Rheniumet

Tungsten possesses several outstanding properties such as high melting point, high density, high recrystallization temperature, excellent high-temperature conductivity, resistance to thermal shock, and erosion resistance. However, tungsten has poor room-temperature ductility and a high ductile-to-brittle transition temperature ranging from 150 to 450°C, making it difficult to achieve machining and cold deformation at room temperature.

On the other hand, rhenium has a high melting point, large elastic modulus, no brittle-to-ductile transition temperature, and exhibits excellent tensile strength, creep resistance, endurance strength, and resistance to thermal shock. By adding rhenium to tungsten, the tungsten-rhenium alloy benefits from the "rhenium effect" and demonstrates a range of excellent properties including high melting point, high hardness, high strength, high ductility, high electrical resistivity, high thermoelectric potential, high recrystallization temperature, low vapor pressure, low electron emission work function, and low ductile-to-brittle transition temperature.

Tungsten-rhenium alloy is one of the best-performing alloys in terms of overall properties. It has extensive application prospects in cutting-edge fields such as aerospace, nuclear industry, electronics industry, and medical applications.

The strengthening effect of rhenium in tungsten-based alloys

Solid solution strengthening tungsten-rhenium alloy.

Solid solution strengthening tungsten-rhenium alloy. Tungsten-rhenium (W-Re) alloy is a solid solution alloy where rhenium is added to tungsten, significantly improving the room-temperature ductility and high-temperature creep resistance of pure tungsten. The added rhenium content is typically in the range of 3% to 26%, with commonly used compositions including W-3Re, W-5Re, W-10Re, and W-25Re. Figure 1 shows the binary phase diagram of tungsten-rhenium alloy, indicating that tungsten and rhenium have a wide range of solubility in each other, forming an α-solid solution in tungsten. Solid solutions can be classified into interstitial solid solutions and substitutional solid solutions. Due to the similar atomic radii of tungsten and rhenium, rhenium forms a substitutional solid solution in tungsten, which has little effect on the lattice constant of the matrix. Tungsten alloy is a body-centered cubic structure of solid solution, as shown in Figure 2. During the formation of the solid solution, tungsten atoms and rhenium atoms migrate to each other, which can repair a considerable number of microcracks. The maximum solubility of rhenium in tungsten is 37% Re (atomic ratio) at 3000°C, and the minimum solubility is 28% Re (atomic ratio) at 1600°C. In practical production, when the rhenium content exceeds 26%, a second phase called W2Re3 precipitates in tungsten alloy. W2Re3 is a high-strength and high-hardness structure, which brings difficulties in the pressure processing and heat treatment of tungsten-rhenium alloys and significantly affects the properties of tungsten-rhenium alloys, especially in terms of material uniformity.

The improvement in properties of solid solution W-Re alloy compared to pure tungsten is mainly attributed to the "rhenium effect." When rhenium is added to tungsten, it forms complex oxide compounds with high surface tension. These oxides do not wet grain boundaries but instead aggregate into spherical shapes, enhancing the grain boundary strength of tungsten-rhenium alloy. This, in turn, improves the ductility of the alloy and promotes the formation of dislocation cells during deformation, reducing the energy of stacking faults.

Fig.1 Tungsten-Rhenium alloy phase diagram

Fig.2 solid solution crystal structure of tungsten-rhenium alloy

Second-phase particle strengthening in tungsten-rhenium alloy

The addition of finely dispersed second-phase particles in W-Re alloy can further enhance the high-temperature mechanical properties of tungsten-rhenium alloy. The second-phase particles are typically oxides and carbides.

Uniformly distributed second-phase particles serve multiple purposes. On one hand, they act as pinning points for dislocations and grain boundaries, leading to grain refinement and increased strength of the tungsten-rhenium alloy. At the same time, they increase the interfacial area of grain boundaries, reducing the concentration of impurities and lowering the ductile-to-brittle transition temperature of the alloy.

Oxides such as ThO2, Y2O3, and CeO2 are commonly used for dispersion strengthening of tungsten alloys. The principle behind this is the use of dispersed oxides to impede dislocation motion and refine grain size, thereby achieving strengthening and toughening effects. The commonly used W-Re-ThO2 series alloys include WRe-1ThO2 and W-Re-2ThO2. However, when the amount of ThO2 exceeds 4%, it can lead to a decline in the alloy's performance. Nevertheless, the size of oxide particles is relatively large, limiting the strengthening effect. Although oxides can improve the strength and lower the ductile-to-brittle transition temperature of tungsten-rhenium alloys, they exhibit lower high-temperature strength and poor resistance to erosion. Since tungsten-rhenium alloys are primarily used in high-temperature applications, the development of oxide-strengthened tungsten-rhenium alloys is limited.

Carbides, on the other hand, possess characteristics such as high hardness, high melting point, and strong thermal stability. By adding carbides, grain refinement can be achieved, leading to improved strength and deformation resistance of tungsten alloys. Additionally, they maintain good resistance to irradiation. The carbides commonly used for precipitation strengthening of tungsten-rhenium alloys include HfC, TaC, NbC, and ZrC. Among them, HfC has the highest melting point, lowest formation energy, and strongest thermodynamic stability. Therefore, W-Re-HfC alloys have been extensively studied, including W-3.6Re-HfC, W-4Re-HfC, W-23.4Re-HfC, and W-24.5Re-HfC alloys.

Applications of tungsten-rhenium alloy

The traditional applications mainly include the following three aspects: 

Thermocouple Wires:

Tungsten-rhenium thermocouple wires are widely used in temperature measurement instruments. They offer a broad temperature range (0 to 2500°C), high thermal electromotive force, fast response speed, and excellent corrosion resistance. These thermocouple wires are used not only in vacuum, reducing, and inert atmospheres but also in oxidizing atmospheres with appropriate oxidation protection measures as a substitute for platinum-rhodium thermocouples.

Manufacturing of Single Crystal Sapphire:

Tungsten-rhenium alloy is used for binding during the manufacturing process of single crystal sapphire.

Electron Tubes, Imaging Tubes, and Lamp Filaments:

Tungsten-rhenium alloy is employed as a material for electrical contact points in electron tubes, imaging tubes, lamp filaments, and other electrical switches. Tungsten-rhenium alloy possesses good low-temperature ductility (favorable for initial wire forming), good low-temperature annealing ductility (favorable for secondary wire forming), good high-temperature ductility (after secondary recrystallization), and excellent high-temperature sag resistance. Therefore, it is particularly suitable for manufacturing high-end lamp filaments. Electrical switches commonly use various contact materials, such as automotive horn contacts, ignition contacts, voltage regulator contacts, telephone contacts, and various other electrical switch contacts. During operation, these contacts experience characteristics such as friction, electrical arcing corrosion, and frequent high-frequency contacts. Consequently, contact materials need to meet the following requirements: low contact resistance, low arc volt-ampere characteristics, and low erosion rates.

Rhniumet Ltd., has been dedicated to the research and development of rhenium and rhenium alloy materials for a long time. In addition to the conventional applications mentioned above, the company has also developed more advanced applications in the aerospace, semiconductor, and friction stir welding industries.

Aerospace High-Temperature Structural Materials:

Tungsten-rhenium alloys are used in aerospace applications such as heat shields, rocket nozzle components, cone parts, and coatings for engines or engine components. Tungsten-rhenium alloy containers, when heated to 2000°C, show no reaction with UO2 and are used as crucible materials for uranium refining. Tungsten-rhenium alloys are also employed in heating elements and insulation screens of high-temperature furnaces, crucibles for evaporating high-purity metals, and components like springs, screws, nuts, support rods, and connecting rods in high-temperature environments, benefiting from their good plasticity.

Semiconductor MOCVD Equipment Components:

In MOCVD (Metal-Organic Chemical Vapor Deposition) equipment, heating components require rapid temperature changes and can reach temperatures as high as 2000°C. Tungsten-rhenium alloys are used to extend the lifespan of these components, and tungsten-rhenium alloy wires with good elongation can also serve as supports for heating filaments.

Friction Stir Welding Tungsten-Rhenium Stirring Heads:

Due to its high hardness, strength, wear resistance, and corrosion resistance, tungsten-rhenium alloys are suitable for manufacturing stirring heads used in friction stir welding. Additionally, these alloys' properties make them suitable for applications such as printer pins, pen tips, plumb bobs for surveying instruments, and wear-resistant components.

High-temperature alloy, also known as heat-resistant alloy or superalloy, refers to a type of metallic material that is composed primarily of iron, nickel, and cobalt. It is capable of long-term operation under high temperatures of 600℃ or above, as well as certain levels of stress. High-temperature alloys possess high strength at elevated temperatures, good oxidation and corrosion resistance, as well as excellent performance in fatigue and fracture toughness. Based on the matrix composition, they can be categorized into three types: iron-based, nickel-based, and cobalt-based alloys. Regarding the production methods, high-temperature alloys can be classified as cast high-temperature alloys, wrought high-temperature alloys, and powder metallurgy high-temperature alloys. Furthermore, they can be strengthened through mechanisms such as solid solution strengthening, dispersion strengthening, precipitation strengthening, and grain boundary strengthening.

Classification and Application Distribution of High-Temperature Alloys

Alloy strengthening characteristics of nickel, iron, and cobalt based high-temperature alloys

Nickel has a face-centered cubic (FCC) structure and does not undergo allotropic phase transformations. Iron and cobalt, on the other hand, exhibit the FCC austenite structure only at high temperatures. Therefore, alloying elements that expand the austenite phase region need to be added to iron-based and cobalt-based alloys.

Nickel has high chemical stability, while cobalt and iron have lower oxidation resistance compared to nickel. However, cobalt has better resistance to high-temperature corrosion than nickel. The addition of chromium can significantly improve the oxidation resistance of nickel-based alloys and the high-temperature corrosion resistance of cobalt-based alloys.

Nickel has the best phase stability, and nickel or nickel-chromium matrices can dissolve a larger amount of alloying elements without forming detrimental phases. Iron has the poorest phase stability, and iron or iron-chromium-nickel matrices can only dissolve a smaller amount of alloying elements, exhibiting a strong tendency to precipitate various harmful phases.

Iron has the lowest density but the highest coefficient of thermal expansion, and it has good thermal conductivity. Compared to nickel, cobalt has better thermal conductivity and a lower coefficient of thermal expansion, making it exhibit superior thermal fatigue performance.

Based on the above comparisons, it is evident that nickel is an excellent base metal, making nickel-based high-temperature alloys the best-performing series of high-temperature alloys. Cobalt-based alloys exhibit high resistance to heat corrosion and thermal fatigue, allowing them to demonstrate their advantages and have a longer service life. They are suitable for static components that require long-term use at high temperatures and low stresses. Iron-based alloys have a lower temperature range of application compared to nickel-based and cobalt-based alloys. Nickel-based high-temperature alloys have the widest range of applications, while iron-based and cobalt-based high-temperature alloys also have some specific applications.

High-temperature alloy materials are generally used in the manufacturing of high-temperature-resistant materials for aircraft engines, especially for the last two stages of the compressor, the initial two stages of turbine blades, combustion chambers, afterburners, turbine discs, turbine blades, and fasteners. Currently, the market demand is mainly for military products. Due to their military significance, these materials are considered strategic resources and their trade is regulated at the same level as weapons trade. The formulation, manufacturing methods, and processing techniques are highly classified, and major aviation powers conduct research and development under extremely secretive conditions.

A fan blade made of nickel-based high-temperature alloy in the Rolls-Royce Turbo-Union RB199-34R Mk103 turbofan engine.

Single crystal high-temperature alloy refers to an alloy that is composed of individual crystals, with a higher degree of alloying than ordinary high-temperature alloys. By eliminating grain boundaries, it reduces the formation of crack initiation sites, thus overcoming the challenges associated with severe segregation in cast or forged high-temperature alloy ingots, poor thermal processing properties, and difficulties in shaping. As a result, single crystal high-temperature alloys exhibit excellent long-term durability, low creep rates, and good resistance to thermal fatigue.

Since the 1980s, single crystal high-temperature alloys have been continuously developed along their unique path. With advancements in alloy design theories and improvements in production processes, second-generation and third-generation single crystal alloys have emerged, with temperature capabilities approximately 30°C and 60°C higher than the first-generation single crystal alloys, respectively. Representative grades of the first-generation single crystal high-temperature alloys include DD2, DD3, DD4, DD26, DD26C, DD402, DD8, while representative grades of the second-generation single crystal high-temperature alloys include PWA1484, CMSX-4, DD6, DD398, DD5, DD99, MO9A, DD13. The third-generation single crystal high-temperature alloys are represented by grades such as CMSX-10, CMSX-11, ReneN6, TMS-75, TMS-82, DD9, DD10, DD32, DD33, DD90.

CMSX-4 is a second-generation, rhenium-containing, high-strength single crystal alloy developed by Cannon Muskegon Corporation. This nickel-based alloy has a higher peak temperature/stress capability and can operate in environments of at least 2125°F (1163°C). CMSX-4 alloy has been extensively produced and utilized. Blades made from CMSX-4 alloy have demonstrated a fault-free service life of 25,000 to 30,000 hours on the 15,000 horsepower Mars 100 industrial gas turbine. This alloy has gained widespread application globally and has gained significant experience in the manufacturing of industrial gas turbine blades.

Lastly, let's talk about the strengthening of high-temperature alloy materials. All high-temperature alloys contain multiple alloying elements, sometimes ranging up to several tens of elements. These alloying elements contribute to a series of alloy strengthening effects.

Alloy Strengthening: The addition of multiple alloying elements interacts with the base elements (nickel, iron, or cobalt) to produce strengthening effects, including:

Solid Solution Strengthening: Alloying elements dissolve in the base matrix, causing lattice distortion and increasing strength.

Second-phase Strengthening (Precipitation Strengthening and Dispersion Strengthening): The formation of precipitates or dispersion of fine particles within the matrix enhances strength.

Grain Boundary Strengthening: Alloying elements can strengthen the grain boundaries, improving resistance to deformation.

Process Strengthening: By adopting new processes or improving steps such as refining, solidification, heat treatment, and surface treatments, the alloy's microstructure can be optimized, resulting in strengthening effects.

High-temperature alloy strengthening = Alloy strengthening + Process strengthening.

Among all the elements that contribute to alloy strengthening in high-temperature alloys, there is one element that plays a crucial role that cannot be overlooked, and that is rhenium. We can use the following analogy to describe the role of rhenium in high-temperature alloys: Before rhenium is added to the alloy, it is like a team of scattered infantry armed with rifles; however, once rhenium is introduced into the high-temperature alloy, it instantly transforms into a formidable space army with the ability to confront the challenges of time and space.

Rhenium, symbol Re, is a chemical element with atomic number 75 and atomic weight of 186.207 u. It is a silver-white, high-density transition metal belonging to the 6th period of the periodic table. Rhenium is one of the rarest elements in the Earth's crust, with an estimated average abundance of one billionth. It is also one of the elements with the highest melting and boiling points. Rhenium is a byproduct of molybdenum and copper refining processes. It exhibits chemical properties similar to manganese and technetium, and it can have oxidation states ranging from -3 to +7 in compounds.

Scientists discovered the element rhenium in 1925, making it the last stable element to be discovered. Its name, "rhenium," is derived from the Latin name for the Rhine River in Europe. Rhenium is one of the rarest elements in the Earth's crust, with an average abundance of about one billionth. When ranking the elements by abundance in the Earth's crust, rhenium occupies the 77th position, indicating its low occurrence compared to other elements.

Nickel-based alloys with rhenium can be used to manufacture combustion chambers, turbine blades, and exhaust nozzles in jet engines. These alloys typically contain up to 6% rhenium, which represents the largest practical application of rhenium. Another significant application of rhenium is as a catalyst in the chemical industry. Rhenium is even rarer and more difficult to obtain than diamonds, which makes it expensive. In August 2011, the average price of rhenium was $4,575 per kilogram ($142.30 per troy ounce). Due to its use in high-performance jet and rocket engines, rhenium holds great importance in military strategy.

The addition of rhenium enhances the creep strength of nickel-based high-temperature alloys. Nickel-based high-temperature alloys typically contain 3% to 6% rhenium. The second-generation nickel-based high-temperature alloys have a rhenium content of 3% and have been used in engines for F-16 and F-15 fighter jets. The third-generation single-crystal nickel-based high-temperature alloys have a rhenium content of 6% and have been used in engines for F-22 and F-35 aircraft. Rhenium-containing nickel-based high-temperature alloys are also used in industrial gas turbines.

The introduction of rhenium in high-temperature alloys leads to the formation of topologically close-packed phases (TCP), which can cause microstructural instability. Therefore, fourth-generation and fifth-generation high-temperature alloys have started to incorporate and utilize the addition of ruthenium to mitigate this undesirable phenomenon.

The F100 turbofan engine utilizing second-generation nickel-based high-temperature alloy with rhenium.

The CFM56 jet engine using nickel-based high-temperature alloy with 3% rhenium content.

According to historical statistics, in 2006, the consumption of rhenium in the field of high-temperature alloys was distributed as follows: General Electric accounted for 28%, Rolls-Royce Limited accounted for 28%, Pratt & Whitney accounted for 12%, all used for the production of nickel-based high-temperature alloys. Additionally, 14% was used as catalysts, and 18% had other uses. Due to the continuous increase in demand for military jet engines, many military research institutions are developing high-temperature alloys with lower rhenium content to maintain low-cost supply. For example, the new CFM56 high-pressure turbine (HPT) blades use an alloy with 1.5% rhenium content to replace the alloy with 3% rhenium content.

Recently, a team of Central South University including Professor Huang Zaiwang and Professor Zhang Lijun published a paper in ACTAMATERIALIA, reporting the latest research trends: the creep resistance and phase stability of nickel-base single crystal superalloy are improved by replacing Re with OS. So, how likely is it that osmium (OS) will replace rhenium (Re) as the most important strengthening element in ni-base single crystal superalloys in practical production and application? Let's read through the core meaning of this paper.

Rhenium (Re) is a paramount strengthening element in nickel-based single crystal superalloys because of its low diffusion coefficient, high melting point and high partition coefficient between γ and γ' phases. However, its detrimental influence on phase stability has remained a long-standing problem to be solved. In this paper, we report a new design strategy to improve creep resistance and phase stability of nickel-based single crystal superalloys by replacing Re by osmium (Os), which contradicts the common belief. The experimental observations clearly show that the creep rate of newly designed CSU-Os-2 alloy is lower than that of commercial CMSX-4 alloy at 980 °C/200 MPa/100 h. Moreover, the CSU-Os-2 alloy also owns the better phase stability after exposure at 1100 °C for 1000 h, compared with CMSX-4 alloy.

Ni-base single crystal superalloys have typical microstructures with a high proportion of coherent precipitated γ' (ordered FCC-L12 structure) cubes separated by a thin channel (disordered FCC-A1 structure) of γ matrix, with excellent mechanical properties and high creep and fatigue resistance at high temperature, so it is widely used as a basic material for high pressure turbine blades. In order to meet the requirements of increasing turbine inlet temperature, Ni-based single crystal superalloy needs higher creep resistance and microstructure stability. In the past few decades, more and more refractory alloy elements, such as Mo, W, Ta, Re, have been introduced into Ni-based single crystal superalloys to improve their creep resistance and phase stability. For example, the total amount of W + Mo + Ta + Re increased from 16.4 wt. % to 20.1 wt. % . Re (rhenium) is the most effective modifier to enhance creep deformation because of its very low diffusion coefficient, high melting point and strong distribution behavior of γ phase. Despite its attractive benefits, the addition of higher amounts of Re in the new generation Ni-based single crystal superalloy may induce the precipitation of brittle topological dense arrangement (TCP-RRB- phase, which accelerates the alloy's failure.

Therefore, the whole world is trying to solve the problem of TCP precipitation. One feasible measure is to introduce Ru (ruthenium) , because its addition can reverse the distribution behavior, increase the solubility of RE and W in γ-matrix phase, and decrease the TCP/γ interface energy and γ/γ' mismatch strain energy. Unfortunately, Ru addition may also lead to topological inversion, especially at high temperature, which in turn will affect the creep fracture life of ni-base single crystal superalloy. Therefore, it is necessary to search for other possible elements to replace Re or Re/Ru, aiming at exploring new ni-base single crystal superalloy. An effective strategy is to search for elements with the same or lower cross-diffusion coefficient as Re in γ-matrix of Ni-based single crystal superalloy. By combining the HITDIC software with the diffusion coupling/multiplex technique, the temperature-and composition-dependent cross-diffusion coefficients in a series of binary, ternary and multicomponent γni-based alloys can be determined effectively. A comprehensive comparison of the interdiffusion coefficients in γni-x and Ni-Al-X (x = Co, CR, Ir, MO, NB, Os, PT, RE, Rh, Ru, Ta, Ti and w) alloys 1423-1573K shows that the co-diffusivity of OS is generally equal to that of Re, and even lower at higher temperatures (i.e. ≥1300 ° C) . As far as the cross diffusion coefficient in γ matrix is concerned, OS may be a potential replacement element for Re in Ni base single crystal superalloy.

The possibility of replacing rhenium with osmium in ni-base single crystal alloy was theoretically analyzed in detail from the aspects of solid solution strengthening, creep resistance and microstructure stability. Under the circumstances of positive confirmation, by replacing Re with OS in a commercial CMSX-4 alloy, Zhang Lijun's team in the State Key Laboratory of Central South University has designed a new Re-free nickel-based superalloy called CSU-OS-2, then the single crystal rod was successfully prepared. With proper solution and aging heat treatment, the segregation is almost alleviated and typical γ/γ' microstructures are produced as in commercial alloys. At 1100 ° C/1000h exposure, the coalescence of γ' precipitates in CSU-Os-2 alloy was slower than that in CMSX-4 alloy, and no TCP phase was observed. After creep deformation at 980 ° C/200 MPA/100 h , the creep rate of CSU-Os-2 alloy is lower than that of CMSX-4 alloy. All attractive properties/properties can be explained by the higher partition coefficient between γ and γ' phases and the likely lower diffusion coefficient of OS than Re in the γ phase. The successful development of a new type of ni-base single crystal superalloy without re and osmium has enhanced creep resistance and microstructure stability, which will undoubtedly open up a new direction for the next generation of ni-base single crystal superalloy.

This sentence reveals the core significance of the paper. After reading the thesis, we return to the question raised at the beginning of this paper: How likely is it that osmium (OS) will replace rhenium (Re) as the most important strengthening element in ni-base single crystal superalloy in practical production and application? To answer this question, we analyze and expound the author's viewpoint from the following three basic aspects: the recoverable amount of Re and OS on the Earth, the cost of extraction and smelting process, the current market price and the production and supply capacity.

Resource Reserves

Osmium (OS) has a density of 22.59 g/cm3, making it the densest metal element on Earth. Osmium is a platinum group metal and the global annual output of osmium is only about 500 kg, less than 1/5000 of gold. According to the USGS 2022 Mineral Commodity Summaries 2022 - Platinum, platinum accounts for about 50 percent of the 70,000 tons of proven platinum-group reserves globally, palladium for about 35 percent, ruthenium, iridium and rhodium for the rest, and osmium is the rarest of the platinum-group elements, in the platinum group content of less than 1% , that is, the global reserves estimated at about 700 ~ 1000 tons.

Rhenium (Re) , with a density of 21.03 g/cm3, is also a rare earth element. The world's proven reserves of rhenium are about 2,500 tonnes, with resource reserves of no less than 10,000 tonnes, less than one-tenth of gold, and annual global production of about 100 tonnes. About 50% of rhenium production is currently used in the production of nickel-based superalloys.

2. The cost of extraction and smelting process

Rhenium is usually associated with copper and molybdenum. In the process of smelting copper and molybdenum, most of rhenium is recovered as a valuable by-product, so the cost of extracting and smelting is relatively low. However, osmium is usually associated with iridium, and the very low content of osmium in platinum group ores, combined with the high toxicity of osmium oxide osmium tetroxide OSO4, leads to high extraction and smelting costs.

3. The current market price and the production and supply capacity

The international price of osmium in US dollars is close to US $20/g, about RMB 150/g and 150,000/kg, while the production of rhenium is 100 tons, and the market price is RMB 170/g, 17,000/kg. In terms of production, rhenium is 200 times that of osmium; in terms of price, rhenium is only a tenth that of osmium.

Price comparison of osmium and rhenium

To sum up, in the practical production and application of ni-base single crystal superalloys, it seems that it is far from simple to make osmium (OS) completely replace rhenium (Re) as the key strengthening element, however, the introduction of osmium into traditional ni-base superalloy is a new scientific research direction, which is worthy of reference for the industry.

Rheniumet Ltd. is in line with the world's major application terminals in the development, production and manufacturing technology of rhenium and its alloys, continuing to serve users in the aerospace, weapons and equipment, nuclear industry, catalysts, semiconductors and microelectronics fields for 14 years, and is committed to the innovative development and application expansion of advanced rhenium and its alloy materials in China's aviation and aerospace technology. Rheniumet focuses on the manufacturing of rhenium-containing superalloys. With the world's largest annual production and processing line of 100 tonnes of metallic rhenium, the company can provide rhenium monolithic metal with a purity of up to 99.999%, and is a high-quality rhenium raw material supplier of nickel-based single-crystal superalloys for aerospace engines. We are also a well-known manufacturer of superalloy materials and their key components, parts and high-performance products made by powder metallurgy.

We look forward to working with you to promote the development of the globle superalloys. Welcome to contact us!

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Introduction: as a rare and refractory metal, rhenium comes into being in nature in association with metallic minerals such as molybdenum and copper. Rhenium has high melting point, high strength, good plasticity and stable mechanical properties and is widely used in single crystal superalloys for aero-engine turbine blades due to the "rhenium effect" caused by the addition of rhenium. Besides, the scarcity of reserves and the difficulty of processing and obtaining result in its high price, so many countries regard it as a strategic element. Currently, the superalloy industry is the largest consumer of rhenium, accounting for about 80% of the total, and with the rapid development of human aerospace industry, the consumer market demand for rhenium resources at a growth rate of 5% annually, leading to more attention to the application of rhenium. In order to alleviate the growing tension in rhenium resources, diversified enrichment and separation methods, the use of alternative elements, low rhenium as well as rhenium recycling and other emerging technologies have been gradually developed to make rhenium metal more fully available.

Basic Properties

The metallic rhenium (Re) is silvery-white, placed at number 75 in the periodic table of elements (belongs to the 6th period of the transition elements) and is not radioactive. Discovered in 1925 by the German scientists Walter Noddack, Ida Tacke-Noddack and Otto Carl Berg, rhenium was the latest stable element discovered by man. Rhenium remains only 10-9 in the earth's crust, making it a rare element. According to the United States Geological Survey (USGS), there are less than 2,500t of proven reserves of rhenium worldwide, and the resources are very unevenly distributed.

Distribution of typical elements in nature

Metallic rhenium is hexagonal close packed crystal structure (hcp) and has a density of 21.0 g/cm3, ranking 4th after osmium (Os), iridium (Ir) and platinum (Pt). Pure rhenium is soft and has excellent ductility. As a refractory metal element, rhenium has a melting point at 3180 °C, second only to tungsten (W), while the melting point of nickel metal is only 1455 °C. Therefore, rhenium has excellent heat resistance and is relatively stable at high temperatures; its creep resistance is better than that of other refractory elements like tungsten (W), molybdenum (Mo) and niobium (Nb) . Rhenium has excellent wear resistance and corrosion resistance, making it ideal for the preparation of aero-engine components which are used in harsh working environments.

Common pure rhenium products: rhenium ingots, rhenium bars, rhenium powder

Nickel-based superalloy

Nickel-based superalloys are a class of highly alloyed metallic materials with nickel as the base. It can be processed and remain stable at high temperatures above 600 °C, and can bear large and complex stresse. It is widely used as hotend component in aero engines and gas turbines because of its high-temperature strength, plasticity, good oxidation resistance, thermal corrosion resistance, good thermal fatigue properties and organizational stability and other comprehensive properties. At present, in advanced engines, the weight of the components prepared by using this type of material can reach 60% of the total weight of the engine. Nickel-based superalloys can be divided into deformed superalloys, cast superalloys and powder metallurgy superalloys according to the preparation process. The cast superalloys are further divided into equiaxed, directional solidification columnar and single crystal superalloys based on the solidification crystalline organisation.

Nickel-based superalloy blades with different solidification crystalline organization: (a) equiaxed crystals; (b) oriented columnar crystals; (c) single crystals

With the gradually increasing requirements in aero-engine capability, due to the complete elimination of grain boundaries, making its temperature capability growing, nickel-based single crystal superalloys developed on the basis of directional solidification superalloys have become the mainstream material for turbine blades of high thrust-to-weight aero-engines and are gradually being applied to the hotend components of advanced ground gas turbines. Currently, single crystal blades made from nickel-based superalloys have become a landmark component of advanced aero engines.

Application of Rhenium

Nickel-based superalloys are complex systems developed on the basis of the Ni-Al binary phase diagram. In general, the comprehensive performance of nickel-based superalloys is improved mainly through composition adjustment and process improvement. Over the decades, nickel-based (single crystal) superalloys have been divided into multiple "generations" due to their compositional characteristics and temperature-bearing capabilities, while the addition of refractory elements represented by rhenium and the platinum group elements represented by ruthenium have attracted more attention. Minerals Summary 2018 released by USGS highlighted that over 70% of rhenium worldwide were used in the manufacture of nickel-based superalloys turbine blades. Clearly, rhenium, as a rare metal, is of much importance for the development of high-performance turbine blades and even the development of the aero-engine industry, and thus becomes a strategic resource for which the aerospace powers are competing.

Single crystal alloy blade Throughout the development of nickel-based superalloys, as a blade material, their use temperature are raised from 700 ℃ in the 1940s to 1150 ℃ in the current, achieving a great leap forward in temperature capability. The technology of single crystal growth in superalloys has developed and make nickel-based superalloy more widespread. So far, single crystal superalloy has been developed to the sixth generations.

(a) Military engines (b) Civilian engines (c) Single Crystal blades

Typical nickel-based single crystal superalloy composition (mass fraction, %)

Starting from the second generation of single crystal superalloys, a prominent feature of the alloy composition is the application of metallic rhenium, and the amount of added rhenium has increased from 2.0% ~ 3.0% (mass fraction, the same below) in the second generation to 4.5% to 6.0% in the third generation of alloys. Take the example of nickel-based single crystal superalloys developed in China, the first generation DD3 did not add rhenium, while the second generation DD6 and the third generation DD9 added 2.0% and 4.5% rhenium respectively to improve the creep performance of the alloy, which is essential for the improvement of the temperature resistance of single crystal superalloys. The results of the study show that at 980 °C/ 250 MPa, the creep fracture life of the third generation DD9 alloy reached 568 hours, while the second generation DD6 alloy was only 275 hours. At 1100 °C/137 MPa, the creep fracture life of the second generation DD6 alloy was 148 hours, while the third generation DD9 alloy reached 274 hours, almost twice as long. This phenomenon of rhenium in single crystal single crystal superalloys is known as the "rhenium effect". As people's understanding of the mechanism of rhenium continues to grow, rhenium has become an essential alloying element for new single crystal superalloys, such as the fourth generation EPM102 with 5.95% rhenium developed by the United States , and the sixth generation TMS-238 developed by the National Institute for Materials Research (NIMS) in Japan, which increased the content of rhenium to 6.4%. However, the high cost and the influence of rhenium on the stability of the alloy's structure have limited the use of rhenium in higher generation single crystal superalloys.

Mechanism of action of rhenium Rhenium is one of the most effective solid solution strengthening elements in nickel-based single crystal superalloys. Rhenium strengthens solid solutions because of its tendency to concentrate in the γ matrix, forming clusters of rhenium atoms that are approximately 1 nm in size and are ordered over short distances, making the strengthening ability of such clusters more prominent than the conventional one. Due to the dislocation movement through the atomic clusters and the disruption of the ordered regions of the rhenium atoms, the resistance to movement increases the strength of the alloy. The addition of rhenium also serves to reduce the diffusion rate of other alloying elements, inhibit the growth of the γ′ phase and increase the γ/γ′ mismatch. What’s more, the addition of rhenium reduces grain defects and surface recrystallisation in single crystal castings, and has a significant effect on the thermal corrosion resistance of the alloy.

However, rhenium is also an important element in the formation of harmful topologically close pack (TCP) phases and the addition of excess rhenium is detrimental to the organisational stability of the alloy. In addition, excess rhenium causes the formation of a secondary reaction zone (SRZ) consisting of P and γ phases under the coating and within the alloy after long service at high temperatures, thus reducing the alloy's durability performance; this, together with the contradicts between Rheniumet’s idea of achieving lightweight blade and alloy’s high density, requires strict control of rhenium addition.

As a rare element, the "rhenium effect" of adding rhenium to single crystal alloys has not yet been clearly explained, and the mechanism of the synergistic strengthening effect of adding appropriate amounts of rhenium and other alloying elements such as ruthenium (Ru) has not yet been investigated in depth. In order to maximise the effect of rhenium and design new single-crystal alloys with higher temperature-bearing capacity and better all-round performance, continued attention is needed in this field.

Recycling and reuse of Rhenium

Rhenium Resources and Prices According to the USGS Minerals Summary 2020, global rhenium production (Table 2) has remained largely stable over the last two years and has not increased with the annual expansion of superalloy production and applications over the years. This is due to three main factors: firstly, rhenium is still very difficult to produce; secondly, countries are protecting their strategic mineral resources more vigorously; and thirdly, there have been advances in rhenium recovery and utilisation technology.

Recent World Rhenium Production and Reserves

In addition to the preparation of superalloy turbine blades, rhenium is also used in the field of petroleum catalysts where rhenium consumption was once as high as more than 60%. With the addition of rhenium in single crystal superalloy for turbine engine blades, its consumption has increased year by year, and now its proportion reaches about 80%. The reason for this is presumably the long manufacturing process of superalloy blades and the harsh inspection standards, resulting in a relatively low overall utilisation rate of the material and thus generating a large amount of return material; secondly, engine overhaul or end-of-life produces large quantities of scrapped superalloy parts, which also generate large quantities of return materials. Therefore, the recycling of return material is of great significance.

Recycling of rhenium As a strategic metal, rhenium has become an important way to control military costs worldwide, given the scarcity of this resource and its importance to the development of defence and military industries. The rational use of return material can achieve the dual purpose of making full use of resources and reducing production costs. In recent years, the global rhenium recycling industry is developing rapidly, and the United States and Germany are the main countries of rhenium resources recycling. In 2020, approximately 20-25t of rhenium were recovered globally, with the US recovering 1/3 of the rhenium.

The recycling of rhenium is technically very difficult and costly. For different forms of rhenium scrap, there are different recovery treatment methods. At present, the process of recovering rhenium from superalloy scrap mainly includes oxidation sublimation, electrochemical treatment, high temperature alkali melting, electrolytic dissolution and other methods. There are advantages and disadvantages of each method. The Institute of Metals in China has adopted the "electrochemical dissolution method" to separate and extract rhenium from superalloy scrap in multiple steps, and explored the key scientific and technical problems in the electrochemical dissolution, precipitation separation, extraction separation, ion exchange separation, metal compound recrystallization and purification, and metal compound gas reduction of superalloy scrap. The goal of separating and recovering rhenium element from superalloy scrap was initially achieved.

It is worth noting that the main problem currently faced in recycling rhenium is how to use technology to achieve high efficiency, low cost and energy saving. Therefore, increasing the development of next-generation recycling technologies that are efficient, low-cost and environmentally friendly is an important research topic that researchers need to tackle urgently.

Rheniumet Ltd. is in line with the world's major application terminals in the development, production and manufacturing technology of rhenium and its alloys, continuing to serve users in the aerospace, weapons and equipment, nuclear industry, catalysts, semiconductors and microelectronics fields for 14 years, and is committed to the innovative development and application expansion of advanced rhenium and its alloy materials in China's aviation and aerospace technology. Rheniumet focuses on the manufacturing of rhenium-containing superalloys. With the world's largest annual production and processing line of 100 tonnes of metallic rhenium, the company can provide rhenium monolithic metal with a purity of up to 99.999%, and is a high-quality rhenium raw material supplier of nickel-based single-crystal superalloys for aerospace engines. We are also a well-known manufacturer of superalloy materials and their key components, parts and high-performance products made by powder metallurgy. 

We look forward to working with you to promote the development of the globle superalloys. Welcome to contact us!

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About Rheniumet

Rheniumet Ltd is a high-tech enterprise who specialize in research, production, processing and operation of rhenium and rhenium alloy materials, together with deep processing parts and terminal components of rhenium element base.

Rheniumet has the world's unique technology research center and laboratory which focus on rhenium element. It has achieved and integrated a number of key technologies to apply in rhenium industry chain. Due to the excellent technological development capability, Rheniumet has built two industrial manufacturing base plants, not only has the ability of extraction and separation rhenium from ore and smelting waste, but also has the capacity of manufacturing high purity rhenium metal and rhenium alloys materials and high precision parts for high-tech equipments,etc.