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Rhenium Strengthened mechanism of single crystal superalloys

Date:2015-03-26 17:56

Nickel-base single crystal superalloy high temperature bearing ability, and has a good overall performance, is considered the most advanced aviation turbine engine blades promising material. In the past 20 years, has successfully developed the first generation free of Re containing 2.0wt Re3.0wt and second generation, third generation containing Re 6.0wt containing Re and Ru simultaneously added a fourth generation single crystal superalloy. Adding a certain amount of rhenium is one of the most advanced single crystal superalloy prominent feature. Because rhenium can significantly increase the creep properties of single crystal superalloys, its strengthening mechanism of single crystal alloy of concern and attention of national research personnel in some developed countries have made - given scientific research, and rhenium High-performance single crystal alloy has been applied. Such as the Eurofighter EFA engines EJ2000 and the US tactical fighter ATF engines and other advanced engines have adopted rhenium containing single crystal superalloy, to equip the Boeing 777 and Airbus A380 engine, which also uses high-performance blade rhenium containing single crystal alloys. But so far, as well as collaborative research strengthening effect of rhenium and other alloying elements to significantly improve rhenium alloy single crystal creep strength reason not deep enough. Currently, rhenium resources scarce, expensive, countries have them as a strategic element. In order to optimize the single crystal alloy composition design, take full advantage of the strengthening effect of rhenium to obtain low-cost high-performance single crystal alloy, it is necessary to strengthen the mechanism of rhenium studies from nature. This paper reviews the rhenium strengthening mechanism in single crystal alloys and discusses the future research directions.

1 basic properties of rhenium
Rhenium, silver-white metal, density at 20 ℃ when 21.0g / cms, melting point 3180 ℃, belong to high-melting metal. The crystal structure of rhenium hcp structure, good plasticity, there is no brittleness at high and low temperatures, the tensile strength and creep resistance than W, Mo, Nb. Add to refractory metal rhenium can improve strength, ductility and weldability of materials, and reduce the ductile-brittle transition temperature and a recrystallization brittleness. These effects are called rhenium rhenium effects [1]. Re and Ni different crystal structure, solubility in Ni Re is very low]. In the nickel-base single crystal alloy commonly used elements, Re has the lowest diffusion coefficient, the diffusion coefficient of Ni in the ratio w smaller one order of magnitude than the Ta small two orders of magnitude, diffusion coefficient Re% W% Mo% Co% Ta % Cr% Ti% A1 ~.
2 strengthening the matrix
Re join the single crystal alloy single crystal alloy appearance is the most significant effects of solid solution strengthening. Studies have shown that], rhenium mainly distributed in 7 matrix phase. A F Gi lion 4_ first studied in 1444 based alloy, were added 2wt, 4wt%, 6wt Re series of single crystal alloy, TEM tests show Re atom is completely distributed in 7 phases; P. Caronc] has also been the same conclusion. But D_Blavette et al [5] research suggests, there is always a small element Re distributed in 7 phase and the alloy content of different Re, Re in the v / v ratio of two distinct phases of distribution. These studies demonstrate that: Although Re distribution in 7 different alloys in the two phases are not the same ratio, but Re mainly distributed in the matrix phase 7 are all single crystal alloy containing Re common features. For 1444 and other alloys 7 phase analysis found: the presence in the form of Re atoms and Mo atoms as aggregation [4] in the matrix. Re added to the CMSX-2 alloy and PWA148O atom probe study also confirmed the presence of Re cluster in 7 matrix [5]. More detailed study shows: The main entry Re 7 formed on the substrate size of about lnm short-range atomic group [4]. An important conclusion for Re atoms exist in the form of special research from U. Glatzel research L7J CMSX-4 alloy, he found Re atoms to about 5 atoms clustered into a series, but not in every atomic level are distributed, but only distributed in some atomic level, in which three atoms the same atomic planes, the other two atoms are located below this three atoms. Recent studies have shown that this unique existence Re atom and its orbit has a special electronic band structure of the [9]. The study rhenium alloy single crystal of solid-solution strengthening of the basis of the presence and distribution of rhenium in the form of matrix view, to explain rhenium can significantly increase the capacity of a single crystal alloy bearing temperature achieved some initial understanding. Rhenium is mainly distributed in the nature of the substrate greatly reduces the SFE substrate, so that the median base error is easy to form extended dislocations, dislocation movement more difficult, and strengthen the substrate. Meanwhile rhenium 7 matrix amounts of dissolved phase, is bound to raise the recrystallization temperature of the substrate, reduce the spread of the diffusion matrix and reinforcing phase between the matrix elements and thus play a significant base for solid solution strengthening effect. on the other hand. Re atoms in the matrix of short range order and the presence of this unique form of three-dimensional distribution of the matrix portion 7 is a major obstacle to dislocation motion and diffusion of elements, so that the creep of dislocation motion and element diffusion hindered radicals, resulting in very It reduces the creep strain rate to a large extent on the alloy. From these findings, the special distribution of Re in the matrix and micro research also exist in the form of a single experiment limited to a certain state, temperature, stress, influence of alloying elements on its very lack of the need to strengthen this research, as well as a detailed study of the content exists in the form of Re on Re atom distribution in the matrix and thus more accurately grasp the real reason Re improved performance.
3 Strengthen Y physiognomy bcc phase single crystal alloy as a reinforcing phase, the number, size and shape as well as alloying elements in which the atoms occupy positions directly affects the performance of a single crystal alloy. Most studies show a, Re little or hardly enter 'strengthening phase, but D. - The study found that about 2O Blavetee etc. The Re enter and strengthening phase. Re impact on the number of phases, different researchers
It has a different conclusion. n Blavetee wait a discovery Re added does not change the volume fraction greater extent. And P. Caron ~ o] from the experimental alloys were found, Re content alloys highest volume fraction is high. More surprisingly, yes. Studies have found one: with the added Re content slightly reduce the number of phases. Effect of Re - size, shape for, researchers from different research Re alloys containing different point of view, despite the different subjects, but the same conclusion: Due to the low diffusion characteristics Re effectively prevented during the heat treatment of 7 grow up, help get small, ^ y a regular phase. For a small amount into 7 phases Re atomic research position in face-centered cubic structure, atom probe analysis showed that the distribution in phase Mo, W, like atoms, Al atoms substituted with Re atoms preferentially, in a face-centered cubic vertex a position. H. Murakami et _] use other methods to CMSX-4 alloy 71 wide research and TMS also get the same result. The raft of single crystal superalloys during creep is an important phenomenon, have a significant impact on performance, has been widely and deeply, but the impact for improving the creep rupture properties and an important element of the 7 Re raft of little research. Frank. R. N. Nabarro noted: 7 'raft of the driving force is proportional to the applied stress, two-phase and two-phase mismatch elastic modulus difference. Re added increases the mismatch single crystal alloy, to a certain extent, an increase raft of driving force. T. Murakumo et l] j have the same view. Since the raft of mechanisms controlled by diffusion raft of rate is also controlled by diffusion of alloying elements, the alloying element Re single crystal has the lowest rate of diffusion, although an increase of 7 raft of driving force, but 7 raft of diffusion process will have a strong positive effect hinder. Clearly, Re increases the raft of activation energy [. S Wdllrner "] and GL Efickson etc. [Re experiments also confirmed that a stable single crystal alloy high temperature process lasting seven phases of the raft; K.Harris et I_ also agreed that: Re hampered ^ y element of the raft. However, the mechanism of obstruction element 7 Re raft of them have different views .K.Harris think D g], Re reduces the diffusion of power on the two-phase interface atoms, thereby increasing the high temperature stability of the alloy microstructure. effectively hindered the coarsening .D.B |. avette a P.Warre ~ and the like but think, Re hinder 7 roughened mainly Re atom segregation y / y two-phase interface hinders the diffusion of elements from. results of the above studies achieved, these results understanding and awareness of the single crystal alloy 7 Re-phase structure as well as the impact of the single crystal alloy Re properties provide important information, but in explaining Re significantly improve the performance of the single crystal alloy as the reason the lack of sure evidence and the impact on the alloy of Re's contribution to the performance of certain organizations is not clear, such as the impact on the relative influence of Re number of creep properties of the alloy case, Re affecting the single crystal alloy 7 the contribution of size and morphology of the alloy properties, and phase 7 Re atom is whether the face-centered cubic vertex position of the phenomenon and strengthen Re 7 with a certain relationship, etc. these are the strengthening mechanism Re exploration require in-depth study issues. in addition, the object of previous studies mostly confined to a single crystal alloy composition containing Re, for the content of Re and 7 with the size relationship between form, quantity, distribution and 7 atomic raft of relatively scarce, it is necessary Re content changes alone, study the effects of change in the amount of 7-phase structure, while in-depth study of Re atoms in distribution and content of 7 Re alloy phase relationships through.
4 strengthen Y / Y interface of the two phases
Deformation behavior of single crystal superalloy is mainly characterized by the y / y two-phase interface decisions l2. Since ^ y / 7 interface is the creep deformation barriers, Re of the two-phase interface atomic state, two-phase coherent influence and status interface dislocation network, determine the deformation behavior of single crystal alloy. J. R ~ sing like. j with a three-dimensional atom probe study sample Ni-Al-Ta-Re alloy two-phase heat treatment after the discovery: in seven phases, from 1.5 to interface a range of 2n, Re mutant atomic concentration, the concentration peak value. H. Murakam a] in CMSX-4 alloy also found a similar phenomenon. But Nelia Wanderka etc. l2IⅢ study sample CMSX_4 after the heat treatment was not found Re atom enrichment at the interface and its vicinity. Even more surprising it is, P. J. warren_2 using an energy compensated three-dimensional atom probe technique in the study sample RR3000 single crystal alloy high temperature creep and low cycle fatigue after the experiment found that: in seven phases, there is a secondary phase precipitation 7, 7 near the second phase and cutting-edge 7 with raft structure are stacked Re atoms. Further research found that Re atom is formed shock solute atoms present in the forefront of raft structure, rather than simply segregation y / y in the two-phase interface. Meanwhile researchers speculate that, with the raft of the course of the growing number of Re atoms accumulate in the vicinity of the interface, so that the formation of the shock wave continues to widen, to more effectively suppress the raft with 7 rows of l2. These studies explore the distribution of Re Re atom-containing single crystal superalloy near v / v phase interface enriched, to explain the increase of single crystal alloy elements Re alloy high temperature creep performance provided some evidence; unfortunately did not reveal Re role atom Y / Y interface of the two phases enriched Re atoms in the distribution T / T interface of the two phases and causes, as well as atomic diffusion and dislocation motion. A. F. Giamei other studies on the effects -Re y / y two-phase interface], and confirmed: with increasing Re content 7 with lattice constants increased steadily. P. Carof 0] study also confirmed: Re accession will greatly increase the lattice constant of 7, a slight increase in the lattice constant, so that the mismatch of a single crystal alloy in the negative direction increases. Japanese scientists H. Harada] said, adding alloy negative mismatches Re alloy single crystal creep strength improving the main reason. The TM has excellent creep properties & 82 + alloy was found to give excellent creep performance is mainly due to: a large negative mismatches accelerated the formation of raft structure and dense dislocation network, thus impeding dislocation motion, especially hindered the bit cutting l2 wrong direction. TM & 75 series of the single crystal alloy of the latest research showed that: the degree of fine 7/7 two-phase interface dislocation network determines the creep performance of the merits, the lower the dislocation network more dense creep rate; resistance hinder dislocation network interface the bits from the matrix slip fault slip y / y phase interface, thus preventing the dislocation of 7 raft tissue cutting l2. The results of this study to increase the alloy negative mismatch is mainly due to increase Re alloy single crystal creep strength provides an important evidence. But P. Caron study L1 0J thought: mismatch for the creep strength of the alloy is not clear mismatch single crystal alloy at a certain temperature and stress environment, the role of the complex, especially 7 phase at a specific temperature and under stress, raft of start and later in the process, the role and mechanism of mismatch played very clear, which means that a single crystal alloy of Re increases is not necessarily a negative mismatch Re strengthened alloy root causes. A. C. Yeh through the study of single crystal superalloy plus Re and Ru people found that: increase the alloy mismatch can really get fine dislocation network. But the contribution of fine dislocation network by increasing the degree of mismatch obtained to increase the creep life is not necessarily large. This is confirmed by Re alloy to improve the mismatch is not necessarily the main reason for Re to improve creep properties. Although many researchers from the two-phase interface may exist Re accumulation and improve the interface Re wrong with both of the two-phase interface of Re strengthening certain research r, provides a detailed study of future ideas and methods worth learning, but from a published literature, in the two-phase interface for Re agglomeration phenomenon of whether there is a contradiction, whether to increase Re alloy negative mismatch dislocation network interface to promote the formation of a single crystal alloy is Re improve the creep strength of the main reasons The study also very lacking. Therefore, a direction for future research is to study the relationship between the distribution of Re and dislocation motion the distribution of two-phase interface, and the Re atoms. Explore the impact of the contribution of the mismatches Re alloy strength.
5 Discussion
Re added so far in the first high-temperature alloys, the study of Re strengthening mechanism has been the focus of a high temperature alloy. From a common literature, for Re strengthening mechanism in single crystal superalloy view there are four kinds: ①Re atoms in the main distribution base and a special presence in the form of radicals, solid solution 7 strengthening, reducing the matrix stacking fault energy and hinder the movement of dislocations base is the key reason Re strengthen superalloy; ② small amount distributed in 7 phase Re atom takes the place of A1 atoms, greatly increasing the 7-phase domain boundaries can reverse , 7 dislocations cutting more difficult phase, reinforces this view Re phase is 7 Re alloy to improve the performance of the main factors; added, Re ③Re atoms in the vicinity of the two-phase interface accumulation hinder 7 phase raft is the Re strengthening the main aspects of a single crystal superalloy; ④Re added greatly increased 7 with lattice constant, so that v / v two-phase interface has a large negative mismatch, but also has fine dislocation network in the two-phase interface, for strengthening v / v phase interface is the most direct cause of the Re-containing alloys of monocrystalline improved. These views show different researchers to strengthen the mechanism of single crystal alloys of Re recognize there is a big difference. However, these studies have in common: the research object limited to one single crystal alloy containing Re, research limited to a unilateral alloy microstructure and performance-related features, most of the conclusion only reveal a certain type of alloy one aspect of characteristics, which for the reasons explained Re improve performance single crystal alloy there is a big limitation. In addition, some researchers in the strengthening of the single crystal alloy mechanism Re explanation is still speculative. For crystal superalloy, at a temperature and stress, either 7 or roughening the movement of dislocations are closely linked with the alloying elements diffusion l1. Diffusion of alloying elements determines the change in the mechanism of single crystal superalloy microstructure]. In the single-crystal nickel-base alloy, the effect of alloying elements on the Re element diffusion process may have the following aspects. (1) its low diffusion coefficient and diffusion activation energy high, resulting in extremely difficult sport itself atoms, while increasing other types of alloying elements diffusion activation energy. (2) a large atomic radius, so that other kinds of metal atoms migrate to bypass the energy increases, thus delaying the diffusion of alloying elements. (3) special electronic band structure so that it can track and Mo atom or atoms combine to form their own short-range order of the Mo-Re, Re-Re radicals, this approach reduces the presence of atomic diffusion channel alloying elements. Effect of alloying elements diffusion is more important is that in 7 matrix Re radicals may not be evenly distributed over the single atom level, but in a three-dimensional way of existence, this particular mode of existence may cause diffusion of atoms in the vicinity of dramatic increase in energy requirements. And so the main obstacle to the formation of diffusion alloying elements, to a large extent reduce the creep strain rate of the alloy. (4) radicals Re formed by gathering in a two-phase near the interface of the base 7, as in the vicinity of 7 edge of the substrate to establish a week the fence, forming a two phase atomic diffusion barrier, effectively hinder the mutual diffusion between the two phases atoms. Re improve the single crystal alloy creep strength through the influence of the diffusion of alloying elements to achieve, it is an important future research is to study the impact of the mechanism and rules of the single crystal superalloy element Re and diffusion redistribution of alloying elements. It is also seeking to strengthen the single crystal alloy Re the real reason the key. From 2O century 8O's, most countries had recognized the prominent role of Re alloys at high temperatures, as well as an important role in aviation turbine engine blade materials. However, due to high prices and Re complexity of the problem, so far, to strengthen the mechanism of single crystal superalloys Re is also only a few developed countries, such as the United States, Britain, Germany, France, Russia and Japan and other countries to carry out a small number of aerospace and defense research institutes. In CMSX-4 alloy containing Re, represented the second generation single crystal superalloy in military and civilian aircraft on the application, as well as a CMSX respectively 10, MC-NG alloy as the representative of the third generation, fourth generation single containing Re successfully developed crystal superalloy shows that for Re strengthening mechanism in single crystal superalloy study these countries have achieved some results. Based emphasis on the important role of Re in single crystal superalloy right, and the developed countries to reduce the gap of aviation technology in the "Ninth Five" period, Beijing Institute of Aeronautical Materials on strengthening mechanism of Re also conducted a preliminary exploration. Though it started later than in developed countries lO ~ 15 years, but in the meantime, we have successfully developed in line with China's national conditions, low-cost, high-performance single crystal superalloy DD6 alloy containing Re. The alloy containing Re was only 2wt, its performance meets or exceeds the amount of foreign-containing Re 3wt second generation single crystal alloy level in some respects, is currently the highest temperature of bearing capacity, the best overall performance of high-temperature alloys. It is worth mentioning that the recent China's first single-crystal turbine parts from the alloy hollow blades produced have been equipped with an advanced engine, and a test assessment. Nevertheless, due to the high prices and Re difficult to control the composition of the alloy, in a very long period of time, to strengthen the mechanism of single crystal superalloys Re carried out not by much, until now, of Re in single crystal superalloy strengthen the understanding of the mechanism is also very thorough, there are many contradictory and unclear. Whether it is from the understanding or application level Re Re mechanism in single crystal alloy in the alloy of view, China and developed countries there is still a gap compared. Of course, whether developed or developing, the high prices are carried Re Re strengthening mechanism and application of important constraints; also containing Re-performance single crystal alloys are widely used in advanced aero engines are facing major challenge.
6 Conclusion
Strengthened Mechanism of Re in single crystal superalloys suggests that research element Re strengthening mechanism of the single crystal alloy, there are still many doubts. In order to optimize the single crystal alloy composition design, take full advantage of the strengthening effect Re, access to low-cost high-performance single crystal alloy, it is necessary to start the detailed study of the following aspects:
(1) Effects of Re on the 7-phase microstructure and organizational stability.
(2) heat treatment study, lasting not broken and creep rupture Re distribution in two phases 7,7 and existing forms under different conditions.
(3) Research on Re phase raft of processes of law.
(4) from the dislocations of the origin, development, investigate the effect of Re on dislocation motion sports.
(5) Research Re interaction with other alloying elements and alloying elements during the movement of high-temperature creep.
(6) to strengthen the mechanism of Re, to explore new design ideas and methods for low-cost high-performance single crystal alloy.

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