Superalloys | In detail

Ahmed Samy

12 Nov, 2023

Materials scientists and engineers are constantly trying to improve the properties of existing alloys and to design and produce new alloys with improved strength, high-temperature strength, creep, and fatigue properties. The existing alloys may be improved by better chemistry, composition control, and processing techniques. For example, by 1961, new and improved nickel-base, iron-nickel-cobalt-base superalloys were available for use in high pressure turbine airfoils in aircraft gas turbines.

What are the superalloys?

Superalloys, also known as high-performance alloys, are a special class of materials that exhibit remarkable mechanical strength, high-temperature resistance, excellent corrosion resistance, and resistance to corrosion. These properties make them ideal for use in extreme environments, such as aerospace, power generation, and chemical processing industries.

The term superalloy was used because of their improved performance at elevated temperatures of approximately 540°C (1000°F) and high stress levels.

Figure (1) shows a PW-4000 gas turbine engine that is made primarily of metal alloys and superalloys. Figures (2a) and (2b) show the fan blades and the compressor bales for gas turbines similar to the PW-4000.

Figure (1)

Figure (2)

What is the composition of superalloys?

Superalloys are primarily composed of iron, nickel, and cobalt, with varying amounts of other elements like chromium, molybdenum, tungsten, tantalum, and aluminum. These alloys are carefully engineered to achieve a combination of desirable properties, so let divide the composition of these alloys to :

1. Base Metals

Nickel: Nickel is a primary constituent of most superalloys, and it forms the base of many of these alloys. It provides excellent high-temperature strength and corrosion resistance.

Cobalt: Cobalt-based superalloys are also common, especially in applications requiring even higher temperature capabilities.

Iron: Some superalloys contain iron as a base metal, although they are less common compared to nickel and cobalt-based alloys.

2. Strengthening Elements

Chromium: Chromium is added to enhance corrosion resistance and oxidation resistance. It forms a protective oxide layer on the surface of the alloy.

Molybdenum: Molybdenum contributes to the high-temperature strength and creep resistance of superalloys.

Tungsten: Tungsten is another element that enhances high-temperature strength and resistance to deformation under load.

Cobalt: In cobalt-based superalloys, cobalt itself acts as a strengthening element, improving high-temperature properties.

3. Stabilizing Elements

Aluminum: Aluminum is often added to form oxide dispersoids that improve the high-temperature strength and stability of the alloy.

Titanium: Titanium is used to stabilize the structure of the alloy and to control the grain size, which affects mechanical properties.

4. Refractory Elements

Tantalum: Tantalum is sometimes added to enhance the high-temperature properties of superalloys, particularly in applications requiring resistance to sulfur-containing environments.

Niobium: Niobium contributes to the stability of the alloy's microstructure at high temperatures.

5. Trace Elements

Carbon: A small amount of carbon may be included for improved high-temperature strength, although excessive carbon can be detrimental to the alloy's properties.

Boron: Boron can be added in trace amounts to improve weldability and high-temperature strength.

Zirconium: Zirconium is used in some superalloys to improve high-temperature creep resistance.

Note that:

Depending on the specific alloy and its intended application, other trace elements like silicon, sulfur, phosphorus, and hafnium may be added in small amounts to modify certain properties.

The specific composition of a superalloy is carefully engineered to meet the requirements of its intended use. Different applications, such as aircraft engine components, industrial gas turbines, or chemical processing equipment, may demand variations in alloy composition to optimize properties like high-temperature strength, corrosion resistance, and thermal stability. Additionally, advanced manufacturing techniques, such as powder metallurgy and directional solidification, are often used to further enhance the properties of superalloys for specific applications.

Superalloys properties

Superalloys properties are characterized by exceptional high-temperature strength, resistance to creep and fatigue, and outstanding corrosion and oxidation resistance.

Exhibit remarkable stability and mechanical integrity in extreme environments, often exceeding 1000°C. Their resistance to degradation under prolonged stress at high temperatures makes them vital for aerospace, energy generation, and industrial applications.

Superalloys are also known for their weldability, thermal conductivity, and tailored compositions, allowing them to excel in a wide range of demanding scenarios, from aircraft engines and gas turbines to chemical processing equipment. (we will mention the properties of each kind of superalloys in details.)

Nickel-based superalloys

Composition :

Nickel-based superalloys are a class of superalloys where nickel serves as the primary base metal. These alloys are known for their exceptional high-temperature strength, corrosion resistance, and other desirable properties. Here is a typical composition of nickel-based superalloys:

Nickel (Ni): Nickel forms the majority of the alloy and serves as the base metal. It provides excellent high-temperature strength and corrosion resistance.

Chromium (Cr): Chromium is added to enhance the alloy's corrosion resistance, particularly in oxidizing environments. It forms a protective oxide layer on the surface of the alloy.

Cobalt (Co): Cobalt is sometimes included in nickel-based superalloys, especially for applications that require even higher temperature capabilities. Cobalt can improve high-temperature strength and creep resistance.

Molybdenum (Mo): Molybdenum is added to enhance the alloy's high-temperature strength and resistance to deformation under load.

Tungsten (W): Tungsten contributes to high-temperature strength and helps resist deformation at elevated temperatures.

Aluminum (Al): Aluminum is included to form oxide dispersoids, which improve high-temperature strength and stability of the alloy's microstructure.

Titanium (Ti): Titanium is used to stabilize the alloy's microstructure and control grain size, which affects mechanical properties.

Niobium (Nb): Niobium contributes to the stability of the alloy's microstructure at high temperatures.

Ta (Tantalum): Tantalum is occasionally added to enhance high-temperature properties, particularly in applications requiring resistance to sulfur-containing environments.

Other Trace Elements: Depending on the specific alloy and its intended application, trace elements like silicon (Si), sulfur (S), phosphorus (P), boron (B), and hafnium (Hf) may be included in small amounts to modify specific properties.

Properties :

High-Temperature Strength: Nickel-based superalloys exhibit remarkable strength and stability at elevated temperatures, often exceeding 1000°C (1832°F). This property is essential for applications like aircraft engines and gas turbines, where components must withstand extreme heat.

High Creep Resistance.

High Oxidation and Corrosion Resistance: including pitting, crevice corrosion, and stress corrosion cracking.

Excellent Fatigue Resistance.

Excellent Weldability.

High Thermal Conductivity.

Thermal Stability: They maintain their mechanical properties and microstructure even after prolonged exposure to high temperatures, ensuring consistent performance over time.

Machinability: While they can be challenging to machine due to their high hardness and toughness, nickel-based superalloys can still be machined using appropriate tooling and techniques.

Cryogenic Properties: Some nickel-based superalloys also exhibit good performance at cryogenic temperatures, making them suitable for applications in extremely cold environments.

Applications :

Aerospace

Nickel-based superalloys are widely used in aircraft engines and gas turbine components. They are employed in critical parts like turbine blades, combustion chambers, and exhaust nozzles due to their ability to withstand high temperatures and mechanical stresses.

Power Generation

They are essential in power generation applications, including gas turbines and steam turbines. These alloys can endure the extreme conditions encountered in power plants, contributing to the efficiency and reliability of energy production.

Chemical Processing

Nickel-based superalloys are used in equipment for the production of chemicals and petrochemicals. They are resistant to corrosion from acids, alkalis, and other aggressive chemicals.

Oil and Gas

In the oil and gas industry, these alloys find use in downhole drilling tools, wellhead equipment, and valves that must operate in high-pressure, high-temperature environments.

Nuclear Reactors

Some nickel-based superalloys are suitable for use in nuclear reactors due to their resistance to radiation and high-temperature stability.

Medical Devices

They are used in certain medical applications, including orthopedic implants and dental prosthetics, where biocompatibility and high strength are required.

Chemical and Petrochemical

Nickel-based superalloys are used in equipment for the production of chemicals and petrochemicals. They can withstand the corrosive environments found in these processes.

Marine Industry

In marine applications, they are used in components exposed to saltwater and seawater environments.

Iron-Based Superalloys

Iron-based superalloys (ferritic superalloys), also known as iron-nickel-based superalloys, are a subset of superalloys that have iron (Fe) as their primary constituent, along with nickel (Ni) and other alloying elements. These alloys offer a unique combination of properties that make them suitable for high-temperature and corrosive environments.

Composition :

These alloys are primarily composed of iron (Fe) and contain various other elements to enhance their properties. The exact composition can vary depending on the specific alloy and application, but here are some common elements found in iron-based superalloys :

Iron (Fe): Iron forms the base of these alloys and provides the primary matrix. It is chosen for its abundance and cost-effectiveness compared to other base metals.

Chromium (Cr): Chromium is added to improve oxidation resistance and corrosion resistance, particularly in high-temperature environments. It forms a protective oxide layer on the alloy's surface.

Aluminum (Al): Aluminum is used to promote the formation of stable oxide scales and improve oxidation resistance. It also contributes to the alloy's high-temperature strength.

Silicon (Si): Silicon is added to enhance the alloy's resistance to oxidation and corrosion. It can also improve the high-temperature mechanical properties.

Molybdenum (Mo): Molybdenum is included to increase the alloy's high-temperature strength, creep resistance, and resistance to corrosion in aggressive environments.

Tungsten (W): Tungsten is sometimes used to further improve the high-temperature strength and creep resistance of iron-based superalloys.

Carbon (C): Carbon content is kept low in these alloys to prevent carbide formation, which can reduce ductility and toughness. However, trace amounts of carbon may be present.

Vanadium (V): Vanadium can be added to enhance high-temperature strength and improve the alloy's ability to resist deformation at elevated temperatures.

Niobium (Nb): Niobium is used to stabilize the alloy's microstructure and can improve its high-temperature properties.

Copper (Cu): Copper may be added in small amounts to enhance the alloy's corrosion resistance and electrical conductivity, although it's not a common element in all iron-based superalloys.

Other Trace Elements: Depending on the specific alloy and its intended application, other trace elements such as boron (B), sulfur (S), and phosphorus (P) might be included to modify certain properties.

Properties :

High-Temperature Strength: Iron-based superalloys exhibit excellent strength at elevated temperatures, making them ideal for applications in environments where other materials would weaken or fail. They maintain their structural integrity and mechanical properties at temperatures ranging from 600°C (1,112°F) to over 1,100°C (2,012°F).

High Creep Resistance.

Oxidation Resistance: Iron-based superalloys are known for their resistance to oxidation, even at high temperatures. This resistance is due to the formation of a protective oxide layer (usually rich in chromium) on the alloy's surface, which helps prevent further corrosion.

High Corrosion Resistance.

Excellent Fatigue Resistance

High Thermal Stability: They maintain their structural stability even in extreme thermal conditions, which is crucial in applications where temperature fluctuations are common.

Magnetic Properties: Iron-based superalloys can be magnetic, which is a desirable property in certain applications like electromagnets and magnetic sensors.

Weldability: These alloys can be welded using appropriate techniques, making them versatile for manufacturing processes.

Controlled Expansion: Some iron-based superalloys have controlled thermal expansion properties, making them suitable for use in applications where dimensional stability is essential, such as in precision instruments.

High Density.

Machinability: These alloys can be machined, although their hardness and toughness can make machining challenging and may require specialized tools and techniques.

Applications :

Industrial Gas Turbines

Iron-based superalloys are frequently used in the construction of components for industrial gas turbines, such as blades, vanes, and combustion chamber liners. These alloys can withstand the high temperatures and stresses encountered in gas turbine engines used for power generation and mechanical drive systems.

Aerospace

In certain aerospace applications, iron-based superalloys are employed in jet engine components, particularly in older engine designs. They are used for parts like turbine blades and vanes, which operate in high-temperature environments.

Petrochemical and Refining Industry

Iron-based superalloys are used in equipment for the petrochemical and refining industries. They are suitable for components like high-temperature piping, heat exchangers, and reformer tubes that handle corrosive and high-temperature environments.

Power Generation

These alloys are used in various components within power generation systems, including fossil fuel power plants and nuclear reactors. Examples of applications include steam turbine components, boiler tubes, and reactor vessel materials.

Chemical Processing

Iron-based superalloys are employed in chemical processing equipment that handles aggressive chemicals at high temperatures. They offer resistance to corrosion and oxidation in such environments.

Automotive Industry

While not as common as other materials like stainless steel, iron-based superalloys are occasionally used in specialized automotive applications where high-temperature resistance is required, such as in exhaust systems.

High-Temperature Fasteners

These alloys can be used for high-temperature fasteners and bolts that need to maintain their integrity and strength under elevated temperature conditions.

Marine Industry

Some marine applications may utilize iron-based superalloys, particularly in parts of marine engines and exhaust systems that require resistance to saltwater corrosion and high temperatures.

Chemical and Petrochemical Reactors

Iron-based superalloys can be employed in the construction of reactors used in the chemical and petrochemical industries. These reactors are exposed to high temperatures and corrosive chemicals.

Heat Treatment Furnaces

These alloys are used in the construction of heat treatment furnaces, which require materials that can withstand high temperatures and maintain their mechanical properties over time.

Automotive and Aerospace Tooling

In tooling applications for the automotive and aerospace industries, iron-based superalloys can be used in dies, molds, and other components that experience high temperatures and wear.

Cobalt-Based Superalloys

Cobalt-based superalloys, also known as cobalt superalloys or simply "cobalt alloys," are a class of high-performance materials primarily used in applications that require exceptional strength, corrosion resistance, and the ability to withstand extreme temperatures. These alloys are commonly used in industries such as aerospace, gas turbines, and medical implants.

Composition :

Cobalt (Co): Cobalt is the base element of these alloys, making up a significant portion of the composition. It provides excellent high-temperature strength and corrosion resistance.

Chromium (Cr): Chromium is added to enhance the alloys' resistance to oxidation and corrosion, especially at elevated temperatures. It also contributes to the alloys' strength.

Tungsten (W): Tungsten is often included to improve the alloys' high-temperature strength and creep resistance. It helps maintain the structural integrity of components at extreme temperatures.

Nickel (Ni): Nickel is commonly added to cobalt superalloys to improve their ductility and thermal stability. It also helps in solid solution strengthening.

Molybdenum (Mo): Molybdenum is added to enhance the alloys' strength, particularly at high temperatures, and to improve resistance to corrosion and oxidation.

Titanium (Ti): Titanium is used to promote the formation of stable, strengthening precipitates within the alloy's microstructure, which contributes to high-temperature strength and creep resistance.

Aluminum (Al): Aluminum is added to cobalt-based superalloys to promote the formation of strengthening phases and improve high-temperature performance.

Niobium (Nb): Niobium is another element used for precipitation hardening in these alloys, enhancing their high-temperature strength and creep resistance.

Carbon (C) and Boron (B): These elements are added in small quantities to aid in precipitation hardening and grain boundary strengthening.

Other Trace Elements: Cobalt-based superalloys may contain trace amounts of other elements like sulfur, silicon, and tantalum to fine-tune their properties.

Properties :

High Temperature Strength: This property makes them suitable for use in the hot sections of gas turbine engines, where temperatures can exceed 1200°C (2200°F).

Corrosion Resistance.

Creep Resistance.

Fatigue Resistance.

High Melting Point: Cobalt has a high melting point, which contributes to the alloys' ability to withstand high temperatures. The melting point of cobalt is approximately 1,495°C (2,723°F).

High Density.

Good Thermal Conductivity: These alloys exhibit good thermal conductivity, which can help in dissipating heat effectively in high-temperature applications.

Excellent Weldability.

Magnetic Properties: Cobalt is ferromagnetic, which means that cobalt-based superalloys are typically magnetic materials. This property can be both an advantage and a limitation depending on the specific application.

High Cost: One significant drawback of cobalt-based superalloys is their relatively high cost. Cobalt is not as abundant as other base metals like nickel and iron, which are used in other superalloy families. This can make cobalt-based superalloys more expensive to produce.

Limited Availability: As cobalt is considered a critical material, the availability of cobalt-based superalloys can be influenced by geopolitical and economic factors. This can create supply chain challenges.

Applications :

Gas Turbine Engines

Cobalt-based superalloys are extensively used in the aerospace industry for manufacturing components in gas turbine engines, such as turbine blades, combustion chambers, and exhaust nozzles. These alloys can withstand the high temperatures and mechanical stresses associated with jet engines and help improve engine efficiency and lifespan.

Power Generation

Superalloys are employed in gas and steam turbines used in power generation plants. The alloys can handle the high temperatures and pressures encountered in these applications, leading to increased efficiency and reduced maintenance.

Chemical Processing

Cobalt-based superalloys are used in chemical reactors, heat exchangers, and other equipment where resistance to high temperatures and corrosive environments is crucial. Their excellent corrosion resistance makes them ideal for handling aggressive chemicals.

Medical Implants

Certain cobalt-based superalloys, like Co-Cr-Mo alloys, are biocompatible and widely used in medical implants such as hip and knee replacements. Their high strength, resistance to wear, and corrosion resistance make them suitable for long-term implantation in the human body.

Oil and Gas Industry

These alloys are used in downhole drilling tools and equipment in the oil and gas industry. They can withstand the high temperatures and pressures encountered during drilling and production processes, contributing to increased drilling efficiency and equipment longevity.

Nuclear Industry

Cobalt-based superalloys are employed in nuclear reactors, particularly for components that are exposed to high radiation levels and elevated temperatures. Their resistance to radiation-induced damage and high-temperature stability make them valuable in this field.

Electronics

Some cobalt-based alloys are used in the electronics industry, particularly for components like magnetic recording heads in hard disk drives. These alloys offer excellent magnetic properties and wear resistance.

Automotive Industry

Superalloys are used in high-performance automotive engines and exhaust systems due to their ability to withstand extreme temperatures and harsh operating conditions. They can improve engine efficiency and longevity.

Aerospace and Defense

In addition to gas turbine engines, cobalt-based superalloys are used in other aerospace and defense applications, including missile components, rocket nozzles, and components for spacecraft that must endure extreme environments in space.

Metal Cutting and Machining

Cutting tools made from cobalt-based superalloys are used in high-speed machining operations for materials like stainless steel and nickel-based superalloys. These tools can withstand high temperatures generated during cutting processes.

High-Performance Racing

In motorsports like Formula 1, cobalt-based superalloys are used in engine components to improve performance and reliability under extreme racing conditions.

Superalloys examples

Inconel (e.g., Inconel 718, Inconel 625): Inconel is a family of nickel-chromium-based superalloys. Inconel 718, for example, is used in aerospace applications like jet engines and rocket propulsion systems due to its exceptional high-temperature strength and resistance to oxidation and corrosion. Inconel 625 is known for its excellent resistance to various corrosive environments and is used in chemical processing and marine applications.

Hastelloy (e.g., Hastelloy C-276): Hastelloy is a family of nickel-based superalloys known for their resistance to a wide range of corrosive chemicals, including sulfuric acid, hydrochloric acid, and chlorine gas. Hastelloy C-276 is one of the most widely used members of this family and is employed in chemical processing equipment and pollution control systems.

Waspaloy: Waspaloy is a precipitation-hardened nickel-based superalloy used in gas turbine engines, particularly for turbine blades and discs. It offers high-temperature strength and creep resistance.

Rene (e.g., Rene 41): Rene superalloys are nickel-based and designed for high-temperature applications. Rene 41, for instance, is used in aircraft engine components due to its strength and thermal stability at elevated temperatures.

Udimet (e.g., Udimet 720): Udimet superalloys, like Udimet 720, are cobalt-based and used in aerospace and gas turbine applications. They offer high-temperature strength and creep resistance.

Haynes (e.g., Haynes 230): Haynes superalloys are nickel-based and have a wide range of applications, including aerospace, chemical processing, and power generation. Haynes 230, for example, is known for its excellent high-temperature strength and corrosion resistance.

Rene N6: Another member of the Rene superalloy family, Rene N6, is used in aerospace applications, particularly for turbine discs and other high-stress components.

Cobalt-Chromium Alloys (e.g., Stellite): Cobalt-chromium alloys, such as Stellite, are used in applications requiring resistance to wear, corrosion, and high-temperature oxidation. They are used in components like cutting tools, valves, and engine parts.

CMSX Alloys (e.g., CMSX-4): CMSX alloys are single-crystal nickel-based superalloys developed for gas turbine components. CMSX-4, for instance, is used in high-temperature, high-stress environments.

Mar-M Alloys (e.g., Mar-M 247): Mar-M alloys are precipitation-hardened nickel-based superalloys used in applications requiring high-temperature strength and creep resistance. Mar-M 247 is used in gas turbine engine components.

Superalloys applications

Superalloys, such as Inconel, Hastelloy, and Waspaloy, find a wide range of critical applications in industries requiring exceptional high-temperature strength, resistance to corrosion, and wear resistance. These advanced materials are integral to the aerospace sector, where they are used in jet engines, rocket propulsion systems, and aircraft components, contributing to increased efficiency and safety. They are also employed in the energy sector for gas and steam turbines, enhancing power generation efficiency. In the chemical processing industry, superalloys like Hastelloy excel in handling corrosive chemicals. Additionally, they play vital roles in medical implants, oil and gas drilling equipment, nuclear reactors, and various high-performance engineering applications, where their unique properties enable reliable performance in extreme conditions.

Note that:

The exactly percentage of the element in the composition of superalloys varies depending on the purpose of alloys and the target application.

Iron-based superalloys are not as widely used as nickel-based or cobalt-based superalloys.

Iron-based superalloys typically contain around 40-60% iron, along with significant amounts of nickel (20-30%), and other alloying elements such as chromium (up to 25%), molybdenum, and sometimes cobalt. Small amounts of elements like aluminum, titanium, and niobium are also added for strengthening and stabilizing the alloy's microstructure.

Iron-based superalloys are generally more cheaper compared to nickel-based or cobalt-based superalloys because iron is more abundant and less expensive. these alloys are known for their wear resistance.

Iron-based superalloys are not as heat-resistant as nickel-based or cobalt-based superalloys, limiting their use in extremely high-temperature applications.

Iron-based superalloys are more susceptible to oxidation and corrosion than some other superalloy types, so they may require protective coatings in certain environments.

Cobalt-based superalloys contain a significant proportion of cobalt, typically more than 50% by weight.

The most using of Iron-based superalloys is in aircraft bearings.

Cobalt superalloys exhibit superior fatigue resistance and greater strength when compared to iron- and nickel-based superalloys when used in high-temperature applications.