Overview

Cobalt-based superalloys are materials used in high temperature and extreme environments with excellent high temperature strength, corrosion resistance and wear resistance. Cobalt-based superalloys are mainly composed of elements such as cobalt, chromium, tungsten, nickel and aluminum.

Cobalt-based superalloys are widely used in aviation, aerospace, energy, medical and other fields. In aviation and aerospace, cobalt-based superalloys are used in the manufacture of components such as turbine engines and gas turbines. In the energy field, cobalt-based superalloys are used to make turbine blades and other components in thermal and nuclear power plants. In the medical field, cobalt-based superalloys are also used to manufacture medical devices such as artificial bones and artificial joints.

In this article, we will introduce the performance advantages of cobalt-based superalloys by comparing other superalloys.

Composition of Cobalt-based Superalloys

Cobalt

In cobalt-based superalloys, cobalt is the most important matrix element, which brings good high-temperature performance, welding performance and other advantages to the alloy. The cobalt content in cobalt-based superalloys is generally 35% to 70%.

Nickel

Due to the different crystal structures of metal cobalt at different temperatures, it is often necessary to add 5% to 25% nickel to the cobalt-based superalloy to stabilize the austenite structure of the alloy. At the same time, the nickel element also brings a certain corrosion resistance to the alloy.

Iron

In addition, some cobalt-based superalloys also use iron as an element to stabilize the austenite structure of the alloy. The content is generally 9% ~ 20%. However, when the iron content is too high, the alloy may precipitate harmful phases.

Chromium

Like other superalloys, cobalt-based superalloys also need to add about 20% chromium to improve oxidation resistance. In addition, chromium can also form carbides with carbon in the alloy to play a role in precipitation strengthening.

Therefore, most cobalt-based superalloys are cobalt-nickel-chromium alloys.

Tungsten and Molybdenum

We mentioned before that other superalloys use more molybdenum for solid solution strengthening. In cobalt-based superalloys, tungsten is the most important solid solution strengthening element. The content of tungsten is about 7% ~ 15%. However, some early developed cobalt-based superalloys still use molybdenum for solid solution strengthening.

Niobium and Tantalum

In addition, in some relatively new cobalt-based superalloys, niobium and tantalum will also replace tungsten to solid solution strengthen the alloy. They also have the effect of increasing the antioxidant capacity.

Carbon

Cobalt-based superalloys have a higher carbon content than other superalloys. The carbon content is usually in the range of 0.25% to 1%. Carbon can play a very good role in carbide precipitation strengthening in cobalt-based superalloys.

Aluminum and Tungsten

In iron-based superalloys and nickel-based superalloys, aluminum and titanium are good precipitation strengthening elements. They combine with nickel to form the γ' phase, which greatly improves the strength of the alloy. But in cobalt-based superalloys, the effect of this method is not obvious. Cobalt-based superalloys often require aluminum and tungsten for γ’ phase precipitation strengthening.

Characteristics of Cobalt-based Superalloys

High Melting Point

The initial melting temperature of cobalt-based superalloys can generally reach above 1300 °C. While the initial melting temperature of most nickel-based superalloys is less than 1280°C. This is mainly because a large amount of refractory metal is often added to cobalt-based superalloys for solid solution strengthening. The high melting point makes cobalt-based superalloys widely used to make guide vanes for aircraft engines. It avoids faults such as burning or deformation of the guide vane at high temperature.

Good Corrosion Resistance

Cobalt-based superalloys tend to have a higher chromium content than other superalloys, which gives them better high-temperature corrosion resistance. Cobalt-based superalloys can form a stable oxide film at high temperatures. It can maintain the strength and stability of its material under the dual action of high temperature and corrosive medium. This characteristic of the cobalt-based superalloy also allows it to work stably in engine blades.

Good High Temperature Durability

Below 900°C, the durability of nickel-based superalloys is better than that of cobalt-based superalloys. However, when the temperature rises above 900°C, the durability of nickel-based superalloys drops sharply. At this time, cobalt-based superalloys have obvious advantages in durability. This is why cobalt-based superalloys are suitable for high-temperature guide vanes.

Good Welding Performance

The welding performance of cobalt-based superalloys is also better than that of nickel-based superalloys. It is suitable for various welding processes. In addition, due to the high melting point of cobalt-based superalloys, appropriate parameters need to be selected during welding.

Conclusion

Cobalt-based superalloys are superalloys based on cobalt. Cobalt-based superalloys often need to add a certain amount of nickel and chromium to stabilize their properties. Unlike nickel-based superalloys, cobalt-based superalloys use more tungsten for solid solution strengthening and precipitation strengthening. Compared with other superalloys, cobalt-based superalloys have a higher melting point, better high-temperature corrosion resistance and durability, and better welding performance.

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