One-step preparation of TaIr3-based material and its ablation performance under extreme environmental conditions

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One-step preparation of TaIr3-based material and its ablation performance under extreme environmental conditions ⁎

Natalya I. Baklanovaa, Victor V. Lozanova, , Anatoly T. Titovb a b

Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze st.18, Novosibirsk, 630128, Russian Federation V S. Sobolev Institute of Geology and Mineralogy SB RAS, Koptyug ave. 3, Novosibirsk, 630090, Russian Federation

A R T I C LE I N FO

A B S T R A C T

Keywords: A. Intermetallics A. Tantalum oxide B. SEM B. X-ray diffraction C. High temperature corrosion C. Oxidation

The solid-state interaction of TaC with Ir within the 1000–1600 °C temperature range leads to the formation of TaIr3 intermetallics. The durability of the TaIr3-based system with a silicon additive was studied under extreme environmental conditions. The TaIr3-based material displays satisfactory ablation resistance at 2000 °C in air in arc-jet. The post-test microstructural and XRD analysis of the developed material allowed us to propose the role of different constituents of the Ta – C – Ir – Si system. The iridium-containing phases play the most significant role in favorable ablation behavior of the developed system.

1. Introduction The trend towards the optimization of efficiency in modern gas turbines, combustors and nose tips requires the development of highstrength materials with structural stability and corrosion resistance during operation at temperatures higher than 2000 °C [1,2]. Platinum group metal-based refractory alloys and intermetallics, especially those composed of iridium and refractory metals of IV-V groups, attract permanent attention of material scientists involved in the development of ultra-high temperature materials [3–5]. Indeed, these materials have a number of very attractive properties, in particular high melting points, often exceeding 2000 °C, high strengths and incompressibility, which are retained up to elevated temperatures, good oxidation resistance due to extremely low recession rate of iridium even at temperatures close to its melting point (2443 °C) [3,6–11]. Traditionally, iridium-based intermetallics composed of iridium and refractory metals of IV-V groups are synthesized from the mixtures of transition metals and iridium using high-temperature methods, e.g. arc melting [3,5,12,13]. High-temperature reaction conditions are combined with long annealing times and frequent regrinding steps for the preparation of powder intermetallics. An alternative approach to synthesize the above-mentioned intermetallics is a solid-state reaction of carbides, borides, and nitrides of corresponding refractory metals [14–17]. The reactions of carbides with iridium had been receiving increased attention in the 60-70′s [14]. Reactions between HfC, ZrC, ThC2 and iridium or rhodium were studied in connection with the development of the protective coating for graphite by Criscione and co-

⁎

workers [14]. The proposed approach to design the protective coatings for graphite involved iridium and rhodium as the external coating separated from the substrate (graphite) by an intermediate layer of some refractory materials that would be chemically stable both with respect to carbon and the external layer. However, as was shown by Criscione and co-workers [14], the reaction between metal carbide and iridium occurs within the temperature range of 1200–2000 °C with the formation of MeIr3 phases alone, even when metal carbides were in a great excess with respect to iridium. Simultaneously, Raub and Falkenburg studied the stability of some carbides including tantalum carbide towards platinum-group metals at high temperatures [18,19]. Later, Strife et al. [17] and Pierre [20] carried out a thorough study of the mechanism of the solid-state reaction of HfC with iridium at temperatures higher than 1650 °C. Based on the calculations of hafnium activity in HfC, HfIr, and HfIr3, they concluded that when stoichiometric HfC is in contact with iridium, only the formation of cubic HfIr3 should be observed. Contrary, the study of the solid-state reaction of TiCx with iridium showed that TiIr3 together with TiIr are formed, and the composition of the obtained product is strongly dependent on the stoichiometry of initial titanium carbide [17]. Unfortunately, another promising intermetallic compound for hightemperature applications, namely, TaIr3 (Tm = 2380 ± 25 °C [21]), has been overlooked. Only a brief mention of the formation of cubic TaIr3 through the reaction of TaC with iridium was presented by Holleck in his Thesis [15] and Raub and Falkenburg [19]. Meanwhile, TaIr3 deserves more attention due to a set of very attractive properties. TaIr3 has a high melting point, high bulk modulus (327 GPa), and hardness

Corresponding author. E-mail address: [email protected] (V.V. Lozanov).

https://doi.org/10.1016/j.corsci.2018.08.044 Received 31 May 2018; Received in revised form 13 August 2018; Accepted 14 August 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Baklanova, N., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.08.044

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3. Results and discussion

(∼26 GPa) [10,22]. One can note that other intermetallic phases of the Ta – Ir system, namely, σ (Ta3Ir) and α1 (TaIr) phases have also melting points higher than 2000 °C, which differs them advantageously from the corresponding analogs of the Zr – Ir and Hf – Ir systems. In addition, TaIr3 appears to be able to withstand high-temperature corrosion in air because one of the components, tantalum, is oxidized with the formation of refractory tantalum oxide (Tm = 1877 °C for Ta2O5), whereas the other component of intermetallics, iridium, forms gaseous iridium oxide with very low velocity even at 2000 °C [7,23–25]. Unfortunately, no data about the oxidation resistance of TaIr3 at 2000 °C were presented before. This work is dedicated to study the peculiarities of the solid-state interaction of tantalum carbide with iridium depending on the experimental conditions, and to test the ablation resistance of the TaIr3-based system obtained by the solid-state reaction of TaC and iridium under severe conditions at 2000 °C.

3.1. Phase composition and morphology of products obtained by solid-state reaction The XRD patterns of TaC:Ir mixture with the 1:1 ratio depending on temperature are presented in Fig. 1. The first features of the appearance of new TaIr3 (cubic AuCu3 structure) phase become noticeable at 1100 °C (Fig. 1, insert a). No intermetallic compounds other than TaIr3 are formed. In addition to the new phase, unreacted TaC and iridium phases were observed too. The lattice parameter of TaIr3 intermetallics was determined to be equal to 3.886 ± 0.001 Å which coincides with the value for stoichiometric TaIr3 [21]. With an increase in the treatment temperature to 1300 °C, the lattice parameter of TaIr3 intermetallics slightly increases and becomes equal to 3.890 Å, which corresponds to the composition TaIr2.86 (74.1% at. Ir). Subsequent temperature rise to 1600 °C, as well as an increase in exposure time to 4 h at this temperature, does not results in the change of the lattice parameter (Fig. 1 insert b; Supplementary Material, Fig. S2). It is noteworthy that the content of TaIr3 phase calculated from the XRD data attains 96% (wt.) for the product obtained at 1600 °C for 4 h (TaC:Ir = 1:3). No carbon phases were found by the XRD analysis. However, intense peaks in the regions of 1200 – 1600 cm−1 and 2500 – 3000 cm−1were detected in the Raman spectra of products obtained under different experimental conditions (Supplementary Material, Fig. S5). The Raman peaks centered at about 1280 (D-peak) and 1592 (Gpeak) cm−1 can be assigned to the carbon phase. As was mentioned above, unreacted initial iridium and TaC phases are present in the products obtained at 1100–1200 °C. As follows from Fig. 1 d (Supplementary Material, Fig. S3a, the 1:3 mixture), the positions of the XRD peaks of iridium are shifted towards the small-angle direction. This suggests that the lattice parameter of iridium is increased (a = 3.846 Å at 1200 °C), which could be explained by partial substitution of iridium atoms by tantalum atoms in the crystal structure of iridium (RTa = 2.00 Å vs. RIr = 1.80 Å). No traces of iridium were detected in the XRD patterns of products obtained at 1300 °C and higher (Fig. 1d). Thus, starting from this temperature, iridium was fully consumed by TaC, whereas tаntalum carbide was detected in all products including those obtained upon long-term exposure at 1600 °C. The excess of TaC phase can be explained by the fact that only TaIr3, i.e., intermetallic compound with the highest iridium content is formed during the reaction of TaC with iridium. As a consequence, unreacted TaC is detected in the reaction products. Taking into account a shift of the Kα1 and Kα2 doublet for the (422) peak (Fig.1c), one can consider that the composition of unreacted TaC approaches the stoichiometric one, as the reaction between TaCx and iridium proceeds. It could be discreetly proposed that carbon eliminating in the course of the reaction of nonstoichiometric TaCx with iridium diffuses inward the TaCx lattice and fills the carbon vacancies. The peculiarities of the formation of TaIr3 for the TaC:Ir = 1:3 powder mixture are the same as those deduced from the XRD results for the TaC:Ir = 1:1 mixture (Supplementary Material, Figs. S3 and S4). Analysis of the SEM images of TaIr3 product (1600 °C, 4 h, TaC:Ir = 1:3) shows that it is composed of grains with irregular shape (Fig. 2 a, b). The sizes of the grains fall within the range 1–10 μm. Together with separate grains, rather large-size aggregates composed of sintered grains are also observed. According to EDS analysis data (Table 1), the composition of product determined in different areas of several samples was estimated to be TaIr2.89 – TaIr3.02, which is in good correspondence with the composition determined by the XRD analysis, namely, TaIr2.86 – TaIr3.15 (74.1–75.9% at.). Also, in some SEM images one can observe that transparent thin sheets are present on the surface of the product grains (Fig. 2 b). These sheets could be related to the carbon phase. One can remind that the presence of carbon phase as transparent sheets was detected in the products obtained from HfC and Ir mixture [27].

2. Experimental Section 2.1. Materials and preparation of the powders The following powder materials were used as the initial substances, tantalum carbide (TU 6-09-03-443-77, Russia) and iridium (GOST 12338-81; purity > 99.96%, Russia). According to the XRD data, the asreceived tantalum carbide phase is nonstoichiometric, and its composition corresponds to TaC0.98. The SEM image of initial TaC powder is shown in Fig. S1 (Supplementary Material). The powders of TaC and Ir were weighted to reach the 1:1 and 1:3 ratios and mixed. The mixtures were placed in grafoil boxes and heated up to a given temperature, T, in the vacuum of 10−5 torr, kept at a given temperature for different time,τ, from 1 to 4 h. The products formed in the TaC – Ir system within the temperature range 1000–1600 °C were analyzed by SEM, XRD, and Raman to study the influence of the composition, the temperature and the duration of the heat-treatment. 2.2. Characterization X-ray diffraction (XRD) patterns of the powders were recorded with a D8 Advance (Bruker AXS, Germany) powder diffractometer using CuKα radiation (λ1 = 1.54056 Å, λ2 = 1.54439 Å in the 30° < 2Θ < 130° interval. Quantitative phase analysis and lattice parameter refinement were performed by the Rietveld method using the Topas 4.2 software (Bruker AXS, Germany) [26] and Inorganic Crystal Structure Database ICSD (FIZ Karlsruhe, Germany, 1996). The morphology and local elemental compositions of samples were examined with high-resolution scanning electron microscopes MIRA 3 LMU, (TESCAN, Czech Republic) equipped with energy dispersive X-ray spectroscopy (EDS) detector (INCA Energy 450 XMax 80) capable of detecting low Z elements. The variation coefficient characterizing the reproducibility of a single determination was found to be ∼ 1% for EDS within the compositional range of the main components. Additionally, TM-1000 (Hitachi Ltd., Japan) scanning electron microscopy coupled with EDS detector Swift-TM (Oxford Instruments Analytical Ltd., GB) was used for the analysis of the morphology. The oxidation tests were carried out in plasma generator EDG200 M. The working current range was 120–400 A; the voltage of the plasma generator was 120 ± 1 V. The tested system was composed of the powdered TaC and Ir mixture with silicon additive for improved sintering of the mixture, the TaC:Ir:Si = 14:16:1 M ratio being used. A powder mixture was loaded into steel die and compacted as a pellet with sizes of Ø 30 × 10 mm. The mixture was compacted and heated up to 1600 °C under vacuum. Then the compact was exposed to the arc-jet in the perpendicular direction. The maximum temperature at the ablation center reached 2000 °C, which was measured with an optical pyrometer focused on the hot zone of the specimen. 2

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Fig. 1. X-Ray diffraction patterns of the TaC:Ir = 1:1 mixtures exposed for 1 h at given temperature.: a – the appearance of the strongest 111 peak of TaIr3; b– splitting of the (133) peak of TaIr3 ± x phase due to the formation of the solid solutions; c – the position of the Kα1 and Kα2 doublet for the (422) peak belonging to the TaC phase with temperature; d – a small shift of the Kα1 and Kα2 doublet for the (220) peak belonging to Ir phase with temperature. All XRD patterns were collected with a step of Δ2Θ = 0.009° and a counting time of 177 s per step.

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Fig. 2. SEM images of the product obtained by the solid-state reaction of TaC with iridium from the mixture Ta:Ir = 1:3 at 1600 °C for 4 h: a – the TaIr3 ± x intermetallic grains; b – carbon sheets on the surface of TaIr3 grain.

1100 °C is far from the melting point of iridium, 2443 °C. The enhanced activity of iridium at 1100 °C could be explained by the fact that this temperature is close to Tamman temperature for iridium (T∼1110 °C) [28], at which the kinetic limitations for iridium are partially taken off, and it actively diffuses inward TaC. Earlier, preferential diffusion of iridium inward refractory carbides (e.g., HfC) was conclusively demonstrated by Pierre and co-workers [20,29,30]. The diffusion of iridium inward TaC is accompanied by the destruction of the TaC crystal lattice, followed by the formation of the TaIr3 crystal lattice. The suggestion is based on the comparison of the TaC unit cell volume per atom (11.07 Å) and the TaIr3 unit cell volume per atom (14.67 Å). Another similarity between the reactions of TaC and HfC with iridium is in the fact that both reactions result in the formation of the only intermetallic compound, MeIr3 (Me = Ta, Hf), although intermetallics other than MeIr3 exist in both Me-Ir systems. Strife and coworkers explained this result by the low activities of hafnium and tantalum at the Ir/MeC interface and supported this conclusion by the calculation of thermodynamic metal activity in MeC and MeIr3 compounds [17]. The experimental results of this work confirm their evaluations.

3.2. Peculiarities of the solid-state reaction of iridium with tantalum carbide Some peculiarities of the solid-state interaction of tantalum carbide and iridium (equation 1) will be discussed and compared with those for the similar reaction of hafnium carbide with iridium studied by us previously [27]. TaC(s) + 3Ir(s) = TaIr3(s) + C(s)

(1)

As was stated in this study, the appearance of the new phase, TaIr3, was detected already at a temperature as low as 1100 °C. The analogous behavior was observed for the hafnium carbide – iridium system, too [27]. With temperature, the rate of the solid-state reaction of TaC with iridium increases, so that the TaIr3 content in products can attain almost 96% (wt.) for the 4-h exposure at 1600 °C (1:3 mixture). Earlier studies of the reaction of precious metals (platinum and palladium) with tantalum carbide at ∼ 1500 °C also showed that tantalum carbide can be readily decomposed by these metals [18]. It is noteworthy that the above-mentioned temperature was close to the melting points of these metals (1555 °C for palladium and 1768 °C for platinum), whereas the temperature at which TaC starts to react with iridium, namely,

Table 1 The SEM/EDS data for the product zobtained from the TaC:Ir = 1:3 mixture (1600 °C, 4 h). Spectrum 1 2 3 4 5 6

Ta, % (at.) 24.9 25.6 25.4 25.2 25.7 25.3

4

Ir, % (at.) 75.1 74.4 74.6 74.8 74.3 74.7

TaIr3 ± x TaIr3.02 TaIr2.91 TaIr2.93 TaIr2.97 TaIr2.89 TaIr2.96

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based on the application of two sequential layers, namely, refractory carbide layer which is adjacent to carbon support, and iridium upper layer needs some revision [14,17,20,29,30]. Indeed, both components, metal carbide and iridium, readily react with each other at temperatures that are far lower than the temperature of operation, 2000 °C. Hence, the real composition of the protection system during operation will greatly differ from the initial one. Intermetallic compounds will be present in such coating system as one of the main components.

Similar to the HfC-Ir system, tantalum atoms released from the TaC crystal lattice also can substitute iridium atoms in the iridium crystal lattice with the formation of Ir(Ta) substitutional solid solution (Fig. 1 d; Supplementary Material, Fig. S3 d). Finally, detection of carbon as a free phase also serves as an evidence of the similarity of both systems. Thus, the observable instability of both high-melting carbides in the presence of iridium even at relatively low temperatures (1100–1600 °C), the phase composition of the products formed in the solid-state reactions between MeC (Me = Hf, Ta) and iridium are common features for both systems under consideration. An insignificant difference between the systems is in the formation of rather broad homogeneity range HfIr3 ± x phase in opposition to the almost stoichiometric TaIr3 phase. It follows from the obtained results that the previously developed concept for the oxidation protection of carbon structural materials

3.3. Ablative behavior of the TaIr3-based systems under arc-jet testing According to the qualitative and quantitative X-Ray analysis data, the composition of the compacted and heated (1600 °C, 2 h) specimens is represented by TaC (∼51% (wt.)), TaIr3 (∼17% (wt.)), and IrSi (∼32% (wt.)) phases (Fig. 3a). One can note that this composition

Fig. 3. The XRD pattern (a) and SEM images (b–d) of the surface of the as-received compact: b – a survey image of the surface; c – solidified melt; d –TaC phase as single crystals. 5

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Fig. 4. Survey SEM image of the cross-section (a). According to EDS analysis data, the surface layer is enriched with the TaC phase (b), as well as the bottom of compact is enriched with the TaIr3 phase (c).

crystals are strongly bonded by the solidified melt. According to the EDS analysis data, the single crystals belong to the TaC phase. Their sizes are approximately 5–8 μm. It is noteworthy that the initial TaC powder, which was used for the preparartion of compact, consisted of very fine particles about 200–300 nm in size (Supplementary Material, Fig. S1). Thus, after heating of compact to 1600 °C, the recrystallization of TaC phase and the formation of large-size single crystals occurred. The explanation could be as follows. According to the Ir – Si phase diagram, several rather low-melting eutectics with melting temperatures lower that 1600 °C (temperature of heat-treatment) exist [31]. One could propose that the initial nanosized TaC particles were dissolved in the melt during heating. During slow cooling, the large-size TaC crystals were grown. Plausibly, the low viscosity of the Ir – Si melt, as well the appropriate diffusion coefficients for tantalum and carbon in the melt promote the formation of rather large-size crystals. The survey morphology of the cross-section of sintered sample is presented in Fig. 4 a. The distinct feature is very homogeneous microstructure. Separate non-uniformities as small pores are seen in the nearsurface region. According to the SEM/EDS analysis data, the TaC phase, TaIr3 and IrSi phases are present (Fig. 4 b,c). At the first seconds of the ablation test at 2000 °C, a liquid was formed. Melting appears to be related with the formation of Ta2O5 phase whose melting point is 1877 °C [23]. One can note that it took more than 20 s to reach the temperature of 2000 °C. Such unsual behavior of TaIr3-based specimen could be explained by a high molar heat of fusion of Ta2O5 (120 kJ/mol vs. 9.58 kJ/mol for SiO2) and a high heat capacity of liquid Ta2O5 at 2300 K (about 243 J/mol⋅K vs. 85.8 J/ mol⋅K for liquid SiO2) [32]. The melt was moved to the periphery of the specimen by a powerful gas flow. After cooling, the color of the oxidized specimen was changed (Fig. 5, after ablation). The central part of

corresponds to the surface part of compacts because of the strong absorption of the X-Ray radiation of heavy elements. Thus, during the preparation of compacts, the reaction of TaC with Ir occurred according to Eq. (1). As a result, intermetallic phase, TaIr3, was formed. A part of initial iridium can be consumed by silicon giving iridium silicide according to Eq. (2), which probably may result in the lowered yield of target TaIr3 phase. Another reason of the apparent lowered yield that was determined by the X-Ray analysis could be the fact that the density of the TaIr3 phase is much higher than those of the TaC and IrSi phases, therefore, it “sinks” in the “Ir-Si” eutectic melt and, as a consequence, the surface layer is enriched by “light” phases. Ir(s) + Si(l) = IrSi(s)

(2)

The photo of specimen is presented in Fig. S6 (Supplementary Material). The surface of specimen is colored. Together with grey areas, the golden areas are observed, too. The XRD pattern and SEM images are shown in Fig. 3 a–d. One can see that the surface of the initial compact is nonuniform. Two types of the morphological features can be distinguished (Fig. 3 b). The first feature is related to the large-size areas which are composed of two phases, namely, light-grey and darkgrey (Fig. 3c). These areas look as solidified melt. Taking into account the XRD and EDS results (Fig. 3a), one can conclude the light-grey area is represented at least by the TaIr3 phase, whereas the dark-grey area is composed of the IrSi phase. It should be stressed that the dark-grey phase is greatly deformed, and parallel shear bands are seen on each particle. Plausible, the observable morphological features could be connected with the greater placticity of IrSi phase compared with TaIr3 phase [22]. Another interesting morphological feature is well-grained crystals that are gathered in rather large-size aggregates (Fig. 3d). Well-grained 6

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Fig. 5. The colored photographs of the compacts before and after ablation tests.

Ta2O5-SiO2 binary almost, but not quite, satisfies the conditions for immiscibility. The droplets are enriched with tantalum, which appears to be the consequence of incomplete diffusion during cooling (Supplementary Material, Fig. S7a). Another morphological feature is periodic and aperiodic pattern of Ta2O5 crystals, as well as the dendrite growth on the surface of oxidized compact (Fig. 7 b, c). The reasons of the formation of a variety of crystal morphologies are beyond the scope of this work. Here, one can note that the variety of the morphological features originates from the motion of solid in melt. Commonly, the formation of dendritic crystals is connected with non-equilibrium environments of crystal growth (temperature gradient etc.) [34–38]. The morphology of the cross-section of the specimen after ablation test at 2000 °C is presented in Fig. 8a–d. The EDS data of the central part of oxide scale show that tantalum, silicon, and oxygen are present (Fig. 8 b). The oxidized specimen is divided in two distinct zones where irregular SiO2-rich glass layer with Ta2O5 crystals lies above the unoxidized part of the specimen (Fig. 8 b). Despite the formation of the movable oxide layer, a part of this layer was kept in the central part of the specimen. It is interesting to note that after cooling the cracks are observed within the oxide scale (Fig. 8 a). They are parallel to the

the ablative surface became dark-grey, and the periphery became bluewhite. According to the XRD analysis data, the central part of the compact is composed of hex-Ta2O5, TaIr3, and TaC phases (Fig. 6 a). In addition, a very broad diffuse peak in the 2Θ = 14 – 30° range is present in the XRD patterns (Fig. 6). Hence, one can propose that a glass-like SiO2 phase is also present in the product of the ablation test. On the periphery of the compact surface (Fig. 6 b), two tantalum oxide phases, namely, triclinic and orthorombic phases, and also iridium phase were detected. A halo in the small-angle range of the XRD pattern is an evidence of the presence of glass-like phase together with crystal phases as oxidation products. Indeed, the elemental analysis confirms that silicon and oxygen are present at the periphery of the compact (Supplementary Material, Table S1). The tested sample demonstrates morphologigal diversity (Fig. 7a–c; Supplementary Material, Fig. S7). Fig. 7 shows the central part of the surface after oxidation test. One can see that large (larger than 100 μm) droplets of glass-like phase are periodically distributed in a partially crystallized glassy matrix (Fig. 7 a). Probably, this could be an indication of glass phase separation. As was noted by Reeve et al. [33], the

Fig. 6. The XRD patterns: a – the central part of sample; b – white oxide scale, detached from periphery area of sample and then ground into powder. 7

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Fig. 7. Morphology of the central part of the surface: a – survey image; b – periodic and aperiodic patterns of Ta2O5 crystals; c – dendrite growth of Ta2O5 crystals.

that very fine microstructure of the compact is consisted of the grains less than 1 μm in size (Fig. 8 c). According to the SEM/EDS data, the non-oxidized part is represented by small-size particles belonging to

surface of the unoxidized part of specimen and do not penetrate inward the compact. This can be an evidence of very weak bonding between the oxide scale and the non-oxidized part of the specimen. One can note

Fig. 8. SEM/EDS analysis data of the cross-section of the specimen after arc-jet testing at 2000 °C: a – survey image; b – SEM/EDS analysis of cross-section of the oxide scale in the central part of sample after ablation test; c – fine microstructure of non-oxidized part; d – SEM/EDS analysis of selected areas of the non-oxidized part. 8

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Notes

different phases including TaIr3, TaC, and IrSi (Fig. 8 d). Summarizing the ablation testing results, one can conclude that the TaIr3-based material developed in this work displays good ablation resistance under extreme environmental conditions. Unfortunately, the absence of the full database on the Ta – Ir – C – Si system within a wide temperature range hinders the understanding of the behavior of this multicomponent system as a whole under extreme conditions. Nevertheless, some considerations can be proposed relatively to the role of components of the developed system. One can note that the IrSi phase plays a significant role in the formation of the most favorable microstructure that provides the durability of Ta – C – Ir – Si system to extreme environmental conditions. During preparation, the transient “Ir-Si” liquid phase can be formed. This phase appears to possess rather good wettability. Therefore, it penetrates in the porous area between particles providing good sintering and forming the pore-free microstructure. A pore-free microstructure of the compact and a low recession rate of iridium-based materials (TaIr3 and IrSi) are responsible for the good ablation behavior of the TaIr3-based system. The appearance of TaC single crystals in the system under consideration is probably an inevitable consequence of the formation of the low-melting Ir-Si eutectic with subsequent dissolution of high-melting tantalum carbide in it and recrystallization under cooling during the preparation of the compact. The effect of the TaC phase as single crystals on the ablation behavior can be twofold. On the one hand, this is a positive factor because of very high melting point of tantalum carbide. On the other hand, TaC is not persistent to oxygen even at moderate temperatures and oxidizes with the formation of gaseous carbon oxides and Ta2O5, which is liquid at testing temperature. This disadvantage is partially compensated, first, by the fact that the rate of oxidation of single crystals appears to be slower than for the corresponding powder [39,40]. Second, despite relatively low melting point (1877 °C), Ta2O5 evaporation rate is very low (3.83 × 10−5 g/cm2s at 2027 °C) [23]. The presence of silicon as the IrSi phase in the compact is a favorable factor because during oxidation the glass-like SiO2 is formed. The formation of a surface layer of immiscible multicomponent SiO2 – Ta2O5 glass appears to play a positive role because of the increased liquidus temperature and viscosity [1], as well as decreased oxygen diffusivity, which finally promotes the good ablation resistance of the developed system. However, the most significant contribution in favorable ablation behavior of the developed system appears to belong to the iridium-containing phases. It is noteworthy that the other Tabased ultra-high temperature materials were degraded under the same conditions [41,42].

The authors declare no competing financial interests. Acknowledgements The authors would like to thank PhD N. Bulina for the X-Ray analysis, D.Sci. I. Prosanov for the recorded Raman spectrum. This work was supported by the Russian Science Foundation under Project No. 1819-00075. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical limitations. Appendix B. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.corsci.2018.08.044. SEM image of initial TaC powder, XRD patterns of mixture with TaC:Ir = 1:3 ratio (the effect of temperature and exposure time at 1600 °C); XRD patterns of mixture with TaC:Ir = 1:1 ratio (the effect of exposure time at 1600 °C); Raman spectrum of product obtained in the TaC:Ir = 1:3 mixture (1600 °C, 1 h); the temperature profile recorded at the center of the surface of compact during ablation test; SEM images of oxide scale at the periphery of the sample after testing; refined ternary phase diagram of the Ta – C – Ir system. References [1] M.M. Opeka, I.G. Talmy, J.A. Zaykoski, Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: theoretical considerations and historical experience, J. Mater. Sci. 39 (19) (2004) 5887–5904. [2] L. Silvestroni, H.-J. Kleebe, W.G. Fahrenholtz, J. Watts, Super-strong materials for temperatures exceeding 2000 °C, Sci. Rep. 7 (2017) 40730. [3] Y. Yamabe-Mitarai, H. Murakami, Mechanical properties at 2223 K and oxidation behavior of Ir alloys, Intermetallics 48 (2014) 86–92. [4] Ishida, K.; Kainuma, R.; Oikawa, K.; Ohnuma, I.; Ohmori, T.; Sato, J. Iridium-Based Alloy with High Heat Resistance and High Strength and Process for Producing the Same. Patent US 7666352 B2, February 23, 2010. [5] Y. Yamabe-Mitarai, Y. Gu, C. Huang, R. Völkl, H. Harada, Platinum-group-metalbased intermetallics as high-temperature structural materials, JOM 56 (9) (2004) 34–39. [6] Y. Yamabe-Mitarai, High-temperature strength of Ir-based refractory superalloys, MRS Proc. 646 (2000). [7] L. Zhu, G. Du, Sh. Bai, H. Zhang, Y. Ye, Y. Ai, Oxidation behavior of a double-layer iridium-aluminum intermetallic coating on iridium at the temperature of 1400°C 2000°C in the air atmosphere, Corros. Sci. 123 (2017) 328–338. [8] Z.B. Bao, H. Murakami, Y. Yamabe-Mitarai, Microstructure and oxidation behaviour of Ir-rich Ir-Al binary alloys, Corros. Sci. 87 (2014) 306–311. [9] Y. Yamabe-Mitarai, H. Murakami, High-temperature oxidation resistance of Irbased alloys, Mater. Jpn. 52 (9) (2013) 440–444. [10] H. Yan, M. Zhang, B. Zheng, Q. Wei, Y. Zhang, Modeling the elastic anisotropies and mechanical strengths of Ir3X intermetallics, J. Alloys 696 (2017) 611–618. [11] O.Y. Kontsevoi, A.J. Freeman, Y.N. Gornostyrev, A.F. Maksyutov, K.Y. Khromov, Dislocation structure, phase stability, and yield stress behavior of L12 intermetallics: Ir3X (X = Ti, Zr, Hf, V, Nb, Ta), Metall. Mater. Trans. A 36 (3) (2005) 559–566. [12] E.K. Ohriner, Processing of iridium and iridium alloys, Platin. Met. Rev. 52 (3) (2008) 186–197. [13] K. Zhang, Y. Ye, L. Zhu, S. Bai, Novel Ir-X thermal protection coatings designed for extreme aerodynamic heating environment, Proceedings (2017), http://dc. engconfintl.org/uhtc_iv/11. [14] Criscione, J. M.; Mercuri, R. A.; Schram, E. P.; Smith, A. W.; Volk, H. F. High Temperature Protective Coatings for Graphite, Part II; Technical Documentary Report ML-TDR-64–173, Part II; Air Force Materials Laboratory: Ohio, USA, 1964; p 156. // http://www.dtic.mil/dtic/tr/fulltext/u2/608092.pdf. [15] Holleck, H. Binäre Und Ternäre Carbide Und Nitride Der Übergangsmetalle Und Ihre Phasenbeziehungen; Habilitationsschrift KfK 3087B; Institut für Material- und Festkörperforschung: Kernforschungszentrum Karlsruhe, Germany, 1981; p 358. // https://publikationen.bibliothek.kit.edu/200015609. [16] H. Holleck, Binäre und ternäre carbid- und nitridsysteme der Übergangsmetalle, Materialkundlich-technische Reihe, Borntraeger, Berlin, 1984. [17] J.R. Strife, J.G. Smeggil, W.L. Worrell, Reaction of iridium with metal carbides in the temperature range of 1923 to 2400 K, J. Am. Ceram. Soc. 73 (4) (1990) 838–845. [18] E. Raub, G. Falkenburg, Die Reaktionen Zwischen Karbiden Und Platin Bzw. Palladium Bei Hohen Temperaturen Im Hinblick Auf Das Sintern von Hartmetall, Z. Metallkde 55 (4) (1964) 190–192. [19] E. Raub, G. Falkenburg, Reaktionen von Platinmetallen mit Carbiden der 4. und 5.

4. Conclusions A one-step preparation of the high-melting intermetallic TaIr3 phase by the solid-state reaction of TaC with iridium is proposed. It was stated that the formation of TaIr3 becomes noticeable at 1100 °C, and the content of the target phase, TaIr3, increases with temperature and exposure time. The durability of the TaIr3-based system with a small amount of silicon additive was tested in air in arc-jet. The morphology, phase and elemental composition of the tested at 2000 °C sample were studied and compared with those for the reference sample. Despite the fact that one of the oxidation product, Ta2O5, has melting point lower than 2000 °C, hence, it is liquid under testing, the developed system displays satisfactory ablation resistance under extreme environmental conditions. The iridium-containing phases appear to play the significant role in favorable ablation behavior of the developed system. Because of the formation of liquid and compliant oxide scale during ablation test at 2000 °C, the proposed Ta – Ir – C – Si system needs in the improvement. As one of the solution of problem, an increase the viscosity of the oxide scale, for example, through the modification of the initial Ta – Ir – C – Si system could be proposed. 9

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