Superalloys: Properties, Processing, and Applications

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.  

Here, you will learn about:

  • The properties of superalloys
  • Examples of common superalloys
  • The composition of superalloys
  • The processing of superalloys
  • Future trends of superalloys

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Properties of superalloys

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]:

Nickel-based superalloys

Cobalt-based superalloys

  • Higher melting point compared to nickel- or iron-based alloys
  • Superior hot corrosion resistance compared to nickel- or iron-based alloys
  • Higher thermal fatigue resistance and weldability compared to nickel-based alloys

Iron-based superalloys

  • High strength at room temperature
  • High resistance to creep, oxidation, corrosion and wear

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].

Examples of superalloys

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

IN-718

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

HS-188

3

22

39

22

--

--

--

--

14

--

--

--

--

--

0.1

--

X-40

--

10

54

25

--

--

--

--

7.5

--

--

--

--

--

0.5

--

Iron-based alloys

A-286

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

--

Processing of Superalloys

Superalloys are generally processed via two separate methods; casting and powder metallurgy [8].

Investment Casting 

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.

Vacuum Induction Melting (VIM) 

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.

Secondary Melting 

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.

Conversion

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.

Directional solidification

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.

Single-crystal growth

This is a slow process in which a monocrystalline superalloy component is grown from a seed crystal.

Powder Metallurgy (P/M)

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].

Applications of Superalloys

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

Future Trends of Superalloys

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].

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Sources

[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.

Fun Fact

During the 1920s and 1930s, stainless steels were developed to provide increased corrosion resistance for high-temperature applications. However, these materials had limited strengths. In response to the demands, the metallurgical community introduced the first superalloys with the adoption of cobalt-based alloys in the aircraft industry. Shortly after the Second World War, nickel-chromium and iron-based alloys were developed [1].