Nickel-based superalloys belong to a special class of high-performance alloys characterised by a significant amount of nickel. Differences between nickel-based superalloys are usually evident in the material composition, which are customised to produce specific properties depending on the intended use.
The uniqueness of superalloys lies in the design features, most notably mechanical strength, durability, and resistance to several factors that may otherwise damage conventional materials.
Properties of nickel-based superalloys
Superalloys possess a face-centred cubic structure  that interlink the pre-identified metals that are designed to be included in the material. The combination of metallic elements depends on the properties that need to emerge from the alloy.
Some of the most common properties of nickel-based superalloys include:
Ni-based superalloy products such as Inconel 625 exhibit a tensile strength of 690 MPa and a yield strength of 275 MPa. Meanwhile, some other products can reach up to 1125 MPa in yield strength. These figures are significantly higher than those of diamond or titanium.
High heat resistance
What’s unique about Ni-based alloys is their resistance to high temperatures. While some materials experience a decrease in mechanical strength when exposed to high heat , superalloys have an intrinsic thermal protection that keeps the material integrity intact.
Some nickel-based superalloys can withstand temperatures beyond 1200°C, depending on the composition of the alloy.
When exposed to oxidation, superalloys of this kind naturally create a protective oxide phase layer on the surface of the alloy. As a result, the material is able to resist oxidative corrosion . This is the reason why some nickel superalloys are also used in industries where the equipment needs to be submerged in seawater.
The weldability of nickel-based superalloys is significantly better than that of other metals and alloys. This may be attributed to the material’s elevated creep resistance and high yield strength.
Examples of nickel-based superalloys
Here are some commonly used nickel-based superalloys available in the market:
- Conventional casting: Rene 80, In-713LC
- Single-crystal alloys: Rene N5, CMSX-10, TMS 138, TMS 173
- Wrought superalloys: Inconel 718, Nimonic 80A
- Powder-processed superalloys: Rene 95, Inconel 100, N18
Inconel is a popular group of Ni-Cr superalloys known for resistance to oxidation and corrosion, as well as high strength even when exposed to extreme temperatures. Some products included in this family of superalloys include Inconel 600, Inconel 625, Inconel 690, and Inconel X-750.
Production and processing
Composition of the superalloy will largely depend on the target use of the material. Nickel superalloys may contain as little as 2 other metal components, or up to as high as 10 alloying elements comprising close to half of the entire alloy weight.
When fused with solutes such as titanium or aluminium, an equilibrium microstructure composed of gamma and gamma-prime is formed. The latter provides the superalloy with its characteristic strength in the presence of elevated temperature .
Superalloys may be manufactured in different forms such as powder, wrought, or cast, and it’s the same condition with nickel-based superalloys .
Ni-based cast superalloys absorb material strength through either solid solution treatment or precipitation strengthening. Common elements that provide solid solution strengthening include chromium, cobalt, iron, molybdenum, niobium, tantalum, and tungsten.
Heat treatment fortifies the intrinsic strength of the superalloy. This process may be carried out through high solution annealing, low solution annealing, and stress relief.
Applications of nickel-based superalloys
Owing to its excellent heat resistance, nickel-based superalloys are perfect components for high-speed and high-friction applications that generate heat. The material has found its way in the manufacturing of engine parts and accessories for aircraft and aerospace equipment.
Combustion vessels are expected to experience extremely high temperatures, and this is where Ni-based alloys are best used.
Similar to combustion equipment, steam turbine power plants require temperature-resistant materials such as nickel-based superalloys to keep the system preserved and intact.
With the combination of corrosion resistance and material strength in the face of extreme heat, this kind of superalloy works well in the nuclear industry. Some nuclear plants use nickel-based superalloys for the reactor core, control rod, and similar parts.