust, flame-retardant and high-temperature resistant high-entropy carbonitride (Ti1/5V1/5Mo1/5Nb1/5Ta1/5)(C0.55N0.45) (HECN) porous ceramic was synthesized and thoroughly characterized. It exhibited a porosity of 93.5 %, a compressive strength of 1.3 MPa and a good high-temperature stability up to 1673 K, which, along with its outstanding fire-retardancy, and low thermal conductivity (as low as 0.150 W m−1 K−1 at 298 K), made it a good candidate material for thermal insulation applications in ultra-high temperature environments. Moreover, the Density functional theory (DFT) calculations predict that the [N]/([C]+[N]) ratio in HECN foam affects its lattice thermal conductivity which is decreased by 67 % upon increasing the ratio from 0 to 1/2.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

On the Microstructure and Properties of Complex Concentrated bcc Solid Solution and Tetragonal D8m M5Si3 Silicide Phases in a Refractory Complex Concentrated Alloy

In this work, the refractory complex concentrated alloy (RCCA) 3.5Al–4Cr–6Ge–1Hf–5Mo–36Nb–22Si–1.5Sn–20Ti–1W (at.%) was studied in the as cast and heat treated conditions (100 h or 200 h at 1500 °C). There was strong macrosegregation of Si in the 0.6 kg button/ingot of the cast alloy, in which A2 solid solution, D8m βNb5Si3, C14-NbCr2 Laves phase and Tiss and a ternary eutectic of the A2, D8m and C14 phases were formed. The partitioning of Ti in the as cast and heat treated microstructure and its relationships with other solutes was shown to be important for the properties of the A2 solid solution and the D8m βNb5Si3, which were the stable phases at 1500 °C. The near surface microstructure of the alloy was contaminated with oxygen after heat treatment under flowing Ar. For the aforementioned phases, it was shown, for the first time, that there are relationships between solutes, between solutes and the parameters VEC, Δχ and δ, between the said parameters, and between parameters and phase properties. For the contaminated with oxygen solid solution and silicide, trends in relationships between solutes, between solutes and oxygen content and between the aforementioned parameters and oxygen content also were shown for the first time. The nano-hardness and Young’s modulus of the A2 solid solution and the D8m βNb5Si3 of the as cast and heat-treated alloy were measured using nanoindentation. Changes of nano-hardness and Young’s modulus of the A2 solid solution and D8m βNb5Si3 per solute addition for this multiphase RCCA were discussed. The nano-hardness and Young’s modulus of the solid solution and the βNb5Si3, respectively, were 9.5 ± 0.2 GPa and 177.4 ± 5.5 GPa, and 17.55 ± 0.5 GPa and 250.27 ± 6.3 GPa after 200 h at 1500 °C. The aforementioned relationships and properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. Implications of synergy and entanglement for the design of metallic ultra-high temperature materials were emphasised.

Powder production, FAST processing and properties of a Nb-silicide based alloy for high temperature aerospace applications

A Nb-silicide based alloy with nominal composition Nb–18Ti–22Si–6Mo-1.5Cr–2Sn-1Hf (at. %), designed for high temperature aerospace applications, was produced via a powder metallurgy (PM) route. The raw elements were arc melted, crushed, and milled to powder, then consolidated using Field Assisted Sintering Technology (FAST). The compressive creep of the alloy was evaluated using electro-thermal mechanical testing (ETMT). The study demonstrated the production of larger 60 mm diameter samples, with potential for further scale up. The microstructure of the FAST alloy, which is comprised of bcc Nbss and tetragonal αNb5Si3 was more homogenous compared with the cast alloy, with some interstitial contamination that occurred during powder production. The FAST alloy had lower density than state of the art Ni-based superalloys and refractory metal complex concentrated alloys (RCCAs) and high entropy alloys (RHEAs), and its yield strength and specific yield strength was higher than those of the latter metallic Ultra high temperature materials (UHTMs) and comparable to those of Nb-silicide based alloys with B addition. The stress exponent n in compressive creep was in the range 1.7–2.6, similar to that of binary Nb–10Si and Nb–16Si alloys and its creep rate at 1200 °C and 100 MPa was similar to that of the MASC alloy (Nb–25Ti–16Si-8Hf-2Al–2Cr (at.%)). Like the latter, the creep of the FAST alloy did not meet the creep goal.

A Perspective of the Design and Development of Metallic Ultra-High Temperature Materials: Refractory Metal Intermetallic Composites, Refractory Complex Concentrated Alloys and Refractory High Entropy Alloys

The paper is a personal perspective on the design of metallic ultra-high temperature materials (UHTMs). Specifically, the alloy design “landscape” of metallic UHTMs was considered from the viewpoint of the alloy design methodology NICE. The concepts of synergy, entanglement and self-regulation and their significance for alloy design/development were discussed. The risks, ecological challenges and material-environment interactions associated with the development of metallic UHTMs were highlighted. The “landscape” showed that beneath the complexities of alloy design lies an elegant and powerful unity of specific parameters that link logically and that progress can be made by recognising those interrelationships between parameters that generate interesting, diverse, and complex alloys.

Effects of solutes on thermodynamic properties of (TMZrU)C (TM = Ta, Y) medium-entropy carbides: a first-principles study

High entropy carbide ceramics have garnered significant interest as a novel class of ultra-high temperature and superhard metallic materials. In the present work, a comparative investigation was conducted for the first time on the stability, mechanical, and thermodynamic properties of two medium entropy carbides (MECs), (TaZrU)C and (YZrU)C, using high-throughput first-principles calculations. Additionally, data from groups IV and V transition metal monocarbides were employed for comparison. The temperature-dependent thermodynamic properties, including bulk modulus (B), constant volume/constant pressure heat capacity (Cv/Cp), Gibbs free energy, volume, entropy, and thermal conductivity, were evaluated using the Debye-Gruneisen model. The results demonstrate that (TaZrU)C and (YZrU)C exhibit similar trends in their thermodynamic properties, with (YZrU)C displaying slightly superior performance as the temperature rises. This work provides valuable insights into the design of innovative high entropy fuels, holding significant implications for the advancement of MEC ceramic fuels in advanced nuclear power systems and nuclear thermal propulsion systems.

On the Nb5Si3 Silicide in Metallic Ultra-High Temperature Materials

Refractory metal (RM) M5Si3 silicides are desirable intermetallics in metallic ultra-high temperature materials (UHTMs), owing to their creep properties and high Si content that benefits oxidation resistance. Of particular interest is the alloyed Nb5Si3 that forms in metallic UHTMs with Nb and Si addition. The choice of alloying elements and type of Nb5Si3 that is critical for achieving a balance of properties or meeting a property goal in a metallic UHTM is considered in this paper. Specifically, the different types of alloyed “normal” Nb5Si3 and Ti-rich Nb5Si3, namely “conventional”, “complex concentrated” (CC) or “high entropy” (HE) silicide, in metallic UHTMs with Nb and Si addition were studied. Advanced metallic UHTMs with additions of RMs, transition metals (TMs), Ge, Sn or Ge + Sn and with/without Al and with different Ti, Al, Cr, Si or Sn concentrations were investigated, considering that the motivation of this work was to support the design and development of metallic-UHTMs. The study of the alloyed silicides was based on the Nb/(Ti + Hf) ratio, which is key regarding creep, the parameters VEC and Δχ and relationships between them. The effect of alloying additions on the stability of “conventional”, CC or HE silicide was discussed. The creep and hardness of alloyed Nb5Si3 was considered. Relationships that link “conventional”, CC or HE bcc solid solution and Nb5Si3 in the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration) were presented. For a given temperature and stress, the steady state creep rate of the alloyed silicide, in which TMs substituted Nb, and Al and B substituted Si, depended on its parameters VEC and Δχ and its Nb/(Ti + Hf) ratio, and increased with decreasing parameter and ratio value, compared with the unalloyed Nb5Si3. Types of alloyed Nb5Si3 with VEC and Δχ values closest to those of the unalloyed Nb5Si3 were identified in maps of alloyed Nb5Si3. Good agreement was shown between the calculated hardness and chemical composition of Nb5Si3 and experimental results.

Microstructure and Thermal Shock Resistance of Zr7/8Ti1/8B2-20 vol% SiC Composites Synthesized by Mechanical Alloying and Spark Plasma Sintering

ZrB2-based composites are promising ultra-high temperature materials, nonetheless their poor damage tolerance and low oxidation resistance hinder their practical applications. In the present work, Zr1–xTixB2-20 vol% SiC composites were prepared based on the mechanical alloying (MA) method and spark plasma sintering (SPS) technique. The results showed that solid solution of ZrTiB4 can be obtained by taking nano-sized ZrB2-TiB2 powders from the MA process. The Zr7/8Ti1/8B2-20 vol% SiC composites presented higher fracture toughness due to various toughening mechanisms, such as ‘crack deflection’ and ‘pull out’ induced by the finer solid solution particles and nano-sized graphite. Consequently, Zr7/8Ti1/8B2-20 vol% SiC composites possessed a residual strength of 180 MPa after thermal shock test with a temperature difference of 1000°C, while the conventional ZrB2-20 vol% SiC composites only remained at 81.4 MPa.

Pyrolysis behaviors of iodine vapor cured fine-diameter polysilazane fibers for the preparation of SiCN ceramic fibers

Polymer-derived SiCN ceramic fibers have been developed as a new ultrahigh-temperature material that can be an alternative to the binary system of SiC and Si3N4 due to the preservation of its amorphous phase at high temperatures. In this work, a novel strategy of iodine vapor curing has been employed to cure the fine-diameter PSZ fibers. The pyrolysis behaviors of the cured PSZ fibers at various temperatures were investigated. The as-prepared fibers were characterized by TG-DSC, PY/GC/MS, FT-IR, FE-SEM, and XRD analysis. The results revealed the fine-diameter amorphous SiCN ceramic fibers have a high tensile strength of 1.08 ± 0.26 GPa after pyrolysis at various temperatures. During the process of pyrolysis, the unstable chemical bonds and active groups (i.e., N-I, Si-I, N-H, Si-H, and Si-CH3) transformed into Si3N structure with the release of iodine, CH4, NH3, cyclotrisiloxane at 300 °C. The dehydrogenation and transamination reactions occurred between the Si-H, N-H, and N-H bonds at the pyrolysis temperature of 400 °C. At the higher temperature (500─600 °C), a rearrangement reaction occurs to form some complex compounds (i.e., 3TMS and 4TMS derivative melamine and other higher molecular weight compounds) and inorganic fibers were attained at temperature ≥ 700 °C. This work revealed an important guiding significance for the pyrolysis of PSZ ceramic precursor polymer fibers.

Recent advances in synthesis of ultra-high temperature ceramic matrix composites

A ceramic material designed for ultra-high temperatures (UHTCs) generally comprised of nitrides, carbides, and borides derived from transition metal elements, with a particular focuson compounds belonging to TaC and Group IVB (Hf and Zr). Hypersonic vehicle nozzles andengine components can take advantage of the unique characteristics of these materials. A broadrange of coatings and composites based on UHTC is currently being developed to conquer theinherent fragility, weak thermal shock resistance, and brittleness of bulk ceramics. Ultra-hightemperature materials with high entropy have gained considerable attention in recent years. Areview of the current state of the art of UHTC composites and coatings will be provided in thisreport. Properties and processing approaches to achieve the microstructure will be discussedfurther.

On the Stability of Complex Concentrated (CC)/High Entropy (HE) Solid Solutions and the Contamination with Oxygen of Solid Solutions in Refractory Metal Intermetallic Composites (RM(Nb)ICs) and Refractory Complex Concentrated Alloys (RCCAs).

In as-cast (AC) or heat-treated (HT) metallic ultra-high temperature materials often "conventional" and complex-concentrated (CC) or high-entropy (HE) solid solutions (sss) are observed. Refractory metal containing bcc sss also are contaminated with oxygen. This paper studied the stability of CC/HE Nbss and the contamination with oxygen of Nbss in RM(INb)ICs, RM(Nb)ICs/RCCAs and RM(Nb)ICs/RHEAs. "Conventional" and CC/HE Nbss were compared. "Conventional" Nbss can be Ti-rich only in AC alloys. Ti-rich Nbss is not observed in HT alloys. In B containing alloys the Ti-rich Nbss is usually CC/HE. The CC/HE Nbss is stable in HT alloys with simultaneous addition of Mo, W with Hf, Ge+Sn. The implications for alloy design of correlations between the parameter δ of "conventional" and CC/HE Nbss with the B or the Ge+Sn concentration in the Nbss and of relationships of other solutes with the B or Ge+Sn content are discussed. The CC/HE Nbss has low Δχ, VEC and Ω and high ΔSmix, |ΔHmix| and δ parameters, and is formed in alloys that have high entropy of mixing. These parameters are compared with those of single-phase bcc ss HEAs and differences in ΔHmix, δ, Δχ and Ω, and similarities in ΔSmix and VEC are discussed. Relationships between the parameters of alloy and "conventional" Nbss also apply for CC/HE Nbss. The parameters δss and Ωss, and VECss and VECalloy can differentiate between types of alloying additions and their concentrations and are key regarding the formation or not of CC/HE Nbss. After isothermal oxidation at a pest temperature (800 oC/100 h) the contaminated with oxygen Nbss in the diffusion zone is CC/HE Nbss, whereas the Nbss in the bulk can be "conventional" Nbss or CC/HE Nbss. The parameters of "uncontaminated" and contaminated with oxygen sss are linked with linear relationships. There are correlations between the oxygen concentration in contaminated sss in the diffusion zone and the bulk of alloys with the parameters ΔχNbss, δNbss and VECNbss, the values of which increase with increasing oxygen concentration in the ss. The effects of contamination with oxygen of the near surface areas of a HT RM(Nb)IC with Al, Cr, Hf, Si, Sn, Ti and V additions and a high vol.% Nbss on the hardness and Young's modulus of the Nbss, and contributions to the hardness of the Nbss in B free or B containing alloys are discussed. The hardness and Young's modulus of the bcc ss increased linearly with its oxygen concentration and the change in hardness and Young's modulus due to contamination increased linearly with [O]2/3.

The investigation of the physical properties and hydrogenated mechanism of TM5Si4 (TM=Ti, Zr, Hf)

The outstanding physical properties make TM5Si4 silicides become the potential silicon-based transition-metal ultrahigh-temperature materials. In present work, we adopt the first-principles scheme to explore the structural stability, mechanical properties and explain the hydrogenated mechanism of Ti5Si4, Zr5Si4 and Hf5Si4 using the electronic structures. And the investigation increases the theoretical support for the developments and applications of TM5Si4 silicides. Three hydrogenated models have shown that the hydrogen displays the stability for hydrogenated TM5Si4 compounds. Furthermore, the introduction of hydrogen occupation has weakened the elastic properties of TM5Si4. The metallic property of TM5Si4 and three hydrogenated models was confirmed by the electronic structures. The localized hybridization between hydrogen and TM5Si4 confirm the hydrogenated structural stability.

A Review of Mo-Si Intermetallic Compounds as Ultrahigh-Temperature Materials

Mo-Si compound-based ultrahigh temperature structural material (UHTM) is a new high-temperature structural material with great potential due to its high melting point, high hardness, and suitable density. As an important material to meet the demands of advanced high-temperature structural applications in the range 1200–1600 °C, in oxidizing and aggressive environments, it has attracted the attention of researchers in aerospace, energy, chemical industry, machinery, mechanical metallurgy, and other fields. This study could provide a reference for the research and application of Mo-Si intermetallic compounds as UHTMs. Based on their crystal structure and properties, we summarized the preparation, limitations, and modification methods of Mo-Si intermetallic compounds and reviewed the research progress in their toughness reinforcement and medium-temperature resistance improvement. Based on the literature review, there has been a certain level of progress in the research on modification of Mo-Si intermetallic compounds worldwide. However, both the high brittleness at room temperature and insufficient strength at high temperature have not been solved yet.

Exploring of the structural stability, mechanical, electronic and optical properties of hydrogenated RE5Si4 (RE=Sc and Y) from first-principles calculations

In order to expand the applications of rare-earth ultrahigh-temperature materials, the overall performances including the hydrogenated mechanism, mechanical and optical properties of hydrogenated RE5Si4 (RE = Sc and Y) were explored and discussed by the first-principles calculations. It provides the theoretical support for the development and application of RE5Si4 silicides. Three hydrogenated models were considered based on the structural properties of RE5Si4 silicides. The results illustrate that the hydrogenated RE5Si4 are thermodynamically stable for the reason of the electronic interaction between hydrogen and RE5Si4 (RE = Sc and Y). Moreover, the elastic modulus and elastic constants demonstrate that the hydrogenated RE5Si4 display the worse mechanical properties than parent RE5Si4. Additionally, the doped H (2) models for Y5Si4 display the ductile behavior. The electronic structures show that all hydrogenated models possess metallic properties. And the stability of hydrogenated models and the localized hybridization between RE5Si4 and hydrogen atoms are closely related. Finally, the adsorption spectrum confirms that the hydrogen atomic doping improves the electronic jump between the occupied states and the empty states.

Microstructure, Mechanical Properties, and Reinforcement Mechanism of Second-Phase Reinforced TiC-Based Composites: A Review

TiC ceramics have the characteristics of high melting point and density, and titanium reserves on earth are extremely large; therefore, TiC ceramics are considered ultra-high temperature materials with great research value. However, the development of TiC-based ultra-high temperature composites has been seriously hindered by their poor mechanical properties. At present, improvement of the mechanical properties of TiC is mainly accomplished by adding a second phase. In this paper, the research status of modified elements-, nitrides-, and metal-reinforced TiC matrix composites is presented. The microstructure, phase composition, and toughening mechanism of TiC matrix composites reinforced by a second phase are described. The influence of the reaction products on the matrix during the toughening process is also discussed.

Refractory Metal Intermetallic Composites, High-Entropy Alloys, and Complex Concentrated Alloys: A Route to Selecting Substrate Alloys and Bond Coat Alloys for Environmental Coatings.

This paper considers metallic ultrahigh-temperature materials (UHTMs) and the alloying behaviour and properties of alloys and their phases by using maps of the parameters δ (based on atomic size), Δχ (based on electronegativity), and valence electron concentration (VEC), and discusses what connects and what differentiates material groups in the maps. The formation of high-entropy or complex concentrated intermetallics, namely 5-3 silicides, C14 Laves and A15 compounds, and bcc solid solutions and eutectics in metallic UHTMs and their co-existence with “conventional” phases is discussed. The practicality of maps for the design/selection of substrate alloys is deliberated upon. The need for environmental coatings for metallic UHTMs was considered and the design of bond coat alloys is discussed by using relevant maps.

Stability, mechanical, and thermodynamic behaviors of (TiZrHfTaM)C (M = Nb, Mo, W, V, Cr) high-entropy carbide ceramics

Multicomponent high entropy carbide ceramics (HECCs) have drawn increasing attention because of their potential applications as ultra-high temperature and super-hard materials. In this work, the stability, mixing behavior, mechanical, and temperature-dependent properties of rock-salt (TiZrHfTaM)C (M = Nb, Mo, W, V, or Cr) HECCs were first systematically investigated by density functional theory (DFT) and Debye-Grüneisen model methods. The five HECCs are thermodynamically stable owing to the negative formation enthalpies and cohesive energies. They could form the single-phase high entropy solid solution ceramics, owing to the evaluation of atom size and lattice differences, as well as mixing enthalpy criteria. Except for the system containing Cr, the others have already been confirmed by experimental findings. Among these, (TiZrHfTaNb)C most readily forms homogeneous solid solution phase, and is the most stable, brittlest, and hardest HECC material. Significantly, (TiZrHfTaV)C behaves slightly better than (TiZrHfTaNb)C with increasing temperature, owing to the comparable bulk modulus and Debye temperature, smaller volumetric expansion, and lower anharmonic effects. This work provides the instructive information for predicting and designing the high-performance ultra-high temperature ceramic materials applicable in extreme environments.

Enhanced Mechanical Properties and Oxidation Resistance of Zirconium Diboride Ceramics via Grain-Refining and Dislocation Regulation.

Zirconium diboride (ZrB2) is considered as one of the most promising ultra‐high temperature materials for the applications in extreme environments. However, the difficulty in fabrication of ZrB2 limits its industrial applications. In this study, fully dense and grain‐refined ZrB2 is prepared under ultra‐high pressure of 15 GPa at low temperature of 1450 °C. The as‐prepared ZrB2 exhibits excellent mechanical and oxidation‐resistant properties. Compared with raw powder, the grain size decreases 56%. Compared with high‐temperature sintered control specimen beyond 2000 °C, the hardness and fracture toughness increase about 46% and 69%, respectively, the dislocation density increase 3 orders of magnitude, while the grain size considerably decrease 96%. According to work hardening, Hall–Petch and Taylor dislocation hardening effects, the refined grains, substructures, and high dislocation density caused by plastic deformation during sintering can enhance the mechanical properties. The unique structure contributes to a threshold oxidation temperature increase of ≈250 °C relative to the high‐temperature sintered ZrB2, achieving one of the highest values (1100 °C) among the reported monolithic ultra‐high temperature ceramics. A developed densification mechanism of dislocation multiplication with grain refining is proposed and proved to dominate the sintering, which is responsible for simultaneous improvements in mechanical and oxidation‐resistant properties.