Nickel base single crystal superalloy has high temperature bearing capacity and good comprehensive properties. It is considered to be the most promising material for advanced aviation turbine engine blades. In the past 20 years, the first generation without re, the second generation with re3.0wt and 2.0wt, the third generation with re6.0wt and the fourth generation single crystal superalloy with RE and Ru have been successfully developed. Adding a certain amount of rhenium is one of the most prominent characteristics of advanced single crystal superalloys. Because rhenium can significantly improve the creep properties of single crystal superalloys, its strengthening mechanism of single crystal alloys has attracted the attention of researchers all over the world. Certain scientific research achievements have been made in some developed countries, and high-performance single crystal alloys containing rhenium have been applied. For example, advanced engines such as European fighter EFA engine ej2000 and American Tactical Fighter ATF engine use rhenium containing single crystal alloy, which is used for engines equipped with Boeing 777 and Airbus A380, and its blades also use high-performance rhenium containing single crystal alloy. However, so far, the research on the reasons why rhenium significantly improves the creep strength of single crystal alloys and the synergistic strengthening effect of rhenium and other alloy elements is not deep enough. At present, 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 alloy, make full use of the strengthening effect of rhenium and obtain high-performance and low-cost single crystal alloy, it is necessary to study the strengthening mechanism of rhenium in essence. In this paper, the strengthening mechanism of rhenium in single crystal alloys is reviewed, and the future research direction is discussed.

1 basic properties of rhenium

Rhenium is a silvery white metal with a density of 21.0g/cms at 20 ℃ and a melting point of 3180 ℃. It is a high melting point metal. Rhenium has a close packed hexagonal crystal structure, good plasticity, no brittleness at high and low temperatures, and its tensile strength and creep resistance are better than W, Mo and Nb. Adding rhenium to refractory metal can improve the strength, plasticity and weldability 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 that of Ni, so the solubility of re in Ni is very low]. Among the common elements of nickel base single crystal alloy, re has the lowest diffusion coefficient. Its diffusion coefficient in Ni is 1 order of magnitude smaller than w and 2 orders of magnitude smaller than TA. The diffusion coefficient is re% w% Mo% Co% ta% Cr% Ti% A1 ~.

2 strengthening matrix

The most obvious effect of re on single crystal alloy is solution strengthening. The results show that] rhenium is mainly distributed in 7 matrix phase. A f GI lion 4_ Firstly, a series of single crystal alloys based on 1444 alloy with 2wt, 4wt and 6wt re were studied. TEM test showed that re atoms were completely distributed in 7 phases; P. Caronc] reached the same conclusion. But d_ The research of blavette et al. [5] shows that a small amount of re is always distributed in 7 phases, and the distribution ratio of re in V / V phases is obviously different for alloys with different RE content. These studies show that although the distribution ratio of re in 7 phases is different in different alloys, re is mainly distributed in 7 matrix phase, which is the common feature of all re containing single crystal alloys. Through the analysis of 7 phases in 1444 and other alloys, it is found that the existing form of re atom is the same as that of Mo atom, which aggregates in the matrix [4]. The atomic probe study of CMSX-2 and pwa148o alloys added with re also confirmed the existence of re clusters in 7 matrix [5]. More detailed studies show that re mainly enters the 7 matrix to form a short-range atomic group with a size of about LNM [4]. An important conclusion on the study of the special existence form of re atoms comes from U. glatzel's study of CMSX-4 alloy l7j. He found that re atoms are clustered into a string of about five atoms, but they are not distributed at each atomic level, but only at some atomic levels. Three atoms are on the same atomic plane, and the other two atoms are above and below the three atoms respectively. Recent studies have shown that this unique existence mode of re atom is related to its special electron orbital energy band structure [9]. These studies are based on the solid solution strengthening effect of rhenium on single crystal alloys. Starting from the distribution and existing form of rhenium in the matrix, some preliminary understanding has been obtained to explain that rhenium can significantly improve the temperature bearing capacity of single crystal alloys. Rhenium is mainly distributed in the matrix, which greatly reduces the SFE of the matrix, makes the dislocations in the matrix easy to react and form extended dislocations, makes the movement of dislocations more difficult and strengthens the matrix. At the same time, the massive dissolution of rhenium in 7 matrix phase is bound to increase the recrystallization temperature of matrix, reduce the diffusion of elements in matrix and the diffusion between matrix and strengthening phase, so as to play a significant solid solution strengthening effect on matrix. On the other hand, the unique existence 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, which hinders the dislocation movement and element diffusion in the creep process, thus reducing the creep strain rate of the alloy to a great extent. From these research results, the microscopic research on the special distribution characteristics and existing forms of re in the matrix is still limited to a single experimental state, and the research on the effects of temperature, stress and alloy composition on it is very scarce. It is necessary to strengthen this research and study in detail the effects of re content on the distribution characteristics and existing forms of re atoms in the matrix, So as to more accurately grasp the real reason why re improves performance.

As the strengthening phase of single crystal alloy, the number, size, morphology and atomic position of alloy elements directly affect the properties of single crystal alloy. Most studies show that 1. Re rarely or even hardly enters the 'strengthening phase, but the studies of D. blavetee et al. ~ found that about 2O re enters and strengthens the phase. For the effect of re on the number of phases, different researchers

There are different conclusions. N blavetee et al. Found that the addition of re did not change the volume fraction of the phase to a great extent. P. Caron ~ o] found that the alloy with the highest RE content has a high volume fraction. More strangely. It is found that the number of phases decreases slightly with the addition of RE content. In view of the effect of re on phase size and morphology, from the research of different re containing alloys by different researchers, although the research objects are different, the conclusions are the same: the low diffusion characteristics of re effectively hinder the growth of 7 during heat treatment, which is conducive to obtain fine and regular ^ y phase one. For the study of the position of a small amount of re atoms entering the 7 phase in the face centered cubic structure, atomic probe analysis shows that, like Mo and W atoms assigned to the phase, re atoms preferentially replace Al atoms and are at the top corner of the face centered cubic structure. H. Murakami et al.] The same results were obtained from the study of CMSX-4 and TMS wide 71 alloys by other methods. Rafting is an important phenomenon in the creep process of single crystal alloys, which has a significant impact on the properties, and has been widely and deeply studied. However, there is little research on the effect of RE, an important element to improve the creep and rupture properties, on 7 rafting. Frank. R. n. Nabarro pointed out that the driving force of 7 'raft is directly proportional to the applied stress, two-phase mismatch and two-phase elastic modulus difference. The addition of re increases the mismatch degree of single crystal alloy and increases the driving force of raft to a certain extent. T. Murakumo et al. L] J have the same view. As the raft mechanism is controlled by diffusion, the raft rate is also controlled by the diffusion of alloy elements. For re with the lowest diffusion rate among single crystal alloying elements, although it increases the driving force of 7 raft, it will have a strong positive effect on the diffusion process of 7 raft. Obviously, re increases the activation energy of rafting [. S wdllrner "] and the experiments of GL efickson et al. [also confirm that re stabilizes the rafting of 7 phases in the high temperature endurance process of single crystal alloy; K. Harris et al. I also agree that re hinders ^ y rafting. However, they have different views on the mechanism of re hindering 7 rafting. K. Harris believes that d g], Re reduces the atomic diffusion force at the two-phase interface, so it increases the high-temperature stability of the alloy microstructure and effectively hinders the coarsening of the alloy. D. B|avette I and P. Warren ~. However, et al. Believe that the main reason why re hinders 7 coarsening is that the segregation of re atoms at the Y / y two-phase interface hinders the diffusion of elements. From the above research results, these results provide important information for understanding and understanding the effect of re on the 7 phase structure of single crystal alloy and the properties of re in single crystal alloy, but there is no conclusive evidence to explain the reason why re significantly improves the properties of single crystal alloy. Moreover, the influence of re on some microstructures of the alloy and its contribution to the properties are not clear, such as the influence of re on the number of phases on the creep properties of the alloy, re affects the contribution of the size and morphology of 7 in the single crystal alloy to the properties of the alloy, and whether the phenomenon that re atoms in 7 phases are at the top angle of face centered cubic has a certain relationship with re strengthening 7 phase, etc. These are the problems that need to be deeply studied to explore the strengthening mechanism of re. In addition, most of the research objects in the past were limited to single crystal alloys containing RE in a certain component. There was a lack of research on the relationship between RE content and the size, morphology, quantity, atomic distribution and 7 rafting of 7 phases. It is necessary to study the effect of RE content change on 7-phase structure by changing RE content alone. At the same time, the relationship between the distribution characteristics of re atoms in 7 phases and the RE content in the alloy was deeply studied.

4 strengthen Y / y two-phase interface

The deformation behavior of single crystal superalloy is mainly determined by the characteristics of Y / y two-phase interface L2. Because the ^ y / 7 interface is the barrier of creep deformation, the influence of re on the atomic state of two-phase interface, two-phase coherent state and interface dislocation network determines the deformation behavior of single crystal alloy. J. R ~ sing et al. J the samples of Ni Al TA re two-phase alloy after heat treatment were studied by three-dimensional atomic probe. It was found that the re atom concentration suddenly changed and the concentration peak appeared in the range of 1.5 ~ 2n from the interface in 7 phases. H. Murakam I] found a similar phenomenon in CMSX-4 alloy. However, nella wanderka et al. L2I Ⅲ have no effect on CMSX_ 4 the enrichment of re atoms at and near the interface was not found in the study of samples after heat treatment. More strangely, P.J. Warren_ 2 using the energy compensated three-dimensional atom probe technology, it is found that there are secondary 7-phase precipitation in the 7-phase and re atom accumulation near the front of the secondary 7-phase and 7-phase raft structure. Further studies show that re atoms are shock waves formed by solute atoms in the front of raft structure, rather than simple segregation at the Y / y two-phase interface. At the same time, the researchers speculate that with the raft process, more and more re atoms accumulate near the interface, which widens the formed shock wave and more effectively inhibits the raft L2 of 7 phases. These studies have explored the distribution characteristics of re atom enrichment near the V / V two-phase interface of single crystal superalloy containing RE, which provides some evidence for explaining that the alloy element re improves the high temperature creep properties of single crystal superalloy; Unfortunately, the distribution law and reason of re atoms at t / T two-phase interface and the role of re atoms enriched at Y / y two-phase interface in the process of atomic diffusion and dislocation movement have not been revealed. A. F. giamei et al. Studied the effect of - re on Y / y two-phase interface], and confirmed that the 7-phase lattice constant increased steadily with the increase of RE content. P. Carof 0] also confirmed that the addition of re will greatly increase the lattice constant of 7, slightly increase the lattice constant, and increase the mismatch degree of single crystal alloy in the negative direction. Japanese scientist h. Harada] believes that increasing the negative mismatch of the alloy is the main reason for re to improve the creep strength of single crystal alloy. It is found that the main reason for the excellent creep properties of TM & 82 + alloy is that the large negative mismatch accelerates the formation of raft structure and fine dislocation network, which hinders the dislocation movement, especially the dislocation cutting L2. The latest research on tm-75 series single crystal alloys shows that the fine degree of dislocation network at the 7 / 7 two-phase interface determines the creep properties. The finer the dislocation network is, the lower the creep rate is; The resistance of the interface dislocation network prevents the slip dislocations from the matrix from sliding through the Y / y phase interface, which hinders the cutting L2 of 7 raft structures by dislocations. This result provides an important evidence that increasing the negative mismatch of the alloy is the main reason for re to improve the creep strength of single crystal alloy. However, P. Caron's research l10j believes that the effect of mismatch on the creep strength of the alloy is not clear, and the role of mismatch in single crystal alloy under certain temperature and stress environment is complex, especially in the process of raft at the beginning and after 7 phases under specific temperature and stress, This means that the negative mismatch of re increasing single crystal alloy is not necessarily the root cause of re strengthening alloy. A. Through the study of single crystal alloys with RE and Ru, C. Yeh found that increasing the mismatch degree of the alloy can indeed obtain a fine dislocation network. However, the fine dislocation network obtained by increasing the mismatch does not necessarily contribute much to the creep life of the alloy. It is also confirmed that the increase of mismatch degree by re is not necessarily the main reason for the improvement of creep properties by re. Although many researchers have conducted r certain research on re strengthening the two-phase interface from the two aspects of re possible agglomeration at the two-phase interface and re improving the interface mismatch degree, the future detailed research provides ideas and methods worthy of reference. However, from the published literature, there are contradictions in the research on whether re is concentrated at the two-phase interface, and re increases the negative mismatch of the alloy There is also a lack of research on whether promoting the formation of interfacial dislocation network is the main reason for re to improve the creep strength of single crystal alloys. 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 dislocation motion. The effect of re on the mismatch degree and its contribution to the strength of the alloy were explored.

5 discussion

Since the first addition of re in superalloys, the research on the strengthening mechanism of re has always been a focus of Superalloy research. According to the existing literature, 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 form of atomic clusters. The key reasons for re strengthening single crystal superalloys are the solid solution strengthening of 7, reducing the stacking fault energy of the matrix and hindering the dislocation movement of the matrix; ② A small amount of re atoms distributed in 7 phases replace the position of A1 atoms, which greatly increases the reverse domain boundary energy of 7 phases and makes it more difficult for dislocations to cut 7 phases. This view holds that re strengthening 7 phases is the main factor for re to improve the properties of the alloy; ③ With the addition of RE, re atoms accumulate near the two-phase interface and hinder the raft of 7 phases, which is the main way of re strengthening single crystal superalloy