Progress of High Temperature Niobium Alloy for Aerospace Applications

Niobium metal has a low density (8.57g/cm3), high melting point (2741 K), high plasticity, good corrosion resistance and low vapor pressure, etc.,. Niobium alloy has a higher temperature (600 ~ 1600 ℃) than the strength and good hot and cold machining performance, you can make the shape of the complex parts, is one of the important candidates for aerospace structural components. It can be used to manufacture critical components for rocket engines, space-earth round-trip airships, supersonic aircraft, satellites, missiles, and nuclear reactors, including shields for combustion chambers of high-thrust space engines, combustion chambers, small-vector or attitude-control nozzles, and extended shields for orbit-control engines, and so on.

In the 1960s, traditional niobium-based alloys began to be used in the aerospace and nuclear industries, with the most widely used alloy being the C-103 (Nb-10Hf-1Ti) niobium alloy, which can be used for high-temperature valves, rocket booster tops, and damper blades for turbine booster units. The extension of the radiation cooling nozzle of the descent engine of the lunar module of the Apollo 11 spacecraft of the United States is also processed with C-103 niobium alloy and coated with anti-oxidizing alumina. the combustion chamber of the R-4D reaction-control engine is processed and made of SCb-291 (Nb-10W-10Ta) and C-103 niobium alloys. In the 1990s, along with the development of aerospace technology, various countries successively developed C-103 niobium alloys for rocket engines, satellite attaches, and rocket engines. In the 1990s, with the development of aerospace technology, countries carried out a new round of competitive development in the fields of rocket engines, satellite attitude control engines and ultra-high speed airplanes, etc., which put forward higher requirements for high-temperature structural materials, and niobium alloys have received further attention again.
In response to the aerospace industry’s needs, the United States and the former Soviet Union have developed more than 20 types of niobium alloys, forming their own system. The niobium alloys of the United States are mainly reinforced with W, Mo and Hf, while those of the former Soviet Union are mainly reinforced with W, Mo and Zr. The second-phase dispersion reinforcement is mainly carbide, which are primarily used in the two-component liquid rocket motors with high specific impulse and adjustable thrust that can be started many times. Among them, the United States to C-103 alloy-based, temperature up to 1200 ~ 1400 ℃; the former Soviet Union to niobium alloy 5 ΒΜЦ (Nb-5W-2Mo-1Zr) alloy based on the density of the alloy and C-103 similar, but the use of the temperature can be reached 1200 ~ 1650 ℃, for a short period of time can be reached 2000 ℃. Based on American and Russian niobium alloys, China has copied and researched and developed niobium alloy structural materials for aerospace engines such as C-103, Cb-752, C-129Y, D43, SCb-291, and Nb521, among which C-103 and Nb521 alloys are the most widely used.
Based on the classification of traditional niobium-based high-temperature alloys, this paper highlights the research progress of C-103, PWC-11 and Nb521 alloys, as well as low-density niobium alloys, discusses the current problems of niobium alloys for aerospace applications, and looks forward to the direction of future development.

1. Classification of Niobium Alloys

Niobium alloys are usually classified into six categories according to their strength levels and functional properties: the first category of high-strength low-plasticity niobium alloys, the second category of medium-strength medium-plasticity niobium alloys, the third category of low-strength high-plasticity niobium alloys, the fourth category of high-strength antioxidant niobium alloys, the fifth category of plastic antioxidant niobium alloys, and the sixth category of corrosion-resistant niobium alloys. According to density, they are divided into high-density and low-density niobium alloys. According to different functions, they are divided into structural alloys, functional precision alloys and corrosion-resistant alloys. The main structural alloys used in the aerospace field are roughly divided into three categories of high strength, medium strength and low strength according to their strength, low-density niobium alloys have been developed in consideration of weight reduction, and high-strength niobium alloys reinforced with interstitial compounds (carbides, oxides, and nitrides) have been developed in order to improve the comprehensive performance of niobium alloys continuously. Niobium alloys of various strengths are currently produced or have been studied as listed in Table 1.

1.1 High-strength, low-plasticity niobium alloys

Generally, high-strength, low-plasticity niobium alloys are strengthened by adding the alloying elements W, Mo and Ta, small amounts of Hf and Zr, and traces of C. These niobium alloys are also known as Cb-niobium alloys. These niobium alloys include Cb-1, As-30, Cb-132M, F48, Su-31, etc., mainly used for gas turbine blades. The W, Mo and Ta elements in these alloys are solid solution strengthened, and Hf and Zr can form a diffuse second-phase strengthening phase with C, giving the alloys higher creep strength. Their solid-phase temperature is higher than that of pure niobium, and their recrystallization temperatures are also higher. Their high-temperature strengths are significantly higher than those of medium- and low-strength niobium alloys, and their operating temperatures are generally in the range of 1,300 to 1,600°C, with even higher temperatures for short periods of time. The antioxidant property is improved somewhat because of W, Hf and other elements. However, with the increase in the content of W, Mo and other high melting point reinforcing alloying elements, the plastic-brittle transition temperature will also rise, and its plastic processing performance deteriorates, deformation processing is difficult. To ensure a good match between high-temperature strength and low-temperature plasticity, the thermomechanical processing of these alloys must be strictly controlled. In addition, high-strength niobium alloys are primarily in the development stage, and the niobium alloys currently in industrial production are primarily medium- and low-strength niobium alloys.

1.2 Medium strength, medium plasticity niobium alloys

Medium-strength and medium-plasticity niobium alloys are mainly composed of niobium as a base with the addition of not more than 10% of metal elements such as W, Mo, Ta, V, Ti, Zr, Hf and a small amount of C. These alloys include C-129Y, SCb-291, D31, D43, FS85, Cb-752, PWC-11, 5ВНЦ and Nb521. The strength of these alloys at room temperature is 400-600 MPa and the elongation is 20-30%; at high temperatures of 1000-1400°C they are still quite strong and can work efficiently, and if the time is short, the working temperature can be higher. Since this kind of alloy contains moderate amount of W, Ta, Ti, Zr, Hf, the recrystallization temperature is increased to 1150~1250℃. At the same time, because the plastic-brittle transition temperature of this type of alloy is higher than pure niobium, the transition temperature in the welded state is generally above room temperature. It is more sensitive to the interstitial elements such as O, N, H, etc., so it is necessary to control the contamination of O, N, H strictly, and the oxygen content in the alloy must be controlled at less than 80 ppm. This kind of alloy has a certain degree of plasticity and good process performance can be used to manufacture various parts and components, such as skin, bolts and, nuts, and other components.

1.3 Low-strength, high-plasticity niobium alloys

Low-strength, high-plasticity niobium alloy is niobium as a substrate, the addition of Ⅳ of the periodic table of elements such as Ti, Zr, Hf and other metal elements to form a solid solution strengthened alloy. The alloys belonging to this category are Nb-1Zr for liquid metal containers and pipelines, C-103 alloy for rocket engine thrust chambers, radiation sleeves, and heat shielding, and Cb-753 alloy for ion engines with honeycomb structures. The recrystallization temperature of these niobium alloys is about the same as that of pure niobium, which is generally 1000-1100°C. The room-temperature strength of the alloys is generally 3,000-3,000°C. The room-temperature strength of the alloys is 320-420 MPa, and the elongation at break is 20-40%. Such alloys have good welding performance. The plastic-brittle transition temperature is low, in the (0.37 ~ 0.47) T melting temperature range and vacuum; after aging treatment of plastic-brittle transition temperature is still lower than room temperature. These alloys, compared with medium and high strength alloys, have good plasticity at room temperature, with excellent process performance, and can be made for liquid alkali metal piping, space and power generation equipment turbopumps, satellites, spacecraft, and missiles, attitude control/orbit control engine thrust chamber body extensions and other components.

Table 1 Niobium alloys of various strength grades

Classification	Alloy Designation	Nominal Composition	Semi-finished Product State	Temperature (°C)	Rm​ (MPa)	Rp0.2​ (MPa)	A (%)
High Strength	Cb-1	Nb-30W-1Zr-0.06C	Annealed rod	1315	–	–	–
	B88	Nb-28W-2Hf-0.067C	–	1315	372	–	–
	VAM-79	Nb-22W-2Hf-0.067C	–	–	–	–	–
	Cb-132M	Nb-15W-5Mo-2Ta-2.5Zr-0.13C	Annealed rod	1315	407	–	–
	As-30	Nb-20W-1Zr-0.1C	Extruded rod (ε80-90%)	1315	633	580	9
	F-48	Nb-15W-5Mo-1Zr-0.1C	Cold working plate (ε80-90%)	1095	450	295	18
	Su-31	Nb-17W-3.5Hf-0.1C	Annealed rod	1315	281	–	–
	C-3009	Nb-22.4Hf-5.9W	Ingot	1200	588	–	–
Medium Strength and Plasticity	Su-16	Nb-11W-3Mo-2Hf-0.08C	–	1100	333	231	–
	Fs-85	Nb-10W-28Ta-1Zr	Annealed	1315	161	–	–
	F-50	Nb-15W-5Mo-1Zr-5Ti-0.1C	Cold working plate (ε80-90%)	1205	146	90	35
	D-31	Nb-10Mo-10Ti	Annealed rod	1095	260	184	8
	D-43	Nb-10W-1Zr-0.1C	Extruded rod	1260	253	225	5
	Cb-752	Nb-10W-2.5Zr	Annealed plate	1649	70.3	63.3	–
	SCb-291	Nb-10W-10Ta	Annealed sheet	1649	70.3	59.3	–
	C-1297	Nb-10W-10Hf-0.1Y	Annealed plate	1649	74.4	60.7	–
	B-66	Nb-5Mo-5V-1Zr	Annealed plate	1315	372	–	–
	B-77	Nb-10W-5V-1Zr	Annealed plate	1315	391	–	–
	As-55	Nb-5W-1Zr-0.1Y-0.06C	–	–	–	–	–
	PWC-11	Nb-1Zr-0.1C	–	927	160	–	–
	5BHt1	Nb-5W-2Mo-1Zr	Annealed rod	1250	120-140	80-110	8-10
	Nb521	Nb-5W-2Mo-1Zr-0.07C	Annealed rod	1600	≥70	≥60	≥4
Low Strength and High Plasticity	Cb-753	Nb-5V-1.25Zr	–	–	–	–	–
	D-369	Nb-10Hf-1Ti-0.7Zr	Annealed plate	1095	199	–	–
				1427	703	–	–
	D-36	Nb-5Zr-10Ti	–	–	–	–	–
	Cb-1Zr	Nb-1Zr	Rotary swaging rod	927	130	–	–

2. Progress and applications of niobium alloys for aerospace applications

At the end of the 1950s, there was a strong interest in nuclear power, aviation and space flight, and the development of related niobium alloys began, and the development of commercial conventional niobium-based alloys began in the 1960s. In the United States, the development of niobium alloys in the early units are mainly CANEL, Wright Patterson AFB (WADD), Boeing Aircraft Company, General Electric Company, Union Carbide Corporation and Westinghouse. Among them, CANEL is primarily engaged in Cb-1Zr and PWC-11 alloy research, Boeing and TWCA developed C-103 and C-129Y alloy, General Electric Company developed As-55, As-30, F48 and F50 niobium alloys, Union Carbide developed Cb-752 alloy. Around 1970, the development of medium and low-strength niobium alloys in foreign countries has been relatively mature, and the niobium alloys that have been used in rocket nozzles or thrust chambers are C-103, SCb-291, C-129Y, FS85, etc. Among these alloys, C-103 alloy has been used in rocket nozzles and thrust chambers, and C-129Y alloy has been used in rocket nozzles and thrust chambers. Among them, C-103 alloy processing, welding performance is excellent, although the room temperature strength is low, but the overall performance is good, especially at high temperatures, can meet the working conditions of the nozzle. SCb-291 alloy plasticity is good and high temperature strength is relatively high, C-129Y alloy also has a good plasticity and welding performance, but the creep strength is low, FS85 alloy creep strength is high, plasticity, welding performance is very good. Performance is excellent. In terms of overall performance, Cb-752 and D43 alloys have high-temperature strength, moderate plasticity, and good machining and weldability and can be selected for rocket nozzles. However, niobium alloys also have a fatal weakness: is easy to oxidize in the atmospheric atmosphere at high temperatures, that is, at about 600 ℃ will be very easy to “pest” oxidation phenomenon, the need to prepare a layer of antioxidant protective coatings on the surface, to meet the high-temperature requirements of the use of the thrust chamber of the space engine.

In the 1970s, people began to study high-strength niobium alloys, the primary strengthening method is still solid solution strengthening or dispersion strengthening two ways mainly; during this period, the former Soviet Union and the United States have carried out a lot of research on the development of high-strength niobium alloys, China’s research in the field of this type of material is still in the blank. With the continuous development of new aerospace products and the need for upgrading, the orbit control/attitude control liquid engine’s specific impulse and weight reduction have put forward higher requirements. Consideration is given to starting from the weight reduction of the base material, which in turn develops the low-density niobium alloy with a density of less than 8g/cm3.
In aerospace, the United States is the most widely used C-103 alloy, using temperatures between 1200 ~ 1400 ℃, followed by PWC-11, Nb-1Zr, SCb-291, FS85, and other alloys. The coatings are mainly of the silicide system, such as R512A (Si-20Cr-5Ti) and R512E (Si-20Cr-20Fe). The most used alloy in Russia is 5ΒΜЦ alloy with a service temperature of 1200-1650°C, usually coated with molybdenum silicide (MoSi2). China’s most used alloys are C-103 and Nb521. The current orbit control/attitude control engine refractory metal materials thrust chamber has formed a “two-generation” series of products. The “first generation” is a niobium-hafnium alloy (i.e., C-103) and “815” coating system, and the “second generation” is a niobium-tungsten alloy (i.e., Nb521) and “056” coating system. The “second generation” is a niobium-tungsten alloy (i.e., Nb521) and a “056” coating system. Physical photos of the thrust and combustion chambers of the two generations of niobium alloy engines in China are shown in Figure 1. The physical properties of the two niobium alloys are compared in Table 2, and their high-temperature tensile properties are shown in Figure 2. From Fig. 2, it can be seen that the high-temperature mechanical properties of Nb521 are much higher than those of C-103 alloy, and its strength is 3-4 times higher than that of C-103 alloy at 1600℃, and it has been successfully applied to many kinds of orbit/attitude control type engines. Some examples of niobium alloys used in the aerospace industry at home and abroad are listed in Table 3.

(a) The first generation of thrust chambers with different specifications(C-103 + 815 coating) ;

(b) The second generation engine thrust chamber(Nb521+ 056 coating);

(c) The second generation engine combustion chamber(Nb521+ 056 coating);

(d) The First (C-103,the specific impulse in 305 seconds)and second (Nb521,the specific impulse in 315 seconds)generation 490N engines

Table 2 Physical properties of typical niobium alloys

Alloy	Melting Point (°C)	Density (g/cm³)	Thermal Conductivity (W/(m·K))	Coefficient of Thermal Expansion (10⁻⁶ K⁻¹) (20–1205°C)	Working Temperature (°C)	Ref.
C-103	2350	8.86	41.9	8.10	1100–1450	[16,27]
Nb521	2452	8.85	48.7	7.40	1370–1650	[27,40]

Table 3 Applications of niobium alloys in the aerospace industry

Nationality	Alloy	Application Examples
USA	C-103	R-4D (490N), R-1E (110N), and R-6C (22N) thrust chambers of rocket engines, attitude control of Apollo lunar module and service module, Agena auxiliary propulsion system, combustion chamber, and nozzle, wings of the Apollo engine, compression cone of a launch vehicle, and structural material for the orbital control engine of the space shuttle.
	PWC-11	The reactor for satellites, space nuclear power systems, and high-temperature reactors.
	Nb-1Zr	Minuteman III MK12 attitude control engine, injector. Poseidon’s final boost control system. Exhaust gas regenerator for Poseidon engine system.
	SCb-291	2670, 1330, 90N shuttle attitude control engine. MinutemanⅢ MK12 attitude control engine and thrust chamber.
	Cb-752	Booster glide manned vehicle. Space shuttle and manned aircraft, screw joints.
	FS-85	Space nuclear energy system, liquid metal container.
	C-129Y	Space shuttle and manned aircraft, screw joints.
Russia	5BMЦ	12, 20, 100, 135, 200, 400N engine thrust chamber is used in satellites such as “Cosmos”, “Crystal”, “Quantum”, “nature” and “Spectrum”, in spacecraft such as “Progress” and “Soyuz-T”, and in space stations such as “Salute” and “Peace”.
China	C-103	490N engine, used for all kinds of satellites, Shenzhou spacecraft, Tiangong and carrier rockets, etc.
	Nb521	The nozzle of the gas-oxygen kerosene engine of a new generation of the high-thrust launch vehicle. 25, 120, 150, 250, 490N engine, Attitude and orbit control engine for Sinosat 6, Nigeria Satellite, etc. China Lunar Rover project engine.

The following is a summary of the progress of C-103, PWC-11, Nb521 alloys, and low-density niobium alloys under research and development.

2.1 C-103 niobium alloy

C-103 (Nb-10Hf-1Ti) alloy is a kind of low-strength niobium alloy with good high-temperature strength, excellent molding performance and welding performance jointly developed by ATI Wah Chang and Boeing, which is widely used in rocket propellers and other fields, and is a kind of niobium alloy with a wide range of applications. Our country imitated this alloy used in two-component liquid rocket engines, and using niobium silicide high-temperature anti-oxidation protective coating, the working temperature can reach 1200 ~ 1300 ℃.

The alloy is a solid solution strengthened by adding Hf, Ti, and Zr alloy elements. Zr strengthening is more significant; Hf and trace W are mainly used to improve high-temperature performance. Panwar et al. investigated the tensile properties and fracture behavior of C-103 alloy in the temperature range between room temperature (RT) and 1200℃ (see Fig. 3), the stress-strain curve of the alloy shows a sawtooth shape and occurs in a high-temperature range from RT to 1200℃, the stress-strain curve of the alloy shows a sawtooth shape and occurs in a low-temperature range. The stress-strain curves of the alloys between room temperature and 1200°C show a sawtooth shape, and the dynamic strain aging (DSA) phenomenon occurs. Among them, dynamic strain aging plays a dominant role in the tensile properties at 900°C, and dynamic recovery and oxidation significantly influence the tensile properties beyond 900°C. The fracture mechanisms were ductile fracture, disintegration fracture and along-crystal fracture from room temperature, 600~900°C and 1000~1200°C, respectively. In addition, the addition of Hf element has another function to improve the resistance of the alloy to internal oxidation. Sankar et al. investigated the effect of internal oxidation on the organization and mechanical properties of C-103 niobium alloy. The microstructure and mechanical properties of alloy samples with different oxygen contents (100~2500 ppm) were characterized, and it was found that the strength and plasticity of the alloy decreased with the increase of the average oxygen content, and the internal oxidation led to surface embrittlement of the alloy. Dhole et al. found that industrial C-103 alloys prepared using the vacuum arc remelting (VAR) method were characterized by a pronounced internal oxidation, that is, at all grain boundaries, there was no internal oxidation in the alloys. Monoclinic HfO2 was present at all grain boundaries and coarsened HfO2 second-phase particles were present at some of the large-angle grain boundaries. Cold rolling cracking occurs even when a small deformation is applied during cold rolling (see Fig. 4). Further microscopic observation reveals selective cold cracking at the Nb-HfO2 interface. Density Functional Theory (DFT) calculations on the metal-oxide interface model indicated that the chemical nature of the termination layer significantly affects the work of separation and depolymerization. In the termination layer of the Nb-HfO2 interface, their separation energies differ significantly due to the difference in atomic bonding, especially the separation work of the single-atom layer interface between Nb and the oxygen bound in HfO2 is very low, about 8% lower than that of the Nb-Nb interface of the substrate. That is, DFT simulations reveal the decisive role of the termination layer at the metal-oxide interface for interfacial depolymerization.

Adding a small amount of Ti to the alloy not only improves the machinability of the alloy, but also improves the antioxidant properties. The interstitial impurities C, N, H, and O elements have a great influence on the mechanical properties of the alloy, and when the oxygen content exceeds 0.1%, it will lead to difficulties in subsequent cold working. In addition, the oxidation resistance of the alloy deteriorates above 450°C. Therefore, care should be taken to avoid thermal oxidation during thermal processing, and inert gas protection or vacuum equipment is usually used. The metal surface is usually coated with silicide antioxidant protective coatings to prevent oxidation when used at high temperatures. Sankar et al. used slurry coating and vacuum diffusion technology to prepare Fe-Cr alloy silicide coating on the surface of C-103 alloy, and observed that the coating microstructure of the three-layer structure, the outer layer of the NbSi2 phase, the inner layer of Nb5Si3 and Nb3Si and other low silicide, the intermediate layer of Fe-Cr alloy is composed of low silica, and the middle layer is composed of Nb5Si3 and Nb3Si, and the middle layer is composed of Nb5Si3 and Nb3Si. The middle layer comprises the Fe-Cr alloy niobium silicide phase and NbSi2 (see Figure 5). The results show that the coating provides good short-term protection of the substrate against high-temperature oxidation in air at 1100 and 1300 °C, and the presence of the coating also increases the tensile strength of the alloy.

ATI Wah Chang prepares C-103 niobium alloy ingots by electron-beam secondary remelting. These are then machined, clad and sheathed, hot-extruded into slabs, and then cold-rolled and processed into thin plates. For smaller components or propellers, the bars are generally processed directly. For the manufacture of larger combustion chambers requiring larger diameter bars, or the use of bar taper die reverse extrusion molding process can effectively improve the components’ yield and the formation rate. The good forming performance and stable reliability make C-103 niobium alloy have excellent cost performance and further promote its application in the aerospace field.

2.2 PWC-11 niobium alloy

PWC-11 (Nb-1Zr-0.1C) alloy is a niobium alloy developed by Pratt & Witney Aircraft Company for high-temperature applications with moderately strong plasticity, good resistance to high-temperature creep, and resistance to corrosion by liquid alkali metals, and it is widely used in high-temperature reactors and space nuclear power systems.

The alloy is simultaneously strengthened by solid solution and precipitation by adding Zr and C elements to Nb to increase the high temperature strength of the alloy. Therefore, the carbide precipitation strengthening of PWC-11 alloy has attracted much attention.The transformation of Nb and C to a mixture of ZrC and NbC during heat treatment has been confirmed by thermodynamic analysis by Farkas et al. Free energy calculations showed that the maximum NbC concentration exists in the mixed carbide particles at a certain aging temperature. The higher the aging temperature, the higher the equilibrium NbC content in the particles.The microstructures of Nb-1Zr-0.1C alloy in as-cast, extruded, and annealed states were studied by Vishwanath et al. The results showed that: the as-cast state organization is a two-phase organization in which the Nb2C precipitation phase is distributed in the matrix in the form of needles; there are two types of precipitation phases in the extruded state specimens (see Fig. 6(a)~(c)), one is needle-like ((Nb, Zr)2C with an orthorhombic crystalline structure, and the other one is rectangular ((Nb,Zr)3C2 with hexagonal structure); and the organization in the annealed state shows an equilibrium structure, i.e., the Nb matrix and (Nb, Zr)C precipitates (see Figs. 6(d)~(f)). Vishwanath et al. investigated the formation of γ-Nb2C and the interrelationships of the other Nb2C carbide phases (α, β) in the Nb-1Zr-0.1C alloy. The results show that the transformation of Nb to γ-Nb2C is realized by carbon atoms occupying octahedral positions in the Nb bcc lattice, whereas the sequence of phase transitions γ-Nb2C → β-Nb2C → α-Nb2C involves the arrangement of vacancies in a lattice that is essentially the same for all three structures (see Fig. 7). Lattice strain calculations show that the strain due to the carbon atoms occupying octahedral positions arises mainly along the [0001] direction of the Nb2C lattice, and this strain also contributes to the dissolution of the substable disordered structural phase, γ-Nb2C phase, at lower temperatures. In order to further clarify the formation of various carbides and the Nb2C→NbC carbide transition in Nb-1Zr-0.1C alloy, Vishwanadh et al. prepared specimens using two methods (i.e., solidified specimen extrusion + recrystallization treatment and solidified specimen + heat treatment) and found that the alloy contains only α-Nb2C carbides in the solidified state, and the Zr phase is uniformly distributed between Nb matrix and Nb2C phase. When Zr is uniformly distributed in the Nb matrix and Nb2C phase, α-Nb2C remains stable; when the alloy is extruded or heat-treated, Zr preferentially diffuses into the α-Nb2C carbide phase, destabilizing the carbide phase, and precipitates out in the subsequent annealing treatment as a more stable (Nb,Zr)C phase with a spherical or acicular morphology, which is specifically related to the thermo-mechanical processing of the alloy; if the precipitated phase nucleates before recrystallization and the If the precipitated phase nucleates before recrystallization, and the growth of the precipitated phase occurs at the same time as recrystallization, it is spherical and has no specific orientation relationship with the matrix phase. If the precipitated phase nucleates and grows after recrystallization or in the uncrystallized sample, it is needle-like and follows a specific orientation with the matrix phase (see Figure 8).

The PWC-11 alloy has very good machinability and fabrication properties compared to other niobium, tantalum, and molybdenum alloys, and Sarkar et al. used vacuum uniaxial compression experiments to study the high-temperature thermal deformation behavior of the Nb-1Zr-0.1C alloy in the range of 700-1700°C and 10-3~ 10s-1 strain rate, and found that the alloy was stable at temperatures greater than 1400°C and at a wide range of strain rates ( 10-3~ 10-1s-1) range will occur dynamic recrystallization, with better processing performance; at the appropriate temperature and strain rate, the dynamic recrystallization grain size of the material can be changed, while below 1000 ℃ and in the range of 10-3~ 10s-1 strain rate are occurring in the region of strain localization, which should be avoided as much as possible in the industrial thermal processing process.Behera et al. also used the Behera et al. also used uniaxial compression experiments to investigate the evolution of dynamic recrystallization with strain in Nb-1Zr-0.1C alloys at 1500 and 1600°C and a strain rate of 0.1s-1. The results show that at strains of 0.6 and 0.9 and temperatures of 1500 and 1600 °C, dynamic recrystallization grains are distributed in a necklace-like manner along the jagged large-grain grain boundaries; at 1500 and 1600 °C, fine grain distribution can be observed at a strain of 0.9, and equiaxial organization is seen at a strain of 1.2 (see Fig. 9); at 1500 and 1600 °C, and at all the measured strains, the dynamic recrystallization evolution of the alloys with strains of 0.1 s-1 and 0.1 s-1 is observed, and the dynamic recrystallization evolution of the alloys with strains of 0.1 s-1 and 0.1 s-1 is observed. at 1500 and 1600 °C and at all measured strains, the recrystallized grains have a strong <001> structure.
Welding is essential for the application of Nb-1Zr-0.1C alloys. Welding of this alloy is a difficult task due to its high melting point and reactive nature. Badgujar et al investigated the electron beam welding process parameters and microstructure of Nb-1Zr-0.1C alloys and found that fine carbides precipitated along the grain boundaries in both the heat-affected zone (HAZ) and the base metal, while carbides dissolved in the weld zone; the microhardness distributions over the width of weld showed that the slight increase in hardness in the weld zone and heat affected zone; and the thermal and residual stress fields of square butt electron beam welded joints were estimated using the finite element method, and a good qualitative match between the calculated and experimental values was obtained. Gupta et al. studied butt laser welding of Nb-1Zr-0.1C alloys by using inert gas protection at the top and bottom. They found that the average fusion zone hardness is much higher than the hardness of the base material. This larger increase in hardness may be due to grain refinement, dissolution of precipitated phases, and formation of brittle phases such as carbides, and established a laser welding technique for preparing niobium alloys under ambient atmosphere.

Similarly, applying Nb-1Zr-0.1C alloys requires corresponding antioxidant coating protection, and sprayed NbSi2 remains the focus of coating design for this alloy. Vishwanadh et al. have shown that a two-layer coating structure is formed on the Nb-1Zr-0.1C alloy substrate above 1300°C, with a thin inner layer of Nb5Si3 and a thicker outer layer of NbSi2 . outer layer. Majumda et al. investigated the static isothermal oxidation of single (NbSi2) and bilayer (Nb5Si3 and NbSi2) coatings between 800 and 1300 °C and derived a kinetic model to predict the growth kinetics of the NbSi2 phase during the filler silicification process. In addition, Majumda et al. prepared MoSi2 coatings on the surface of Nb-1Zr-0.1C alloys by deposition of Mo using magnetron sputtering followed by deposition of Si using chemical vapor deposition (i.e., silica-in-packing), and it was found that a bilayered coating with an outer layer of MoSi2 and an inner layer of NbSi2 was formed. The MoSi2 coatings were formed at 1100 °C with a thickness of about 25 μm consisting of a fine crystalline organization.

2.3 Nb521 Niobium Alloy

Due to operating temperature limitations and high-temperature mechanical properties, C-103 alloy and niobium silicide antioxidant coatings have been challenging to meet the ever-developing needs of spacecraft. For this reason, China has developed Nb521 (Nb-5W-2Mo-1Zr) niobium alloy based on 5ΒΜЦ niobium alloy of the former Soviet Union. The alloy adopts molybdenum silicide high-temperature anti-oxidation protective coating, making the working temperature increased to about 1550 ℃, significantly reducing the flow rate used to cool the combustion chamber of the propellant, thus effectively enhancing the engine-specific impulse, and becoming the second generation of niobium alloy substrate for the thrust chambers of the refractory metal materials of China’s current orbit control/attitude control engines.

Nb521 niobium alloy is a medium-strength plastic niobium alloy with the addition of W, Mo, Zr alloying elements and a small amount of C elements in the niobium matrix, which further improves the room-temperature and high-temperature mechanical properties of niobium alloys through a combination of solid-solution strengthening and precipitation strengthening. It is a medium-strength plastic niobium alloy. The alloy can be prepared as ingots by vacuum electron beam melting or vacuum electron beam + vacuum self-consumption melting, and bars, forgings and plates of various specifications can be prepared by hot extrusion, forging, cold spinning, stretching and rolling. The addition of W and Mo elements to the alloy is mainly due to their high melting point and similar atomic radius, which can form a replacement solid solution to improve the alloy matrix’s creep resistance and high-temperature strength. Zr and C elements are added to the alloy because C can easily generate diffuse precipitated reinforcing phases with Zr and Nb, which can further improve the high-temperature strength of the alloy by playing the role of precipitation strengthening.

Usually, niobium alloys are diffusely strengthened by stable and fine carbides, oxides and nitrides, and this strengthening method is very effective in improving the high-temperature strength. Chunji Zhang et al. investigated that the carbide strengthened phases of Nb521 alloy are diffusely distributed (Nb,Zr)C, ZrC and Nb2C phases, ZrC is a diffusely distributed stabilized carbide phase while Nb2C is a sub-stabilized carbide phase. Xia Mingxing et al. studied the effect of different contents and sizes of Nb2C particles on the organization and properties of Nb-W-Mo-Zr-C niobium alloy, and concluded that the addition of Nb2C particles with a content of 0.4 wt% and a size of about 5 μm can make the alloy reach the highest high-temperature strength.

Nb521 niobium alloy has good room-temperature forming properties, and the uniform transition nozzle extensions can usually be prepared by plate spinning, but most of the nozzles are obtained by machining the bars, which is difficult and the material utilization is relatively low. Therefore, the study of near-net-shaping and additive manufacturing of Nb521 niobium alloy is an effective way to improve the material utilization of this alloy. Zhang et al. successfully prepared nano Nb521 alloy powder by using a high-energy ball mill at room temperature and carried out the corresponding analytical characterization and the results show that the ball milling speed is decisive when the ball milling speed reaches 450 rpm, after 60 hours, the grain size of 14 nm nanopowder can be obtained. The results show that the ball milling speed is the decisive factor. Liu Baoqi et al. studied the Nb521 alloy powder prepared by the plasma rotary atomization method, and found that the alloy powder prepared by this method is mostly spherical and has a high degree of sphericity, the surface of the large particle powder is relatively rough, and the surface of the small-sized particles of the powder is relatively smooth (see Fig. 10), and the distribution of the particle size is in accordance with the standard normal distribution; analyzed by the XRD and nano-indentation experiments, the results show that only Nb diffraction peaks exist, and the powder size decreases with the decrease of the particle size of the powder. As the powder particle size decreases, the nano-hardness and the maximum load increase.Yang et al. prepared Nb521 alloys by the electron beam selective melting (EBSM) method and compared the microstructure characteristics of the prepared specimens with those of the ingot specimens, as shown in Fig. 11. The results show that in the Nb521 samples prepared by EBSM, the content of precipitated phases increases gradually from top to bottom. The point-like or rod-like precipitated phases with different morphologies are distributed inside the grains or along the grain boundaries. In contrast, needle-like precipitated phases have large aspect ratios in the ingots of Nb521 alloys. The precipitated phases in the specimens prepared by the EBSM method are mainly (Nb, Zr)C and Nb2C. With the extension of the thermal equilibrium holding time, the finer and the more compact precipitated phases are found. In the specimens prepared by the EBSM method, the precipitated phases are mainly (Nb, Zr)C and Nb2C, and with the prolongation of the thermal equilibrium holding time, the elongated precipitated phases are partially fragmented and the grains become fine and uniform.

In addition, the molybdenum silicide high-temperature antioxidant coating prepared on Nb521 alloy material has a certain self-healing ability, and the coefficient of linear expansion of the alloy material is relatively close to that of Nb521 alloy material, so it has good antioxidant protection and bonding properties. In order to further improve the comprehensive performance of the molybdenum silicide coatings, Sun et al. prepared multiphase MoSi2 coatings on the surface of niobium-based alloys by using two plasma spraying techniques (SAPS and SPS) and defined the melt index (i.e., M.I. value) of the plasma-sprayed MoSi2 coatings in order to characterize the multiphase effect of the coatings, and it was found that the higher the M.I. value was, the stronger were the mechanical properties of the coatings, and the better were the antioxidant properties. Sun et al [80] investigated the preparation of MoSi2-based/NbSi2 double coatings on the surface of Nb521 alloy, which could form a continuous SiO2 barrier on the coating surface after pre-oxidation at 1500°C for 10h, and could effectively retard the thermal corrosion between MoSi2-based coatings and Na2SO4 salt. Xiao et al. used a new two-step method to prepare a MoSi2-NbSi2 dual coating on the surface of Nb521 alloy. MoSi2-NbSi2 coating and Ce-modified MoSi2-NbSi2 coating, and the effective protection times of the two coating specimens at 1600 °C were 24.7 h and 28.5 h, respectively, indicating that the Ce-modified coating exhibited better antioxidant properties. Zhang et al. prepared mullite-modified MoSi2(MM) coating, WSi2 and mullite on the surface of Nb521 alloy Co-modified MoSi2 (WMM) coatings and their antioxidant properties were investigated, and it was found that the antioxidant properties of MM coatings with the addition of 10 wt% mullite were very good, whereas the antioxidant properties of WMM coatings were even better, with the effective protection time at 1500 °C (500 h) being at least 2.8 and 1.5 times longer than that of the MoSi2 single coatings (175 h) and MM (346 h) coatings, respectively. as shown in Fig. 12. In order to further develop ultrahigh-temperature niobium alloy coatings for higher temperatures, Zhang et al [84] used a novel three-step method to prepare 10% ZrB2 + 5% YSZ modified Si-Mo-18%W coatings with boride diffusion barrier, and found that the coatings with the NbB2-Nb3B2 diffusion barrier could effectively protect the Nb521 alloy up to 1850 °C for 8 h above (as shown in Fig. 13), while the lifetime of the coating without diffusion barrier is only 3.5 h. The contribution of the superior coating performance is mainly due to the diffusion barrier layer and the formation of the self-healing SiO2-B2O3-ZrSiO4-ZrO2 oxide scale, which effectively prevents the mutual diffusion of the coating and the substrate and can reduce inward depletion of the silicon element, thus improving the lifetime of the coating.

Through the practice and application in recent years, Nb521 niobium alloy and its supporting high-temperature antioxidant coating have been successfully used in the new generation of high-thrust carrier rocket gas-oxygen kerosene engine nozzles, Nigerian satellites, Xinnuo 6, etc. Attitude control/orbit control engine and Chinese lunar vehicle engine and other projects, and it is the first choice of the domestic attitude control/orbit control engine refractory metal materials for the thrust chamber.

2.4 Low-density niobium alloy

Low-density niobium alloy is also known as lightweight niobium alloy. Due to the addition of a large number of Ti, Al and other lightweight elements and W, Mo and other reinforcing elements, has a low density (<7g/cm3), high room temperature and high-temperature strength, and room temperature plasticity is good (grain refinement can be cold machining molding) and other excellent characteristics, is a can not be coated in the atmosphere at 550 ~ 800 ℃ direct use, coating can be applied to the atmosphere at 800 ~ 1200 ℃ It is a new type of high-temperature structural material that can be used directly without coating in 550～800℃ atmospheric environment and with coating in 800～1200℃ atmospheric environment. The United States and the former Soviet Union have developed dozens of low-density niobium alloys in accordance with the requirements of the use of temperature and strength, with Nb-Ti-Al, Nb-Ti-Al-Cr, Nb-Ti-Al-Hf, Nb-Ti-Al-Cr-Hf and other systems. In foreign countries, low-density niobium alloys have been used in rocket and aviation engine heated parts, such as the U.S. Pratt & Whitney company to manufacture military aircraft engines with plates for pressurized nozzles, Russia for aircraft engine exhaust piping, and similar new stamping engine thermal structure components and other aspects of the application has also been used. Table 4 shows some of the foreign low-density niobium alloy grades and their properties of the domestic since 2005, the development of low-density niobium alloys, so far has been in the research and development stage, there is no engineering application of the alloy grades and products. Up to now, the research units of such alloys are Zhongnan University, Nanjing University of Aeronautics and Astronautics, Northwestern Polytechnical University, Northwestern Nonferrous Metals Research Institute and Ningxia Oriental Tantalum Industry Co. Among them, Zhongnan University, Northwest Research Institute of Nonferrous Metals and Ningxia Oriental Tantalum Industry Co., Ltd. are all using the Nb-Ti-Al system, mainly for the development of a material that can be used in the side skirt part of the thrust chamber of the liquid rocket engine, the use of the temperature of 1100 ~ 1200 ℃, in order to meet the urgent requirements of the liquid rocket engine on the lightweight. Table 5 shows the composition and properties of some domestic low-density niobium alloys.

Table 4 Tensile properties and density of some low-density niobium alloys abroad

Alloy	Density (ρ/g/cm³)	Test Temp. (°C)	Tensile Strength (Rm/MPa)	Yield Strength (Rp₀.₂/MPa)	Elongation (A/%)
Nb-40Ti-12Al	6.20	25	811	799	25
		1200	56	56	166
Nb-40Ti-10Al-10Cr	6.35	25	1010	1010	14
		1200	56	56	153
Nb-40Ti-10Al-10Cr-C-Y (at%)	–	25	1050	1000	22
		1000	150	130	75
Nb-38Ti-12Al-12Hf	6.76	25	901	901	15
		1200	77	77	91
Nb-34Ti-8Al-7Cr-2Hf	6.04	25	915	915	23
		1200	97	62	51
Nb-27.5Ti-5.5Al-6Cr-4.5Hf-2.5V	5.49	25	930	930	24
		1200	141	141	83
Nb-35Ti-6Al-5Cr-8V-1W-0.5Mo-0.3Hf (at%)	–	23	1117	1107	16
		1000	280	280	244
Nb-18Al-20V (at%)	–	25	–	1200	8
		1000	–	550	–
Nb-11Al-41Ti-1.5Mo-1.5Cr (at%)	–	25	–	788	11
		1000	–	875	–
Nb-35Ti-6Al-5Cr-8V-1C (at%)	–	1000	–	137	–

Northwest Institute of Nonferrous Metals (NIN) developed low-density niobium alloys are mainly Nb-31Ti-7Al-(3~10)V-1.5Zr, Nb-35Ti-6Al-(2~10)Cr-(3~8)V-1W-0.5Zr and Nb-37.5Ti-5Al-4.5V-0.5Zr alloys and alloy rods, plates and machining processes. were studied. Among them, Cai Xiaomei et al. studied the hot rolling process of Nb-37.5Ti-5Al-4.5V-0.5Zr alloy plate, and concluded that the alloy has good room temperature and high-temperature properties when hot rolled at temperatures of 1200 and 1100 ℃ and that the tensile strength decreases and the plasticity increases as the rolling temperature increases, and that when hot rolled at 1000 ℃, the mechanical properties of the alloy have lower room temperature and high temperature properties, and the room temperature tensile strength is lower than that of the alloy, and the mechanical properties of the alloy have lower than that of the alloy when hot rolled. Lower, and the room temperature tensile fracture showed brittle fracture. Wang Feng et al. Nb-35Ti-5Al-5V-1Zr and NbW521 alloy vacuum electron beam welding process research, the results show that the alloy and NbW5-2 alloy have good welding performance in the state of weld without heat treatment after welding the room temperature tensile strength of weld samples reached 468 MPa, close to the tensile strength of the Nb521 alloy matrix. Cai Xiaomei et al. on the Nb-30Ti-5Al-4.5W-0.5Mo alloy bar forging process research by changing the forging process to prepare a room temperature mechanical property of good low-density high-strength niobium alloy bar.

Ningxia Oriental Tantalum Industry Co., Ltd (OTIC) has successively developed a variety of alloys such as Nb-40Ti-15Al, Nb-40Ti-10Cr, Nb-40Ti-20Cr, and Nb-40Ti-10Cr-10A1, etc. The micro-alloying process of the alloys and the effect of the B element on the refinement of the alloy grains were investigated, and the alloys Nb-Ti-Al-Al-Cr and Nb-Ti-Al-Al-Cr and Nb-Ti-Al-Al-Al-Cr and Nb-Ti-Al-Al-Al-Cr and Nb-Ti-Al-Al-Al-Cr alloys were formed. The Cr and Nb-Ti-Al-Zr series of new low-density niobium alloys have a production capacity of φ30~150mm alloy bars and 1~3mm×500mm×500mm plates. Fig. 14 shows the actual parts of Nb-Ti-Al-Cr-M3 and Nb-Ti-Al-Zr-M8 plates, which were pressed and spun at room temperature. In recent years, Nb-Ti-Al-Mo-W-Zr niobium alloys have been developed based on the original. Zhao Hongyun et al. studied the heat deformation behavior and microstructure evolution of low-density Nb-Ti-Al-Mo-W-Zr niobium alloy. They obtained the flow stress curves and constitutive equations of the new low-density niobium alloy for high-temperature deformation. Liu Yanchang et al. studied the annealing process of Nb-Ti-Al-Mo-W-Zr niobium alloy plates. They concluded that the alloy can obtain good comprehensive performance by annealing at 900-950℃.

Table 5 Tensile properties and density of some low-density niobium alloys in China

Alloy	Density (ρ/g/cm³)	Test Temp. (°C)	Tensile Strength (Rm/MPa)	Yield Strength (Rp₀.₂/MPa)	Elongation (A/%)	Institution
Nb-31Ti-7Al-xV-1.5Zr	6.0	25	937	–	16.2
		1100	81	–	56.8
Nb-35Ti-6Al-xCr-yV-1W-0.5Zr	6.0~6.9	25	1088	–	21.0	NIN
		1200	152	–	62.6
Nb-37.5Ti-5Al-4.5V-0.5Zr	6.01	25	895	–	12.0
		1100	90	–	39.4
Nb-Ti-Al-Cr-M3	6.15	25	900	877	32.8
		1100	87	86	62.0
		1200	53	52	62.0	OTIC
Nb-Ti-Al-Zr-M8	6.15	25	804	804	28.0
		1100	77.8	73.2	59.5
		1200	47.8	45.2	74.0

In addition, Zhao et al. investigated that the addition of Si could enhance the strength of Nb-35Ti-15Al alloy. Shi et al. prepared Nb-23Ti-15Al (at%) alloy by mechanical alloying (MA) and hot pressing (HP) method and studied the organization evolution of powder particles during the MA process and its effect on the organization and mechanical properties of the hot-pressed alloy. Meanwhile, Shi [108] et al. also investigated the laser forming of this alloy and its microstructure and mechanical behavior, and the results showed that almost defect-free Nb-Ti-Al alloys with fine dendrites could be obtained by laser forming. The presence of β, δ, and Ti(O,C) phases in the alloys (see Fig. 15), and the relatively high microhardness and fracture toughness when the β/δ phases are refined and Ti(O,C) dispersed indicate that laser forming is a potential preparation method that can be used to prepare high-performance Nb-Ti-Al alloys. Chaia [109] et al. deposited aluminum compounds on the surface of Nb-Ti-Al alloys using chloride, fluoride, aluminum-silicon, and aluminum-silicon coatings on the surface of Nb-Ti-Al alloys. Silicide coatings and the high-temperature oxidation test of the Al-Si coatings in the air at 1000°C proved that the coatings have specific protective properties. Wei et al. investigated the microstructure evolution and mechanical behaviors of Nb-35Ti-4C, Nb-35Ti-4C-15Al, Nb-25Ti-8C, and Nb-25Ti-8C-15Al. They found that the use of C and Al elements C and Al elements were found to significantly improve the mechanical properties of the as-cast and heat-treated specimens at room and evaluation temperatures (see Fig. 16). The plasticity of the alloys was reduced by the appearance of large brittle carbides and the formation of Nb3Al with the introduction of the strengthening phases (Nb, Ti)C and Nb3A. In addition, Xiao Lairong’s team at Central South University has also developed low-density niobium alloys such as Nb-40Ti-7Al and Nb-38Ti-12Al and investigated the hot deformation behavior, recrystallization kinetics, high-temperature oxidation behavior, compatibility with Si-Cr-Ti coatings, and the role of the coating oxidation resistance, and the effect of the addition of the C element on the organization and properties of the Nb-20Ti-16Al alloys. The impact of the addition of C elements on the organization and properties of Nb-20Ti-16Al alloy was studied. It was determined that the recrystallization temperature of Nb-Ti-Al alloy was 880~1000℃. The C atoms were solidly dissolved in the form of replacement atoms in Nbss and Nb3 Al, and with the increase of C elements, the volume fraction of the heat-treated alloys of Nbss decreases, the high-temperature compression strength increases, and the room-temperature fracture toughness gradually decreases.

3. Problems and prospects

With the progress and development of aerospace technology, the performance of the engine (such as combustion efficiency, payload and in-orbit operation life) requirements continue to improve, which requires further improvement of the engine-specific impulse performance so that the temperature of the combustion chamber body is getting higher and higher, resulting in the use of the temperature of the engine body materials and mechanical properties are also more and more demanding. Compared with other kinds of high-temperature alloys, niobium alloy has the advantages of low density, high specific strength, good cold forming, and welding performance, and it can be processed to form thin-walled and complex-shaped parts. Therefore, niobium alloy has been widely used in the aerospace field. But at the same time, there are some unavoidable problems:

1) Currently used niobium alloys are mostly low—and medium-strength niobium alloys. To upgrade engine body materials, in-depth research on the mechanism of high-temperature strengthening of high-strength niobium alloys and toughening mechanism is needed to develop a new generation of niobium alloys of higher strength, higher toughness, or higher strength plasticity to provide theoretical support.

2) With the increase in operating temperature and life time requirements, the current high-temperature antioxidant protective coating technology is also difficult to meet such requirements. In order to meet the requirements of the new generation of niobium alloys with higher strength or toughness in higher temperature environments, it is necessary to develop long-life protective coatings with higher operating temperatures and better antioxidant properties.

3) Although niobium alloys are among the lightest of the high melting point metals, they are still slightly heavier in density compared to the development of aerospace engineering. The lightness of niobium alloys is of great importance for increasing the specific impulse of space engines, further extending the range and increasing the spacecraft payload. Therefore, developing low-density niobium alloys and the thinning and lightweight of high-strength niobium alloys is also an essential direction for the future development of aerospace engineering.

Therefore, in order to meet the growing demand for high performance of space engines, niobium alloys, which are commonly used in engine thrust chambers, need to continuously improve and solve their own problems and further develop new niobium alloys with higher strength, higher toughness, and lighter weight, as well as higher temperature and long-life high-temperature oxidation-resistant protective coatings, so as to promote further the application of niobium alloys in the aerospace field.