Superalloys are special materials which perform extremely well in harsh conditions. These special alloys give extraordinary strength and much greater resistance than ordinary metals.
In this blog post we will cover what superalloys are and their main properties. We will also see types, manufacturing processes & industrial applications of superalloys.
What are Superalloys?
Basically, superalloys are advanced metallic materials which are designed to perform reliably in harsh environments. They can withstand high mechanical pressures, temperatures above 500°C and corrosive conditions.
They are used in many applications such as chemical processing and aerospace due to their excellent thermal stability, strength and oxidation resistance.
Also See: What is an Aluminium Alloy
The Composition of Superalloys
The basic elements found in superalloys are iron, nickel and cobalt. They also contain some additional elements like aluminum, chromium as well as titanium which act as important alloying components.
Nickel based superalloys consist of 50 to 70% nickel and 5 to 20 % chromium. Within these materials aluminum content is up to 6% which provide oxidation resistance as well as better strength. Moreover tungsten, molybdenum and tantalum are also added for solid solution strengthening as well as for formation of carbides which keep the alloy stable even at high temperatures.
Types of Superalloys
Cobalt Based Superalloys
Cobalt based superalloys are very important due to their outstanding wear resistance and thermal stability properties. Their composition includes 50 to 60% cobalt and some amount of tungsten and chromium. These alloys form stable oxide layers which protect against corrosion and can withstand temperatures above 1300°C.
These alloys are well suited for low pressure turbine guide vanes as well as for components which are subjected to temperature fluctuations because of their resistance to thermal fatigue.
Nickel Based Superalloys
These superalloys are used in many industries because they can withstand high temperatures without losing their structural integrity. Mainly they are composed of nickel as well as some additional elements such as chromium, titanium and aluminum.
These alloys are particularly used in gas turbines and jet engines because their strength and corrosion resistance remain intact even at temperatures above 1000 °C.
Iron Based Superalloys
Iron based superalloys provide a cost effective solution for applications with medium temperature requirements. These alloys are largely used in aero‐engine parts and aircraft bearings which are subjected to long term stress. They deliver outstanding mechanical properties at temperatures below 750°C, even though their high temperature capabilities are inferior as compared to nickel or cobalt based superalloys.
High Entropy Alloys (HEAs)
High entropy alloys represent the latest advancement in superalloy technology. They contain equal amounts of more than five elements. They have properties like outstanding oxidation resistance, thermal stability & creep resistance at high temperatures.
Due to these properties they are used as an alternative to nickel based superalloys in aerospace industry as well as in gas turbines.
Titanium Based Superalloys
Titanium based superalloys are composed of titanium combined with elements such as molybdenum and aluminum. They are used to make strong, lightweight materials.
Due to their outstanding corrosion resistance and high strength to weight ratio, they are very important in aerospace applications particularly in landing gear. Moreover these alloys are also used in medical industry for implants due to their biocompatibility.
Main Properties of Superalloys
Oxidation Resistance
Superalloys resist oxidation by forming special protective oxide scales such as Cr2O3 and Al2O3. These scales form protective barriers which inhibit further oxidation. That is why they maintain structural integrity even at high temperatures.
High Temperature Strength
Superalloys have the ability to maintain their mechanical strength even at temperatures beyond 500°C. Precipitation hardening and solid solution strengthening methods account for this ability in which nickel forms the stable gamma prime phases.
Rust Resistance
The presence of chromium and molybdenum gives superalloys their high corrosion resistance. Chromium forms a protective Cr2O3 layer on the alloy’s surface. On the other hand molybdenum improves passivation. Due to these elements, superalloys remain protected from harsh chemical environments.
Thermal Fatigue Resistance
The distinctive microstructure of superalloys possess stable grain boundaries and coherent precipitates. These characteristics prevent the formation of cracks during cyclic thermal stresses. That is why these materials are ideal for components which undergo repeated cooling and heating.
Oxidation Resistance
Superalloys provide very extraordinary creep resistance due to presence of finely distributed precipitates and coherent particles within their metal matrix. Due to this property these alloys retain their structural integrity under high temperature and pressure for long periods of time.
Solid Solution and Precipitation Hardening
The extraordinary strength of superalloys is due to two mechanisms. The first is solid solution strengthening in which alloying elements dissolve into base metal. The other one is precipitation hardening which forms fine particles. These particles block movement of dislocations which greatly increases the strength of alloy.
5 Manufacturing Processes for Superalloys
1. Casting Techniques
The most commonly used method for making superalloys is casting. Three main techniques which dominate the industry are investment casting, directional solidification & single crystal casting.
- Investment casting uses ceramic molds created in vacuum conditions to reduce impurities.
- Directional solidification uses a slow cooling rate to control grain growth which increases creep resistance.
- Single crystal casting further refines directional solidification process in which grain boundaries are completely removed. This process improves resistance to both thermal fatigue and creep.
2. Additive Manufacturing
Additive manufacturing allows you to build complicated shapes directly from CAD models.
Selective laser melting is a type of additive manufacturing in which high energy lasers fuse metal powders layer by layer. This process can create some holes in the final product but these holes can be reduced by using post processing methods such as heat treatment or HIP.
AM helps to make hollow designs & lattice structures which cannot be made using traditional manufacturing methods.
See Also: Additive vs. Subtractive Manufacturing
3. Powder Metallurgy
Another important method for making superalloys is powder metallurgy. The process begins with atomization in which the molten metal is broken into fine powder particles. Manufacturers then combine these powders by using extrusion or hot isostatic pressing. This approach leads to more homogeneous alloys with smaller grain sizes. Manufacturers mostly use this method for the production of turban discs.
4. Heat Treatment Processes
Heat treatment processes perform an important part in improving the properties of superalloys. Its main processes are solution treatment in which precipitate is dissolved at high temperatures to make a uniform microstructure. Precipitation hardening is then used to produce strengthening particles through a controlled aging and cooling process.
To prevent oxidation manufacturers use vacuum heat treating processes. Moreover hot isostatic pressing is also used to eradicate internal pores. These methods increase thermal stability, strength and creep resistance of superalloys.
5. Thermo Mechanical Processing
Thermo mechanical processing combines heat treatment and deformation to improve the microstructures of superalloy. This process needs multiple stages of forging, compression & rolling at specific temperatures. TMP makes fine grained structures which have better mechanical properties. Manufacturers mostly apply this method to nickel based superalloys used in turbine blades.
Industrial Applications of Superalloys
Power Generation
Power plants use superalloys in heat exchangers, steam and gas turbines and boiler components. In these parts Inconel 718 and other nickel based alloys are particularly used because they resist mechanical fatigue and high temperature corrosion.
Moreover superalloys are also used in renewable energy systems such as concentrated solar power receivers & wind turbines which work at temperatures up to 500 °C.
Aerospace
The use of superalloys is very important in aerospace industry. These materials are used to make important components such as combustors, exhaust & turbine blades. These parts maintain their durability even at temperatures above 1000 °C.
Marine Applications
Superalloys are well suited for marine applications because they resist seawater fouling and corrosion. Engineers use them in subsea systems for wave and tidal energy generation. These materials are particularly used to protect those ship components which are subjected to extreme marine conditions.
Chemical Processing
Chemical factories need super alloys for pipelines, reactors as well as heat exchangers because these components are exposed to high pressures and corrosive chemicals. These alloys are used for long term operations in harsh chemical environments due to their crevice corrosion & pitting resistance properties.
Automotive Industry
In automotive industry superalloys are used in exhaust systems, turbocharger wheels & high performance engine parts. These materials retain their properties even under intense heat and pressure which leads to increased engine efficiency. Moreover superalloys are used as a lightweight solution for structural parts such as chassis components.
Considerations and Challenges
Manufactures face a lot of challenges during application and manufacturing of superalloys, even though they perform well in extreme conditions.
- Superalloys often suffer from stress cracking because they conduct heat poorly. To avoid this problem, engineers have to carefully plan heat distribution in their designs.
- Low thermal diffusivity & high hardness of superalloys lead to many obstacles during the machining process. Due to concentration of heat in cutting zone, the tools deteriorate quickly and surface finish is substandard. In order to solve these problems you have to use advanced cooling methods & special carbide tools.
- The need for heat treatments and complex casting methods have greatly increased the production cost of superalloys. However advanced production technologies like additive manufacturing and powder metallurgy prove very helpful in reducing these difficulties.
To Sum Up
In short superalloys are indispensable in modern industry due to their extraordinary properties such as high temperature, corrosion resistance & thermal stability. These materials remain important for aerospace, energy & automotive industries, even though they present manufacturing challenges.
If you need any kind of custom CNC services for superalloys then Richconn is your best option. You can contact us anytime.
Related Questions
How do superalloys withstand high stress environments & extreme temperatures?
Superalloys use three main methods to maintain their strength at high temperatures which include precipitation hardening, solid solution strengthening & refractory elements.
How do regular alloys and superalloys differ?
The main difference is that regular alloys do not have advanced properties like extraordinary corrosion resistance, high temperature strength & outstanding thermal stability which superalloys possess.
How do superalloys improve the performance of aerospace components & jet engines?
Superalloys increase engine efficiency in three ways – they minimize fuel consumption, withstand high temperatures and increase thrust to weight ratios.
What is the reason for using nickel based superalloys in turbine engines?
Nickel based superalloys are well suited for turbine engine applications due to their properties like thermal stability, high strength and corrosion resistance.
What is the role of cobalt in the properties of cobalt based superalloys?
Cobalt gives three main benefits which are—improved corrosion resistance, better thermal stability & higher creep resistance. Due to these properties these alloys show reliable performance under intense mechanical stress and heat.
