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Rhenium, a panacea to solve the "heart disease" of aero engines

Rhenium, a panacea to solve the "heart disease" of aero engines

(Summary description)To simply prove the conclusions of the title clearly, you need to explain from two levels. First of all, what are superalloys and directionally solidified single crystal superalloys? Secondly, why must rhenium-containing single crystal superalloy turbine blades be used?

Rhenium, a panacea to solve the "heart disease" of aero engines

(Summary description)To simply prove the conclusions of the title clearly, you need to explain from two levels. First of all, what are superalloys and directionally solidified single crystal superalloys? Secondly, why must rhenium-containing single crystal superalloy turbine blades be used?

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Figure 1 Aero Engine

To simply prove the conclusions of the title clearly, you need to explain from two levels. First of all, what are superalloys and directionally solidified single crystal superalloys? Secondly, why must rhenium-containing single crystal superalloy turbine blades be used?

As the name implies, high-temperature alloys refer to alloys that can withstand a certain stress and have the ability to resist oxidation or corrosion at a high temperature of 600°C to 1200°C. It can be simply divided into iron-based superalloys, nickel-based superalloys and cobalt-based superalloys. Tracing back to the history of superalloys and turbine blades, in 1940, the first batch of jet engines using superalloys to make turbine blades replaced piston engines. Since then, the development of the aviation industry has entered a new historical period. The higher the temperature of the gas flowing into the rotating blades of the gas turbine, the greater the thrust of the engine. According to statistics, in the nearly 80 years from 1940 to the present, the working temperature of nickel-based ultra-high temperature alloys has increased from about 850°C (polycrystalline alloys) to about 1150°C (single crystal fourth-generation alloys). The amount of ultra-high temperature nickel-based alloys used in engines is increasing. Now 40% of the total weight of the engine has been used, and 50% to 60% of the total weight has been used in some new military engines. After the 1940s, in order to further improve the high temperature strength of the alloy, tungsten, molybdenum, cobalt and other elements were added to the nickel-based alloy to increase the content of aluminum and titanium, and a series of alloys were developed. Nickel, tungsten and other elements are added to cobalt-based alloys to produce a variety of high-temperature alloys. After the 1970s, superalloys were mainly used to manufacture high-temperature components for aviation, naval and industrial gas turbines, as well as for high-temperature components such as aerospace vehicles, rocket engines, nuclear reactors, and petrochemical equipment.

The study found that there are two main ways to improve the temperature-bearing capacity of nickel-based superalloys: 1. Directional solidification technology and single crystal solidification technology are used in the preparation of nickel-based superalloys to eliminate low melting point grain boundaries; 2. In smelting nickel A large amount of high melting point alloy elements such as rhenium, tungsten, tantalum, and molybdenum are added to the base superalloy, which further increases the melting point of the alloy, and also makes the use temperature of the nickel-based single crystal superalloy close to the melting point of the alloy matrix (the theoretical melting point of nickel is 1453℃) . That is to say, the combination and balance of these two factors have made the research of nickel-based superalloys in my country, from the first generation of DD3, to the third generation of DD9, and the fourth and fifth generations of superalloys under study.

Then back to the first question, the advantages of directional solidification and single crystal microstructure and macro performance can be explained simply by the following picture. Ordinary pouring and solidification result in equiaxed crystal system, that is, polycrystal of cubic crystal system. Each unit cell is randomly, disorderly, etc. for crystallization and solidification. The creep resistance at this time is the worst. Secondly, through the directional solidification technology, all the unit cells grow in one direction, such as [001], [011] and [111], at this time, the creep resistance will be significantly improved. In the end, on the basis of directional solidification, the entire blade becomes a single crystal, and the creep resistance at this time will be optimal.

 

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Figure 2 Comparison of creep resistance of blades with different crystal structures

That’s why people say that single crystal superalloys are the material basis for the development of advanced aero-engines. Without single crystal superalloys, there would be no advanced aviation engines. Historically speaking, since the 1980s, the first generation of single crystal superalloys and the second generation of single crystal superalloys have been widely used in military and civil aviation engines. Since the late 1990s, the United States has successfully developed the third-generation single-crystal superalloys CMSX-10 and RenéN6, and passed the test run of advanced aero-engines. The superalloy DD9 with independent intellectual property rights developed by my country is the representative of the third-generation single crystal superalloy. The grade here, DD9 is based on the naming rules stipulated in the national standard "GB/T 14992-2005 Classification and grades of high temperature alloys and intermetallic compounds". "D" and "D" stand for the Chinese characters "定" and "single The first letter of the pinyin of ".

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Table 1 Application of single crystal superalloys on turbine blades

Returning to the second technological means "composition design" to improve the temperature-bearing capacity of nickel-based superalloys, it is to add a large amount of high-melting alloy elements such as rhenium, tungsten, tantalum and molybdenum when smelting nickel-based superalloys to further increase the melting point of the alloy , Can also make the use temperature of nickel-based single crystal superalloy close to the melting point of the alloy matrix. The most magical element in it is rhenium. The addition of rhenium can significantly improve the creep properties of nickel-based superalloys, especially nickel-based single crystal superalloys. This phenomenon is called the "rhenium effect". Studies have shown that every time 3% (mass fraction) of rhenium is added, the working temperature of nickel-based single crystal superalloys can be increased by about 30 ℃. The working temperature of superalloys is defined as a new generation of alloys for every increase of 30 ℃. The rhenium content is regarded as an intuitive parameter for the generation of superalloys due to its significant strengthening effect. For example, the first generation of nickel-based single crystal superalloys does not contain rhenium, and the second and third generations contain 3% and 6% (mass fraction) respectively. Rhenium element. The third-generation nickel-based single crystal superalloy has a life of up to 2500 hours at 750 ℃/500 MPa creep, and the life of the first-generation alloy without rhenium is only 250 hours under the same test conditions. The main difference in composition between the third generation and the first generation of nickel-based single crystal superalloys lies in the presence or absence of rhenium, while the content of other alloying elements is basically the same, that is to say, adding 6% (mass fraction) of the alloying element rhenium makes the superalloy creep life Improved by a full 10 times. In other words, this is the magic of the "rhenium effect".

The advancement of single crystal alloy preparation technology has significantly improved the temperature-bearing capacity of nickel-based superalloys, and changed the design direction of superalloy designers: instead of adding a large amount of alloy elements that strengthen the grain boundary to the superalloy, try to add a large amount of high-temperature alloy elements. The melting point alloying element further improves its temperature-bearing capacity. The discovery of the "rhenium effect" makes it possible to prepare super temperature-resistant superalloys, and exploring the internal strengthening mechanism of the "rhenium effect" has important guiding significance for further optimizing the alloy composition, preparing high-performance superalloys with excellent properties and moderate prices.

In general, "the addition of rhenium determines a generation of single crystal blades", and "a generation of blade materials determines a generation of engines", so it is said that rhenium Re is a panacea for curing the "heart disease" of aero engines.

 

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Figure 3 Morphology of high-purity 99.99% metal rhenium powder

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