Preparation and Applications of Tungsten‑Rhenium Alloys
Release time:
2026-03-06
Preparation and Applications of Tungsten‑Rhenium Alloys
Solid‑solution‑strengthened tungsten‑rhenium (W‑Re) alloys with a body‑centered cubic (BCC) structure, formed by the dissolution of rhenium in tungsten, exhibit a series of excellent properties, such as high melting point, high strength, high hardness, high ductility, high recrystallization temperature, high electrical resistivity, low vapor pressure, low electron work function, and low ductile‑brittle transition temperature. They are among the tungsten‑based alloys with outstanding comprehensive performance.
Since Geach and Hughes first reported in 1955 that adding rhenium to tungsten could improve its ductility, W‑Re alloys have attracted extensive attention and developed rapidly. At present, they are widely used in thermocouples, electronic vacuum devices, and electrical contact materials. Promising progress has also been made in their application as plasma‑facing materials for the first wall in fusion reactors.
The preparation of W‑Re alloys mainly includes powder metallurgy and melting methods. Powder metallurgy is the dominant production route due to its high yield and low cost. The typical powder metallurgy process for W‑Re alloys is: pre‑alloyed powder preparation → compacting → sintering → plastic working. This process has been reviewed in detail by Yin Xieshi. However, the selection of mixing methods for different Re contents, as well as the effects of reduction temperature and holding time on alloy billets, have not been fully described. Song Lin, Cai Jingyu, et al. conducted systematic studies on these issues and obtained a series of important conclusions.
With the expanding applications of W‑Re alloys, increasingly stringent requirements have been imposed on their properties, leading to the development of new production techniques in powder metallurgy to meet performance demands. The first part of this paper reviews the research by Song Lin, Cai Jingyu, et al. on powder mixing and reduction processes, and introduces emerging technologies potentially applicable to the production of nanostructured W‑Re alloys. The second part summarizes the research status and development trends of W‑Re alloys in thermocouples and plasma‑facing materials.
1 Preparation Processes
1.1 Powder Mixing Methods for W‑Re Alloys
Mixing is the first critical step in preparing W‑Re alloys. The homogeneity of mixing directly determines the uniformity of alloy properties. Inhomogeneous mixing and segregation may induce the formation of brittle σ‑phase, which severely deteriorates subsequent workability.
Common mixing methods for W‑Re alloys include dry mixing (direct mechanical blending of W powder and ammonium rhenate) and wet mixing (mixing W powder in an aqueous ammonium rhenate solution). In wet mixing, W particles are immersed in the solution and coated with a dense layer of ammonium rhenate, forming “core‑shell composite powder”. Therefore, wet mixing is regarded as the most desirable pre‑alloying approach.
Cai Jingyu et al. investigated the homogeneity of WAl‑1%Re, WAl‑3%Re, WAl‑5%Re, and W‑26%Re alloys prepared by dry and wet mixing. SEM observations revealed that WAl‑1%Re and WAl‑3%Re powders produced by wet mixing exhibited uniform ammonium rhenate coatings on tungsten particle surfaces. EDS analysis of individual WAl‑3%Re particles showed overlapping signals of W and Re, confirming the reliability of wet mixing for low‑Re alloys. Nevertheless, a considerable number of free ammonium rhenate particles were also observed, indicating that wet mixing still has certain inhomogeneities.
Experiments by Song Lin, Cai Jingyu, et al. showed that with increasing Re content, segregation of precipitated ammonium rhenate during water evaporation becomes more severe. Consequently, wet mixing yielded poor performance for WAl‑5%Re and W‑26%Re, whereas dry mixing enabled successful fabrication of finished products, including fine wires as thin as 0.021 mm.
The homogeneity of wet‑mixed WAl‑1%Re, WAl‑3%Re and dry‑mixed WAl‑5%Re, W‑26%Re powders was verified by analyzing Re content at three sampling ports of a V‑type mixer, with relative deviations listed in Table 1. It was concluded that WAl‑1%Re and WAl‑3%Re are suitable for wet mixing, while WAl‑5%Re and W‑26%Re are better prepared by dry mixing. It should be noted that with optimized mixing processes, wet mixing can also be applied to W‑Re powders with Re content above 5%. Further optimization of mixing technology is still required.
1.2 Reduction Temperature and Holding Time
Reduction of W‑Re composite powder is another key process after mixing. The degree of reduction directly affects subsequent compaction and sintering, and strongly influences the final properties of W‑Re alloys.
The objectives of reduction are:① To fully reduce ammonium rhenate to metallic rhenium and remove oxide layers on W and Re surfaces, promoting intimate contact and pre‑alloying.② To control powder agglomeration and particle size distribution by adjusting reduction temperature and time, since fine powders possess high activity and tend to agglomerate at elevated temperatures. A near‑normal particle size distribution helps achieve appropriate green density and improves degassing during sintering.
Song Lin, Cai Jingyu, et al. performed sintering studies on low‑Re and high‑Re alloy powders under identical reduction conditions. It was found that low‑Re alloys sintered normally, while high‑Re alloys exhibited severe swelling after sintering. SEM examination revealed that incomplete reduction of Re was responsible for the swelling. By increasing reduction temperature and prolonging holding time, this problem was successfully resolved.
It was concluded that insufficiently reduced W‑Re powder containing residual rhenium oxides causes swelling during sintering, making the billets unworkable. Compared with low‑Re alloys, high‑Re W‑Re powders require higher reduction temperatures and longer holding times to meet subsequent sintering requirements. The corresponding process parameters are listed in Table 2.
1.3 Novel Processes for W‑Re Alloys
With the advancement of nanomaterials research, nanostructured W‑Re alloys show great application potential. Although few reports focus specifically on nanostructured W‑Re alloys, extensive studies on nano‑crystalline W provide valuable references for their preparation.
Nanostructured alloys can be fabricated via powder metallurgy and severe plastic deformation (SPD). For ultra‑fine‑grained / nano‑crystalline W‑Re alloys produced by powder metallurgy, mechanical alloying is usually employed prior to sintering to obtain precursor powders, followed by appropriate sintering techniques.
Since tungsten has a relatively low recrystallization temperature, high‑temperature sintering makes it difficult to retain nano‑sized grains. Thus, the sintering method is critical. Ultra‑high‑pressure electric current sintering is effective for preparing nano‑crystalline W, featuring rapid heating/cooling, high thermal conversion efficiency, and short sintering duration. Zhou Z. et al. sintered commercial pure W powders with particle sizes of 0.1 μm, 1 μm, and 10 μm via ultra‑high‑pressure electric current sintering and achieved negligible grain growth and relative density above 95%.
Severe plastic deformation (SPD) refines coarse grains into ultra‑fine or nano‑crystals by imposing intense shear strain on bulk materials. SPD techniques include equal‑channel angular pressing (ECAP) and high‑pressure torsion (HPT), among which the latter two are the most mature. Their principles are illustrated in Figure 2.
SPD‑processed pure W exhibits enhanced fracture strength and ductility. Wei Q. suggested that SPD introduces abundant non‑equilibrium high‑angle grain boundaries in coarse‑grained W, along which impurities redistribute and reduce the average grain‑boundary impurity concentration, thereby improving ductility. Meanwhile, Valiev R. Z. proposed that non‑equilibrium high‑angle grain boundaries induced by SPD are responsible for the simultaneous high strength and high toughness.
The excellent mechanical properties of SPD‑processed pure W provide a valuable reference for fabricating ultra‑fine‑grained W‑Re alloys. However, reports on nanostructured W‑Re alloys prepared by SPD remain scarce, requiring further investigation.
2 Applications of W‑Re Alloys
2.1 Thermocouples
In modern high‑temperature measurement, W‑Re thermocouples offer advantages over Pt‑Rh thermocouples such as lower cost, wider temperature range, larger thermoelectric power, stronger output signal, and faster response. Consequently, research on W‑Re thermocouples has gained increasing attention.
As early as 1979, ASTM issued the standard ASTM E696‑79 for W‑3Re/W‑25Re thermocouples, ten years earlier than China’s first W‑Re thermocouple standard ZBN N05003‑88. With technological development, a complete industrial system for production, calibration, and application of W‑Re thermocouples has been established in China.
However, W‑Re thermocouples are limited to non‑oxidizing atmospheres, and their mechanical and thermoelectric properties are still unsatisfactory. In recent years, extensive efforts have been devoted to improving service atmospheres, mechanical properties, and thermoelectric stability.
Doping W‑Re alloys with Si, Al, and K to increase recrystallization temperature and high‑temperature strength is a common method to enhance thermocouple performance. Moon and Koo first investigated potassium bubble formation in doped W‑based alloys.After compaction and sintering, residual pores exist in the billet. During sintering in hydrogen, additives are reduced to metals. Since K atoms are larger than W atoms, K can neither evaporate nor dissolve in the W lattice, but attaches to pore walls.During subsequent deformation, elongated pores become unstable above a certain aspect ratio and decompose into spherical potassium bubbles at high annealing temperatures.Potassium bubbles improve mechanical properties but adversely affect thermoelectric uniformity, as selective deformation and preferential alignment of bubbles cause inhomogeneous K distribution. The absolute thermoelectric power S of the alloy follows the Nordheim‑Gorter rule.
To improve strength while preserving thermoelectric performance, especially for low‑Re thermocouple wires, Chen Demao, Liu Qi, et al. studied the effect of Co doping on W‑3Re thermocouples.Co addition effectively deoxidized the alloy and refined grains. The W‑3Re alloy fabricated via high‑dispersion Co doping exhibited excellent workability, high yield, and thermoelectric performance superior to the requirements of JB 5401‑91.
Regarding service atmosphere limitations, Wang Kuihan et al. investigated thermoelectric stability and oxidation failure mechanisms of W‑Re thermocouples in air, and developed oxidation‑resistant W‑Re thermocouples using gas‑filled sealed protection technology.
2.2 Plasma‑Facing Materials
Controlled thermonuclear fusion is widely recognized as a major solution to future energy shortages due to its minimal radioactive pollution and abundant deuterium fuel in seawater.The scientific feasibility of fusion energy has been verified with the construction of Tokamak magnetic confinement devices.The International Thermonuclear Experimental Reactor (ITER) project was launched in the 1980s, and its design outline was finalized in the early 21st century, marking the transition of fusion technology from basic research to engineering feasibility.
Plasma‑facing materials (PFMs) in fusion reactors require high thermal conductivity, excellent thermal‑shock resistance, low sputtering yield, low radioactivity, low vapor pressure, and high melting point.Tungsten and W‑based materials are considered the most promising candidates for the first wall (FW), divertor, and limiter armor in future commercial fusion devices such as DEMO.
Recently, the development of ultra‑fine‑grained / nano‑crystalline W‑Re alloys has become an important approach to advancing PFMs.On the one hand, ultra‑fine‑grained / nano‑crystalline materials show superior ductility compared to coarse‑grained counterparts.On the other hand, nanomaterials exhibit excellent resistance to irradiation‑induced swelling and embrittlement.
David E. J. Armstrong et al. studied the effect of dislocation density on irradiation embrittlement resistance of nano‑crystalline W‑Re alloys.They found that the irradiation hardening resistance of rolled nanostructured W‑Re alloy sheets was approximately 50 times higher than that of annealed coarse‑grained sheets.This was attributed to the high dislocation density in nanostructured alloys, which allows vacancy aggregation during irradiation without forming new dislocation loops, greatly enhancing resistance to irradiation hardening.
Reports on nano‑crystalline W‑Re alloys as PFMs are still limited, requiring further experimental research. Nevertheless, their outstanding performance promises enormous development potential.
3 Conclusion
This paper reviews the research by Song Lin, Cai Jingyu, et al. on powder mixing and reduction processes for W‑Re alloys, and introduces emerging technologies for preparing nanostructured W‑Re alloys.The research status of W‑Re alloys in thermocouples and plasma‑facing materials is summarized.
For thermocouples, which have been widely industrialized, further studies are needed to balance service atmosphere compatibility, mechanical properties, and thermoelectric performance, especially for low‑Re thermocouple wires.For fusion energy, W‑Re alloys as plasma‑facing materials have great prospects. The preparation and development of nanostructured W‑Re alloys will be an important research direction in the future.
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