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.

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.