Nickel-based single crystal superalloy has high temperature-bearing capacity and good overall performance, and is considered to be the most promising material for advanced aero-turbine engine blades. In the past 20 years, the first generation without Re, the second generation with Re 3.0 wt and 2.0 wt, the third generation containing Re 6.0 wt and the fourth generation containing Re and adding Ru at the same time have been successfully developed. Generation of single crystal superalloy. Adding a certain amount of rhenium is one of the most prominent features of advanced single crystal superalloys. Because rhenium can significantly improve the creep properties of single crystal superalloys, its strengthening mechanism for single crystal alloys has attracted the attention and attention of scientific researchers from all over the world. High-performance single crystal alloys have been applied. Advanced engines such as the EFA engine EJ2000 for the Eurofighter and the ATF engine for the American tactical fighter are all made of rhenium-containing single crystal alloys. They are used to equip Boeing 777 and Airbus A380 engines, and their blades are also made of high-performance rhenium-containing single crystal alloys. But so far, research on the reasons why rhenium significantly increases the creep strength of single crystal alloys and the synergistic strengthening effect of rhenium and other alloying elements has not been in-depth. Currently, rhenium resources are scarce and expensive, and all countries regard it as a strategic element. In order to optimize the composition design of single crystal alloys, make full use of the strengthening effect of rhenium, and obtain high-performance and low-cost single crystal alloys, it is necessary to study the strengthening mechanism of rhenium in essence. This article reviews the strengthening mechanism of rhenium in single crystal alloys and discusses future research directions. 

1. Basic properties of rhenium

Rhenium is a silver-white metal with a density of 21.0 g/cms at 20°C and a melting point of 3180°C. It is a high melting point metal. The crystal structure of rhenium is a close-packed hexagonal structure, with good plasticity, no brittleness at high and low temperatures, and its tensile strength and creep resistance are better than those of W, Mo, and Nb. Adding rhenium to refractory metals can improve the strength, plasticity, and welding performance of the material, and reduce the ductile-brittle transition temperature and recrystallization brittleness. The above effect of rhenium is called rhenium effect [1]. The crystal structure of Re is different from Ni, so the solubility of Re in Ni is very low]. Among the common elements in nickel-based single crystal alloys, Re has the lowest diffusion coefficient. Its diffusion coefficient in Ni is one order of magnitude smaller than w and 2 orders of magnitude smaller than Ta, and its diffusion coefficient is Re%W%Mo%Co%Ta ％Cr％Ti％A1~.

2. Strengthen the matrix

The most obvious effect of adding Re to the single crystal alloy is solid solution strengthening. Studies have shown that rhenium is mainly distributed in 7 matrix phases. A F Gi Lion 4_ Firstly, a series of single crystal alloys with 2wt, 4wt%, and 6wt Re added on the basis of 1444 alloy were studied. Transmission electron microscopy tests showed that Re atoms are completely distributed in 7 phases; P. Caronc] also reached the same conclusion. However, the research of D_Blavette et al. [5] showed that there is always a small amount of Re element distributed in 7 phases, and for alloys with different Re content, the distribution ratio of Re in the v/v phase is significantly different. These studies have proved that although the distribution ratio of the two phases of Re in different alloys is not the same, the main distribution of Re in the 7 matrix phase is a common feature of all single crystal alloys containing Re. The analysis of 7 phases in alloys such as 1444 found that Re atoms exist in the same form as Mo atoms and aggregate in the matrix [4]. Atom probe studies on CMSX-2 and PWA148O alloys with Re added also confirmed the existence of Re clusters in the 7 matrix [5]. A more detailed study shows that Re, which mainly enters the 7 matrix, forms a short-range atom group with a size of about 1 nm [4]. An important conclusion of the research on the special existence form of Re atom comes from U. Glatzel's research on the CMSX-4 alloy L7J found that Re atoms are clustered into a bunch of about 5 atoms, but they are not distributed at every atomic level, but only at certain atomic levels, of which 3 atoms are in On the same atomic plane, the other two atoms are located above and below the three atoms. Recent studies have shown that this unique way of existence of Re atoms is related to its special electron orbital band structure [9]. These studies are based on the solid solution strengthening effect of rhenium on single crystal alloys, starting from the distribution and existence of rhenium in the matrix, to explain that rhenium can significantly improve the temperature-bearing capacity of single crystal alloys. The property that rhenium is mainly distributed in the matrix greatly reduces the SFE of the matrix, making the dislocations in the matrix easy to react to form extended dislocations, making the movement of the dislocations more difficult, and strengthening the matrix. At the same time, the massive dissolution of rhenium in the 7 matrix phase will inevitably increase the recrystallization temperature of the matrix, reduce the diffusion of elements in the matrix and the diffusion between the matrix and the strengthening phase, thereby exerting a significant solid solution strengthening effect on the matrix. on the other hand. This unique form of short-range order and three-dimensional distribution of Re atoms in the matrix is ​​the main obstacle to the dislocation movement and element diffusion in the 7 matrix. The dislocation movement and element diffusion during the creep process are hindered by the atomic group, which is very difficult. The creep strain rate of the alloy is greatly reduced. Judging from these research results, the microscopic research on the special distribution characteristics and existence form of Re in the matrix is ​​still limited to a certain single experimental state, and the research on the influence of temperature, stress, and alloy composition on it is very lacking. It is necessary to strengthen this Research on the aspects, as well as the detailed study of the influence of Re content on the distribution characteristics and existence of Re atoms in the matrix, so as to more accurately grasp the real reasons for the improvement of Re performance.

3. Strengthen Y-phase face-centered cubic

As the strengthening phase of the single crystal alloy, the number, size and shape of the phase and the atomic position occupied by the alloying elements directly affect the performance of the single crystal alloy. Most studies have shown that one, Re rarely or almost never enters the strengthening phase, but D. Research by Blavetee et al. found that about 20 Re entered and strengthened the phase. Regarding the influence of Re on the number of phases, different researchers
has different conclusions. n Blavetee et al. found that the addition of Re did not change the phase volume fraction to a large extent. And P. Caron~o] From the series of alloys tested, it is found that the alloy with the highest Re content has a high volume fraction. What is even more strange is. Some studies have found that: with the addition of Re content, the number of phases slightly decreases. Regarding the influence of Re on the phase size and morphology, from the research of different researchers on different Re-containing alloys, although the research objects are different, the conclusion is the same: the low diffusion characteristics of Re effectively hinder the 7 in the heat treatment process. Growing up is conducive to obtaining small and regular phases. For the research on the position of a small amount of Re atoms entering the 7 phases in the face-centered cubic structure, atom probe analysis shows that, like the Mo and W atoms distributed in the phases, Re atoms preferentially replace Al atoms and are at the top corner of the face-centered cubic structure. Location one. H. Murakami et al.] used other methods to study CMSX-4 and TMS Canton 71 alloys and obtained the same results. Rafting is an important phenomenon in the creep process of single crystal alloys. It has a significant impact on performance and has been extensively and in-depth studied. However, the effect of Re, which is an important element for improving creep and durability, on 7 rafting Little research. Frank. R. N. Nabarro pointed out: The driving force of 7'rafting is proportional to the applied stress, the degree of mismatch of the two phases, and the difference in the elastic modulus of the two phases. The addition of Re increases the mismatch of single crystal alloys, and to a certain extent increases the driving force for rafting. T. Murakumo et al. have the same view. Since the mechanism of rafting is controlled by diffusion, the rate of rafting is also controlled by the diffusion of alloying elements. For Re, which has the lowest diffusion rate among single crystal alloying elements, although the driving force for rafting is increased by 7 7 The process of rafting and diffusion will produce a strong positive effect. Obviously, Re has increased the activation energy of rafting[ ). The experiments of S Wdllrner"] and G L Efickson et al[ But they have different opinions on the mechanism of Re element hindering 7 rafting. K. Harris believes that D g], Re reduces the diffusion power of atoms at the two-phase interface, thus increasing the high temperature stability of the alloy microstructure Effectively hinder the coarsening. D. B|avette and P. Warre~. But they believe that the main reason why Re hinders the coarsening is that the segregation of Re atoms at the y/y two-phase interface hinders the diffusion of elements. Judging from the results obtained from the above research, these results provide important information for understanding and understanding the influence of Re on the 7-phase structure of single crystal alloys and the properties of Re in single crystal alloys, but they provide important information for explaining how Re significantly improves the performance of single crystal alloys. There is still a lack of convincing evidence for the reasons. Moreover, the contribution of Re to certain microstructures of alloys is not clear. For example, the effect of Re on the number of phases on the creep properties of alloys. Re affects 7 in single crystal alloys. The degree of contribution of the size and morphology of the alloy to the properties of the alloy, and whether the phenomenon of Re atoms in the face-centered cubic apex angle of the 7 phase has a certain relationship with the Re-strengthened 7 phase. These are all to explore the strengthening mechanism of Re that requires in-depth study In addition, the research objects in the past are mostly limited to single crystal alloys containing Re in a certain composition. There is a lack of research on the relationship between the Re content and the size, shape, number, atomic distribution and 7 rafts of the 7 phases. It is necessary to study the influence of the change of Re content on the 7-phase structure by individually changing the Re content. At the same time, in-depth study of the relationship between the distribution characteristics of Re atoms in the 7-phase and the Re content in the alloy.

4.Strengthen the Y/Y two-phase interface

The deformation behavior of single crystal superalloys is mainly determined by the characteristics of the y/y two-phase interface. Since the ^y/7 interface is a barrier to creep deformation, the influence of Re on the atomic state of the two-phase interface, the coherence of the two phases, and the dislocation network of the interface determines the deformation behavior of the single crystal alloy. J. R~sing, etc. j Using a three-dimensional atom probe to study the sample after heat treatment of the Ni-Al-Ta-Re two-phase alloy, it is found that within the 7 phases, within the range of 1.5-2n from the interface, the Re atom concentration abruptly and its concentration peaks. H. Murakam 1] also found a similar phenomenon in CMSX-4 alloy. But Nelia Wanderka et al. l2IⅢ did not find the enrichment of Re atoms at the interface and its vicinity on the samples after heat treatment of CMSX_4. What is even stranger is that P. J. warren_2 Using energy-compensated three-dimensional atom probe technology, the study of samples after high-temperature creep and low-cycle fatigue experiments of the RR3000 single crystal alloy found that in the 7 phases, there are secondary 7 phases precipitated, and near the secondary 7 phases. There is a accumulation of Re atoms at the front of the 7-phase raft structure. Further research found that Re atoms exist in the front of the raft structure as a shock wave formed by solute atoms, rather than simply segregating at the y/y two-phase interface. At the same time, the researchers speculated that as the rafting process progresses, more and more Re atoms accumulate near the interface, which causes the shock wave formed to continue to widen, and more effectively suppresses the 7-phase rafting l2. These studies have explored the distribution characteristics of the enrichment of Re atoms in single crystal superalloys containing Re near the v/v two-phase interface, and provided certain evidence for explaining that the alloying element Re improves the high-temperature creep properties of single crystal alloys; unfortunately, it is not revealed. The distribution law and reason of Re atoms in the T/T two-phase interface, and the role of Re atoms enriched in the Y/Y two-phase interface during the atomic diffusion and dislocation movement. A. F. Giamei et al. studied the influence of -Re on the y/y two-phase interface] and confirmed that with the increase of Re content, the 7-phase lattice constant increased steadily. P. Carof 0]’s research also confirmed that the addition of Re will greatly increase the lattice constant of 7, and slightly increase the lattice constant, thereby increasing the mismatch of the single crystal alloy in the negative direction. Japanese scientist H. Harada] believes that increasing the negative mismatch of the alloy is the main reason that Re increases the creep strength of single crystal alloys. The study of TM&82+ alloy with excellent creep properties found that the main reason for obtaining excellent creep properties is: large negative mismatches accelerate the formation of raft structure and fine dislocation network, thereby hindering the movement of dislocations, especially It hinders the cutting of dislocation l2. The latest research on TM&75 series single crystal alloys shows that the fineness of the 7/7 two-phase interface dislocation network determines the quality of the creep performance. The finer the dislocation network, the lower the creep rate; the resistance of the interface dislocation network hinders Therefore, the slip displacement from the matrix slips across the y/y phase interface, which prevents the dislocation from cutting the raft tissue. This research result provides an important evidence that increasing the negative mismatch of the alloy is the main reason that Re increases the creep strength of single crystal alloys. But P. Caron's research L1 0J believes that the influence of mismatch on the creep strength of alloys is not clear. The mismatch of single crystal alloys plays a complicated role under certain temperature and stress environment, especially the 7 phases at a certain temperature. Under the combined stress, the role and mechanism of the mismatch degree during the beginning and subsequent processes of rafting is very unclear, which means that the negative mismatch of single crystal alloys by Re increases is not necessarily the root cause of Re-strengthened alloys. A. C. Yeh's research on single crystal alloys with addition of Re and Ru found that increasing the degree of mismatch of the alloy can indeed obtain a fine dislocation network. However, the fine dislocation network obtained by increasing the degree of misfit does not necessarily contribute much to the increase of the alloy's creep life. This also confirms that the improvement of the mismatch of the alloy by Re is not necessarily the main reason for the improvement of the creep performance by Re. Although many researchers have conducted certain studies on Re-enhanced two-phase interface from the two aspects of Re may exist in the two-phase interface and Re to increase the degree of interface mismatch, the detailed research in the future provides ideas and methods that are worth learning from. According to the published literature, there is a contradiction in the research on whether Re agglomerates at the two-phase interface. Whether Re increases the negative mismatch of the alloy and promotes the formation of interfacial dislocation network is the main reason that Re increases the creep strength of single crystal alloys. Research is also very lacking. Therefore, a future research direction is to study the distribution of Re at the two-phase interface and the relationship between the distribution of Re atoms and the movement of dislocations. Explore the contribution of Re's influence on the degree of mismatch to the strength of the alloy.

5.Discussion

Since the first addition of Re in superalloys, the research on the strengthening mechanism of Re has been a focus of research on superalloys. From the literature that has been seen, there are four main viewpoints on the strengthening mechanism of Re in single crystal superalloys: ① The main distribution of Re atoms in the matrix and the special existence of atomic groups, solid solution strengthening of 7 and reducing the matrix The stacking fault energy and hindering the movement of matrix dislocations are the key reasons for Re strengthening the single crystal superalloy; ② A small amount of Re atoms distributed in the 7 phases replaced the A1 atoms, greatly increasing the reverse domain boundary energy of the 7 phases It makes it more difficult for dislocations to cut the 7 phases. This view believes that Re strengthens the 7 phases and is the main factor that Re improves the properties of the alloy; ③The addition of Re, the accumulation of Re atoms near the two-phase interface, hindering the rafting of the 7 phases is Re Strengthen the main aspects of single crystal superalloys; ④ The addition of Re greatly increases the lattice constant of the 7 phases, so that the v/v two-phase interface has a large negative mismatch, and at the same time, there is a fine dislocation network on the two-phase interface. The strengthening of the v/v two-phase interface is the most direct reason for the improved performance of Re-containing single crystal alloys. These points of view indicate that different scientific researchers have great differences in the understanding of the mechanism of Re-strengthening single crystal alloys. However, these studies have common characteristics: the research object is limited to a certain type of Re-containing single crystal alloys, and the research content is limited to a certain unilateral microstructure characteristic related to the properties of the alloy. Most of the conclusions obtained only reveal the characteristics of a certain type of alloy. A certain aspect of the characteristics, this has great limitations for explaining the reasons that Re improves the performance of single crystal alloys. In addition, some researchers still speculate on the explanation of the mechanism of Re-strengthening single crystal alloys. For single crystal superalloys, under the action of temperature and stress, both the coarsening and the movement of dislocations are closely related to the diffusion of alloying elements. The diffusion of alloying elements determines the change and mechanism of the microstructure of single crystal superalloys]. In nickel-based single crystal alloys, the influence of Re element on the diffusion process of alloying elements may have the following aspects. (1) Its low diffusion coefficient and high diffusion activation energy make the movement of its own atoms extremely difficult, and at the same time increase the diffusion activation energy of other types of alloying elements. (2) The large atomic radius increases the energy for other types of alloy atoms to migrate and bypass, thereby delaying the diffusion of alloy elements. (3) The special electron orbital energy band structure makes it combine with Mo atoms or its own atoms to form short-range ordered Mo-Re and Re-Re atomic groups. This way of existence reduces the atomic diffusion channels of alloying elements. The more important effect on the diffusion of alloying elements is that the Re atomic group in the 7 matrix may not be uniformly distributed on a single atomic level, but exist in a three-dimensional manner. This special way of existence may cause nearby atoms to diffuse. The energy required increases dramatically. The main obstacle to the diffusion of alloying elements in this way will reduce the creep strain rate of the alloy to a large extent. (4) The atomic clusters formed by Re accumulate in the 7 matrix near the two-phase interface, just like building a fence near the edge of the 7 matrix, forming a barrier for the diffusion of atoms between the two phases, effectively preventing the mutual diffusion of atoms between the two phases. The improvement of the creep strength of single crystal alloys by Re is realized by affecting the diffusion of alloying elements. Therefore, an important research content in the future is to study the influence mechanism and law of Re on the diffusion of single crystal superalloys and the redistribution of alloying elements. This is also the key to seeking the real reason for Re-strengthened single crystal alloys. Since the 1980s, most countries have realized the prominent role of Re in superalloys and the important position of Re in aero turbine engine blade materials. However, due to the high price of Re and the complexity of the problem, so far, the research on the strengthening mechanism of Re in single crystal superalloys has only been carried out in a few developed countries, such as the United States, Britain, Germany, France, Russia and Japan. A small number of national defense aviation scientific research units carried out. The application of second-generation single crystal superalloys containing Re represented by CMSX-4 alloy in military and civil aircraft, as well as the third and fourth-generation single crystals containing Re represented by CMSX-10 and MC-NG alloys respectively The successful development of crystal superalloys shows that these developed countries have made certain achievements in the research on the strengthening mechanism of Re in single crystal superalloys. Based on the importance of Re's important role in single crystal superalloys, and in order to shorten the gap with the aviation technology of developed countries, during the "Ninth Five-Year Plan" period, my country's Beijing Institute of Aeronautical Materials also conducted preliminary explorations on the strengthening mechanism of Re. Although it started 10-15 years later than the developed countries, during this period, we successfully developed a low-cost, high-performance Re-containing single crystal alloy DD6 alloy in line with my country's national conditions. The Re content of this alloy is only 2wt, and its performance reaches or even exceeds the level of the second-generation single crystal alloy with Re content of 3wt abroad in some respects. It is also the superalloy with the highest temperature bearing capacity and the best comprehensive performance in my country at present. It is worth mentioning that my country's first single-crystal turbine hollow blade made of this alloy has recently been equipped with an advanced aero engine and has undergone a test run. Nevertheless, due to the high price of Re and the difficulty of controlling the composition of the alloy, for a long period of time, little research has been carried out on the strengthening mechanism of Re in single crystal superalloys. Until now, the research on the strengthening mechanism of Re in single crystal superalloys has not been carried out much. The understanding of the strengthening mechanism is also very in-depth, and there are many contradictions and ambiguities. Whether it is from the understanding of the mechanism of Re in single crystal alloys or the application level of Re in alloys, there is still a certain gap between my country and developed countries. Of course, regardless of whether it is a developed or developing country, the high price of Re is an important constraint factor in the development of Re strengthening mechanism and application research; at the same time, it is also the main factor facing the widespread application of Re-containing high-performance single crystal alloys in advanced aero-engines. challenge.

6.Concluding remarks

The research status of the strengthening mechanism of Re in single crystal superalloys shows that there are still many doubts about the strengthening mechanism of Re in single crystal alloys. In order to optimize the composition design of single crystal alloys, make full use of the strengthening effect of Re, and obtain high-performance and low-cost single crystal alloys, it is necessary to conduct detailed research from the following aspects:
(1) Study the influence of Re on the microstructure and stability of the seven phases.
(2) Study the distribution characteristics and existence forms of Re in the phases 7, 7 under different states of heat treatment, persistent creep and unbroken fracture.
(3) Research on Re vs. Phase Rafting