Superalloys are complex, high-performance alloys, which have a high tolerance of oxidising environments and high temperatures. They are typically classified according to their predominant matrix element; nickel, cobalt, or iron, and they contain multiple alloying elements including the refractory metals (Nb, Mo, W, Ta), chromium, and titanium. They exhibit high mechanical strength, creep resistance and corrosion resistance, especially at high temperatures [1]. These properties make them more challenging to produce and costlier than other alloys, but they are also critical for components in industries such as aerospace.
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Superalloys are intended for use in high-temperature applications, which means they need to maintain their shape at elevated temperatures close to their melting points (above 650 °C or 1200°F). When alloyed with certain elements, at extreme temperatures superalloys can maintain high strength, stability, and corrosion and oxidation resistance [2].
Superalloys are classified into three main categories [1, 4, 5]:
Of these three categories, nickel-based alloys have the widest range of applications, particularly in the aerospace industry. The essential solutes in the nickel-based alloys are aluminium (Al) and titanium, with concentrations of less than 10 wt. %. This allows the generation of a two-phase equilibrium microstructure that consists of the phases known as gamma (γ) and gamma-prime (γ’). The matrix of superalloys is composed of the γ-phase, while their primary hardening is a result of the γ’-phase. The high-temperature strength, as well as other mechanical properties of superalloys, are also a result of the presence of the γ’-phase [6].
The high-temperature properties of superalloys are provided by alloying the matrix element (Ni, Co or Fe) with various other elements such as chromium (Cr), titanium (Ti), aluminium (Al), boron (B), and iron (Fe). In some cases, refractory metals are added, such as molybdenum (Mo), cobalt (Co), niobium (Nb), zirconium (Zr), amongst others. The composition of some of the most common superalloys is presented in the table below [7].
Alloy |
Fe |
Ni |
Co |
Cr |
V |
Nb |
Ta |
Mo |
W |
Re |
Zr |
Al |
Ti |
B |
C |
Hf |
Nickel-based alloys |
||||||||||||||||
19 |
53 |
-- |
19 |
-- |
5.2 |
-- |
3 |
-- |
-- |
-- |
6.6 |
0.8 |
0.006 |
0.05 |
-- |
|
Mar-M 247 |
-- |
62 |
10 |
8.2 |
-- |
-- |
3 |
0.6 |
10 |
-- |
0.09 |
5.5 |
1.4 |
0.001 |
0.006 |
-- |
Udimet-700 |
-- |
53 |
19 |
15 |
-- |
-- |
-- |
5.2 |
-- |
-- |
-- |
4.3 |
3.5 |
0.03 |
0.08 |
-- |
CM SX-2 |
-- |
66 |
4.6 |
8 |
-- |
-- |
5.8 |
0.6 |
7.9 |
-- |
-- |
5.6 |
0.9 |
-- |
0.005 |
-- |
IN 713C |
-- |
74 |
-- |
12 |
-- |
2 |
-- |
4.2 |
-- |
-- |
0.1 |
6.1 |
0.8 |
0.012 |
0.12 |
-- |
PWA 1480 |
-- |
63 |
5 |
10 |
-- |
-- |
12 |
-- |
4 |
-- |
-- |
5 |
1.5 |
-- |
-- |
-- |
Waspaloy |
-- |
58 |
13 |
19 |
-- |
-- |
-- |
4.3 |
-- |
-- |
0.06 |
1.3 |
3 |
0.006 |
0.08 |
-- |
N-4 |
-- |
63 |
7.5 |
9.2 |
-- |
0.5 |
4 |
1 |
6 |
-- |
-- |
3.77 |
4.25 |
-- |
0.005 |
-- |
Rene 150 |
-- |
58 |
12 |
5 |
3 |
-- |
6 |
1 |
5 |
2.2 |
0.03 |
5.5 |
-- |
0.0015 |
0.06 |
1.5 |
Cobalt-based alloys |
||||||||||||||||
3 |
22 |
39 |
22 |
-- |
-- |
-- |
-- |
14 |
-- |
-- |
-- |
-- |
-- |
0.1 |
-- |
|
X-40 |
-- |
10 |
54 |
25 |
-- |
-- |
-- |
-- |
7.5 |
-- |
-- |
-- |
-- |
-- |
0.5 |
-- |
Iron-based alloys |
||||||||||||||||
53 |
26 |
-- |
15 |
0.2 |
-- |
-- |
1.25 |
-- |
-- |
-- |
0.2 |
2.15 |
-- |
0.05 |
-- |
|
N-155 |
30 |
20 |
20 |
21 |
-- |
1 |
-- |
3 |
2.5 |
-- |
-- |
-- |
-- |
-- |
-- |
-- |
CG-27 |
38 |
38 |
-- |
13 |
-- |
0.6 |
-- |
5.5 |
-- |
-- |
-- |
1.5 |
2.5 |
0.01 |
0.05 |
-- |
Superalloys are generally processed via two separate methods; casting and powder metallurgy [8].
Also known as lost-wax casting, this process uses wax models or replicas to create a casing for the molten metals and is mainly used for complex shapes. It was the first method to improve upon the formerly prevalent cold-rolling techniques.
This is a standard melting practice where raw metallic materials are melted within a vacuum using electric currents. It is an improvement upon investment casting as it offers greater control of chemical composition.
In some applications, VIM can leave ceramic inclusions in the material affecting fatigue properties. Secondary melting is an additional melting process, which increases the chemical homogeneity and is applied after the VIM process. This increases chemical homogeneity and reduces problems associated with the initial process.
Also known as ingot conversion, this process involves several stages of thermal-mechanical deformation to make the superalloy ingots produced by secondary melting suitable for mechanical applications.
In this method, the alloy is allowed to nucleate on a low-temperature surface via the presence of a thermal gradient. This provides greater creep resistance along the grain direction.
This is a slow process in which a monocrystalline superalloy component is grown from a seed crystal.
This is a group of processes for producing alloys used in critical fatigue applications and consists of forming superalloys from a mixture of metal powders. Sintering can be applied to convert these metal powders into parts by applying chemical pressure to bond them [9]. It is now also possible to use additive manufacturing, also known as 3D printing, to print parts from superalloy powders from a 3D model [10].
Superalloys have many applications. These mainly include aircraft components, chemical plant equipment and petrochemical equipment. The table below shows some of the applications of superalloys [1].
Aircraft gas turbine components |
Nuclear power components |
Chemical products |
Disks, bolts, shafts, cases, blades, and vanes Combustors Afterburner |
Control-rod drive mechanisms Valve stems Springs Ducting |
Bolts, valves Reaction vessels Springs Ducting |
Power plant components for steam turbines |
Metal processing products |
Medical components |
Bolts and blades
|
Hot forming tools and dies |
Dentistry components Prostates |
Automobile components |
Aerospace components |
Heat treating equipment |
Turbine-driven chargers Exhaust valves |
Aerodynamically heated skins Rocket-engine parts |
Trays and fixtures Conveyor belt furnaces |
The demand for superalloys is continuously increasing, primarily driven by the aerospace industry. One challenge is the high cost of production for unique and complex parts. This is partly being met by the printing of complex parts using additive manufacturing.
Another interesting focus of superalloy research is the synthesis of nanoparticles. This has been performed via the radiolysis process, a radiation method in which the molecular structure of substances is broken down to form nanoparticles. This approach is a flexible and versatile method to manufacture large quantities of superalloy nanoparticles that can't be easily created by other methods [11, 12].
[1] Donachie, M.J. and Donachie, S.J., 2002, ASM International, Superalloys: A Technical Guide, 2nd Edition, Materials Park, Ohio, USA
[2] El-Bagoury, N. 2016, “Ni Based Superalloy: Casting Technology, Metallurgy, Development, Properties And Applications”, International Journal of Engineering Sciences and Research Technology, Vol. 5, No. 108.
[3] Encyclopedia Britannica, “Superalloy”, https://www.britannica.com/technology/superalloy.
[4] Nickel Institute, “Nickel based alloys”, https://www.nickelinstitute.org/about-nickel/nickel-alloys.
[5] Cobalt Institute, “Superalloys”, [Online] https://www.cobaltinstitute.org/superalloys.html.
[6] Kracke, A., 2010, "Superalloys, The most successful alloy system of modern time – past, present and future", 7th International Symposium on Superalloy 718 and Derivatives, TMS.
[7] Reed, R.C., 2008, The Superalloys, Fundamentals and Applications, Cambridge University Press
[8] Materials Technology TMS, “Processing of Superalloys”, [Online] https://www.tms.org/Communities/FTAttachments/Superalloys%20Processing%20Summary.pdf.
[9] General Electric, How sintering works powder metallurgy, [Online] https://www.ge.com/additive/additive-manufacturing/information/materials/powder-metallurgy-sintering.
[10] European Powder Metallurgical Association, EPMA, Additive Manufacturing, [Online] https://www.epma.com/additive-manufacturing
[11] Weber, J.H., Banerjee, M. K., 2016, Nickel-Based Superalloys: Alloying, Reference Module in Materials Science and Materials Engineering, Elsevier.
[12] Science Daily, [Online] https://www.sciencedaily.com/releases/2007/06/070613140415.htm.