What is Nickel Base Superalloy
Nickel-based cast superalloys refer to cast superalloys with nickel as the main component. With the increase of service temperature and strength, the degree of alloying of superalloys (see superalloy strengthening) is getting higher and higher, and hot forming is more and more difficult, and casting processes must be used for production. In addition, the internal complex cavity of the hollow blade using cooling technology can only be produced by the precision casting process. In this way, the nickel-based wrought superalloy is transformed into a nickel-based cast superalloy.
Elements That Can Be Added
Nickel-based casting superalloys take γ phase as the matrix, and add aluminum, titanium, niobium, tantalum, etc. to form γ' phase (see intermetallic compound phase of superalloy materials) for strengthening. The number of γ' phases is large, and some alloys are as high as 60 %;
Adding cobalt can increase the dissolution temperature of the γ' phase, thereby increasing the service temperature of the alloy;
Molybdenum, tungsten, and chromium have the effect of strengthening solid solutions, and chromium, molybdenum, and tantalum can also form a series of carbides that strengthen grain boundaries; aluminum and chromium contribute to oxidation resistance, but chromium reduces the dissolution temperature of γ' phase and high temperature strength, so the chromium content should be lower;
The addition of hafnium improves the medium temperature plasticity and strength of the alloy; in order to strengthen the grain boundary, an appropriate amount of elements such as boron and zirconium are added.
The fuels used in industrial gas turbines contain high levels of sulfur and vanadium, which will cause serious thermal corrosion to the alloys during long-term operation. If gas turbines are used on ships, the sodium salts in the marine atmosphere will accelerate sulfidation corrosion.
Therefore, by appropriately increasing the chromium content, the ratio of titanium and aluminum, and the content of refractory metal elements, a series of alloys that can maintain high creep strength and improve heat-resistant corrosion resistance have been developed, such as IN738, IN792, Rene'80, MarM432 and Chinese K4537 etc.
Properties of Nickel-based Superalloys
Some of the most common properties of nickel-based superalloys include:
Exceptional strength
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.
Corrosion resistance
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.
Machinability
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.
Applications of Nickel-based Superalloys
Nickel-based superalloys are commonly used in the manufacture and production of the following fields.
Engine parts
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 chambers
Combustion vessels are expected to experience extremely high temperatures, and this is where Ni-based alloys are best used.
Steam turbines
Similar to combustion equipment, steam turbine power plants require temperature-resistant materials such as nickel-based superalloys to keep the system preserved and intact.
Nuclear applications
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.
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