Alloys whose basic structure consists of iron, cobalt (Co) or nickel (Ni) and contain a high percentage of chromium (Cr) and a low content of tungsten (W), molybdenum (Mo), aluminum (Al) and titanium (Ti) are called superalloys.
Properties of Super Alloys
Super alloys have a high resistance under high temperatures, that is, they have a high thermal resistance. Super alloys have high oxidation and corrosion resistance at high temperatures, have superior rupture and friction resistance. 15%-20% of superalloys are materials with superior corrosion resistance and high amounts of chromium.
History of Superalloys The first examples of superalloys
are modified versions of austenitic stainless steels, and many of the superalloys we use today were developed between 1950 and 1970.By the 1980s, advances in technology allowed superalloys to combine some elements with superalloys. These alloys, which can be added with different elements, have acquired many specific mechanical properties that are demanded today, such as high temperature resistance.
Where are Super Alloys Used?
Superalloys, which were developed for use on gas turbines in the 1900s, are used in many areas of industry today. These alloys are generally used in the following areas:
Aircraft and Gas turbines
Thermal power plants
Steam power units
Ferrous Based Super Alloys
Iron-based superalloys contain between 15% and 60% iron (Fe), 25% to 45% nickel (Ni), and 15% to 28% chromium (Cr) to increase oxidation resistance under high temperature. In addition, 1% to 6% mobidlen (Mo) can be added to the alloy for solid solution strength. Different elements such as cobalt (Co), boron (B), zirconium (Zr) and carbon (C) can be added to iron-based superalloys.
Ferrous superalloys are resistant to temperatures up to 650 °C. Of course, since the durability of these alloys is lower than nickel and cobalt based superalloys, they are not used in applications with high mechanical and thermal processes.
Because iron-based superalloys are cheaper than other alloys, they are used in discs, shafts, gas turbine engines and some parts in steam turbines.
Nickel Based Super Alloys
Since the melting temperature of nickel is 1453 °C, alloys created with this element are used in high temperature applications. Nickel-based superalloys consist of 50-70% nickel (Ni) element. Cobalt (Co), zirconium (Zr), iron (Fe), molybdenum (Mo), niobium (Nb) and boron (Bo) elements were added to these alloys in order to increase their efficiency under conditions of use.
These alloys are used in applications in the temperature range of 815 °C to 900 °C. These alloys are used in aircraft and gas turbines, rocket engines, spacecraft, thermal power plants, nuclear reactors and other different high-temperature applications such as these.
The production of nickel-based superalloys is carried out by powder metallurgy and single crystal casting method.
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.
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.
Cobalt Based Super Alloys
The element cobalt is very similar to nickel in terms of physical properties so that the melting temperature of cobalt is 1495 °C. Cobalt-based superalloys consist of 50% to 60% cobalt (Co), chromium (Cr) between 20% and 30%, Tungsten (W) between 5% and 10% and carbon (C) between 0.1% and 1%.
These alloys are used in static parts that must have a long working life at high temperatures and low stresses. Due to the high melting temperature of cobalt-based superalloys, it has a higher strength in high temperature applications than iron and nickel based superalloys.
These alloys, which also have the ability to be welded, also have high hot corrosion resistance due to the high amount of chromium in their composition. Therefore, they are especially suitable materials for the parts of gas turbines that are open to the atmosphere. They have a higher thermal fatigue resistance than nickel-based superalloys.
Today, the development of super alloys, which are of vital importance for many industries, is still continuing, and their efficiency and usage areas are increased with many R & D studies.