RU2786768C1 - Refractory high-entropy alloy with bcc-b2 structure - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy and can be used for the production of structural elements and parts operating at high temperatures in aircraft and rocket engines. Alloy Nb_xMo_xHf_50-xCo_50-x, where x takes the values of 12.5 or 37.5 at.%.

EFFECT: alloy has high strength characteristics from 395 to 460 MPa at 1000°C, plasticity at room temperature of at least 5%, and a stable structure at a temperature of 1200°C.

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Description

The invention relates to the field of metallurgy, and can be used for the production of structural elements and parts operating at high temperatures in aircraft and rocket engines.

To date, superalloys based on nickel, iron, and cobalt have found wide application as heat-resistant materials. Such alloys are used at temperatures above 650°C, the alloys have high strength and structural stability. For more than 60 years, scientific and engineering efforts have been aimed at increasing the productivity and efficiency of superalloys, but the possible ways of alloying these alloys have long been exhausted and operating temperatures are fundamentally limited by the melting point of the main element. A promising alternative is the so-called high-entropy alloys, which have been actively studied in the last decade. Existing experimental data show that high entropy alloys can provide the high performance required for the aviation and missile industries. Most of the recorded HEAs have a single-phase structure; however, the mechanical properties of such alloys are difficult to control [Senkov et al. Development and exploration of refractory high entropy alloys—A review, Journal of Materials Research, 2018, v. 33, p. 3092-3128]. Using nickel superalloys as an example, it has been shown that obtaining balanced properties can be achieved through the formation of a coherent structure [Reed et al. The superalloys: fundamentals and applications, 2006, p. 1-372]. Based on this concept, a large class of materials has been created - high-entropy superalloys [Senkov et al. Effect of aluminum on the microstructure and properties of two refractory high-entropy alloys, Acta Materialia, 2014, v. 68, p. 214-228]. Some of these alloys show high strength along with high ductility. Such characteristics are provided by the bcc matrix and coherent B2 particles. However, most Al-containing high-entropy superalloys are composed of an ordered matrix of B2 and bcc particles, which makes these alloys strong but brittle. But the main problem of such alloys is the low stability of the microstructure: at T > 700°C, coarsening, dissolution, or disordering of (Al, Zr)-rich in B2 particles and the release of additional intermetallic compounds occur [Soni et al. Phase stability as a function of temperature in a refractory high-entropy alloy, Materials Research Letters, 2018; v. 33, p. 3235–3246].

By changing the approach in the development of high-entropy superalloys, it is possible to obtain alloys with an operating temperature of ≥1200 °C. The strength of metal alloys usually decreases sharply at a melting point above _0.6Tmelt ( _Tmelt is the absolute melting point), so alloys with a higher melting point have higher operating temperatures. Several papers have reported binary B2 compounds of CoTi, CoZr and CoHf with high melting point and high ductility [Takasugi et al. Anomalous temperature dependence of the yield strength in IVa-VIII intermetallic compounds with B2 structure, J. Mater. Sc., 1991, v. 26, p. 2941–2948; Yamaguchi et al. Room-temperature tensile property and fracture behavior of recrystallized B2-type CoZr intermetallic compound, Scr. Mater., 2005, v. 52, p. 39–44; Agnew et al. Determination of the dislocation-based mechanism(s) responsible for the anomalous ductility of a class of B2 intermetallic alloys, IOP Conf. Ser. mater. sci. Eng., 2019 p. 580]. However, these compounds have a low yield strength. Significant hardening can be achieved by introducing a harder bcc phase, consisting of elements of groups 5 and 6 of the periodic table.

Patent KR20200093826 (A) (publication date 2020-08-06) describes an alloy Ti ₂₅ Zr ₂₅ Nb ₃₀ Al ₂₀ (at.%) having a bcc-B2 structure. This alloy shows a high ductility of about 30% while maintaining a high strength of about 1400 MPa at room temperature and a stable structure in the temperature range of 1300-1600°C. However, there is no information on mechanical properties at high temperatures. Also, a deviation from the calculated ratio of elements can lead to the formation of intermetallic compounds, which will significantly worsen the mechanical properties of the alloy.

We studied the Nb ₄₀ Ti ₂₅ Al ₁₅ V ₁₀ Ta ₅ Hf ₃ W ₂ (at.%) alloy, which also has a bcc-B2 structure. Plastic at room temperature, it shows low strength (σ _YS = 237 MPa) at 1000°C. Also, the stability of the structure of this alloy is not fully understood [Pang et al. A ductile Nb ₄₀ Ti ₂₅ Al ₁₅ V ₁₀ Ta ₅ Hf ₃ W ₂ refractory high entropy alloy with high specific strength for high temperature applications, Materials Science & Engineering A, 2022, v. 831, p. 142290].

Al ₁₀ Nb ₁₅ Ta ₅ Ti ₃₀ Zr ₄₀ based on the BCC-B2 structure shows high strength and ductility at room temperature, the yield strength is 1075 MPa and the ductility is 60%. At a temperature of 600-750°C, the formation of AlZr ₂ particles occurs, which leads to significant softening. The yield strength of the alloy at 1000°C is 45MPa [Soni et al. Phase stability and microstructure evolution in a ductile refractory high entropy alloy Al ₁₀ Nb ₁₅ Ta ₅ Ti ₃₀ Zr ₄₀ , Materialia, 2020, v. 9, p. 100569].

A refractory high-entropy alloy Nb ₃₀ Mo ₃₀ Hf ₂₀ Co ₂₀ was chosen as a prototype. This alloy contains 30 at.% niobium, 30 at.% molybdenum, 20 at.% hafnium, 20 at.% cobalt. The alloy has high mechanical properties at room temperature and a stable structure at T ≥ 1200°C. The main disadvantage is the low strength at 1000°C [Yurchenko et al. Refractory high entropy alloy with ductile intermetallic B2 matrix / hard bcc particles and exceptional strain hardening capacity, Materialia, 2021, v. 20, p. 101225].

The technical objective of the invention is to create a refractory high-entropy alloy with high strength characteristics at a temperature of 1000°C, with sufficient ductility at room temperature and a stable structure at a temperature of 1200°C.

EFFECT: high strength characteristics of the proposed alloy: from 395 to 460 MPa at 1000°C, with sufficient plasticity at room temperature of at least 5%, as well as a stable structure at a temperature of 1200°C.

The technical result is achieved by the proposed refractory high-entropy alloy Nb _x Mo _x Hf _50-x Co _50-x, where x is 12.5 or 37.5 (at.%).

The invention is characterized by the images shown in the figures:

fig. 1 - microstructure of the Nb ₅ Mo ₅ Hf ₄₅ Co ₄₅ alloy in the cast (a) and annealed state after annealing at 1200°C for 24 hours (b), obtained using an FEI Quanta 600 FEG scanning electron microscope;

fig. 2 - microstructure of the Nb _12.5 Mo _12.5 Hf _37.5 Co _37.5 alloy in the cast (a) and annealed state after annealing at 1200°C for 24 hours (b), obtained using an FEI Quanta scanning electron microscope 600 FEG;

fig. 3 - microstructure of the Nb _37.5 Mo _37.5 Hf _12.5 Co _12.5 alloy in the cast (a) and annealed state after annealing at 1200°C for 24 hours (b), obtained using an FEI Quanta scanning electron microscope 600 FEG;

fig. 4 - Table 1 - Comparative characteristics of samples of alloys.

The claimed invention meets the conditions of novelty and inventive step, since it is not known from the prior art that the Nb _x Mo _x Hf _50-x Co _50-x alloy, where x takes the values of 12.5 or 37.5, provides high strength characteristics : from 395 to 460 MPa at 1000°C, with sufficient plasticity at room temperature of at least 5%, as well as a stable structure at a temperature of 1200°C, and the assumption that such a ratio of elements can make it possible to obtain the claimed technical result is obvious to a specialist way does not follow from the prior art.

Compliance with the condition of industrial applicability of the invention is confirmed by the following examples of implementation.

Samples of the alloy according to the invention Nb _x Mo _x Hf _50-x Co _50-x where x takes the values of 5, 12.5 or 37.5 (at.%), were made by vacuum arc remelting.

Fusion of high-purity (≥99.9 at.%) charge materials was carried out in an argon atmosphere in a water-cooled copper mold. The time for maintaining the melt in a liquid state is no more than 20 seconds. The resulting ingots were remelted 5 times to obtain a uniform distribution of elements over the volume.

The samples were annealed at 1200°C for 24 hours in a Nabertherm muffle furnace. To prevent oxidation of the alloy during annealing, the ingots were pre-soldered into a quartz tube at a pressure of ~1.3 Pa.

To conduct microstructural studies and tests to study the mechanical properties, 4x4x6 mm samples were cut from ingots by the electroerosive method. There were no structural macrodefects in the obtained ingots: shells, cracks, pores.

The microstructure was studied using an FEI Quanta 600 FEG scanning electron microscope. Mechanical tests were carried out on an Instron 5882 tensile testing machine in accordance with GOST 8817-82. Metals. Draft test method.

Examples of the invention.

Example 1

For the study, an Nb alloy is used_fiveMo_fivehf₄₅co₄₅, with the following ratio of components, at.%: 5 niobium, 5 molybdenum, 45 hafnium, 45 cobalt, which has a two-phase bcc-B2 structure. It can be noted that no significant changes in the microstructure are observed after annealing. This ratio of elements allows you to get a stable structure (Fig. 1) and plasticity values exceeding the corresponding values of the prototype. The value of the yield strength at room temperature is 640 MPa, plasticity 16%. The value of the yield strength at a temperature of 1000°C is 365 MPa, ductility>50% (figure 4). Since the strength value at a test temperature of 1000°C does not exceed the strength value of the prototype alloy at the same temperature, the technical result is not achieved.

Example 2

For the study, an Nb _12.5 Mo _12.5 Hf _37.5 Co _37.5 alloy is used, having the following ratio of components, at.%: 12.5 niobium, 12.5 molybdenum, 37.5 hafnium, 37.5 cobalt , which has a two-phase bcc-B2 structure. This ratio of elements allows to obtain a stable structure (Fig. 2), as well as the values of strength and ductility at temperatures from 600 to 1000°C, higher than that of the prototype. The value of the yield strength at room temperature is 1155 MPa, ductility 10%. The value of the yield strength at a temperature of 1000°C is 395 MPa, ductility >50%. (Fig. 4)

Example 3

For the study, an Nb alloy is used_37.5Mo_37.5hf_12.5co_12.5, having the following ratio of components, at.%: 37.5 niobium, 37.5 molybdenum, 12.5 hafnium, 12.5 cobalt, which has a two-phase bcc-B2 structure. This ratio of elements allows to obtain a stable structure (Fig. 3), as well as the strength value at temperatures from 22 to 1000°C, higher than that of the prototype. The value of the yield strength at room temperature is 1280 MPa, ductility 5%. The value of the yield strength at a temperature of 1000°C is 460 MPa, ductility >50% (Fig. 4).

Thus, the claimed technical result is high strength characteristics: from 395 to 460 MPa at 1000°C, with sufficient ductility at room temperature of at least 5%, as well as a stable structure at a temperature of 1200°C, Nb _x Mo _x Hf _{50- x} Co _50-x, where x takes on the values of 12.5 or 37.5 (at.%), is reached.

Claims (1)

Refractory high-entropy alloy Nb _x Mo _x Hf _50-x Co _50-x, where x takes values of 12.5 or 37.5 at.%.

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