Incoloy 903, Incoloy 907 and Incoloy 909 are all low expansion superalloys. They have low coefficient of linear expansion. At the same time, they have high high-temperature strength.

This article will fully explain the origin of these three alloys and their similarities and differences.

At the end of the 19th century, studies found that when the nickel content in iron-nickel alloys was about 36%, the linear expansion coefficient of the alloy was very low. Based on this characteristic, a series of alloys with low linear expansion coefficients have been developed. Among them, Invar alloy is the most famous. It plays an irreplaceable role in some specific applications.

However, Invar alloys have significant limitations in application. Its low expansion properties only exist below 200°C. When the temperature rises above 200°C, the material reaches the Curie temperature, and the magnetic properties of the alloy are transformed, thereby losing the low expansion characteristic.

In order to solve the above problems, people added cobalt to nickel-iron alloys. The addition of cobalt makes the Curie temperature of the alloy higher, thereby reducing the high temperature linear expansion coefficient of the alloy. This iron-nickel-cobalt alloy is called Kovar alloy. It has low expansion properties in the range of 20 ~ 450°C.

In the 1970s, international oil prices rose sharply. People began to develop more efficient engines. The spacing between engine parts has a relatively large impact on the operating efficiency of the engine. If the part material uses a low expansion alloy, the control of the part spacing becomes possible.

Although Kovar alloys can achieve low expansion properties at high temperatures, the strength of this alloy is not high. In an engine, parts are subject to cyclic stress at high temperatures for extended periods of time. And the Kovar alloy is clearly not up to the job. Therefore, increasing the strength of low-expansion alloys becomes the key.

Precipitation strengthening is an effective method to increase the strength of alloys. Therefore, people try to add aluminum, titanium, niobium and tantalum to the iron-nickel-cobalt alloy. After aging treatment, the alloy reaches a state of precipitation strengthening. This greatly increases the strength of the low expansion alloy at high temperatures. Incoloy 903 was developed in this context. It is called the first generation low expansion superalloy. The high temperature strength of Incoloy 903 can reach the level of Inconel 718, and the coefficient of linear expansion is only half of that of Inconel 718.

Incoloy 903 has excellent high temperature strength and low coefficient of linear expansion. But since it does not contain chromium, Incoloy 903 has poor oxidation resistance. Oxidation, on the other hand, reduces the durable properties of the material and increases notch sensitivity. To avoid this, Incoloy 903 had to be produced using special processing techniques to reduce the chance of oxidation diffusion. However, under long-term high temperature use, it still cannot completely solve the problem of durable performance.

Alloy researchers are constantly looking for a solution to this problem. In the meantime, they have successively developed Incoloy 904, Incoloy 905 and Incoloy 906 alloys. Eventually, the Incoloy 907 came out. It solves the durable performance issues with Incoloy 903.

The study found that aluminum content has a detrimental effect on the durable properties of Incoloy 903 alloy. Therefore, Incoloy 907 alloy strictly controls the aluminum content (below 0.2%). Aluminum also plays a role in precipitation strengthening. To balance the reduction in aluminum content, Incoloy 907 is strengthened by adding more niobium instead of aluminum. Incoloy 907 is also known as the second generation low expansion superalloy.

With optimized chemical composition, Incoloy 907 no longer requires special processing like Incoloy 903. However, it requires a longer aging treatment time than ordinary precipitation strengthened alloys to ensure durable performance. This overaging treatment reduces the transient properties of the alloy.

Shortly after the Incoloy 907 alloy was introduced, the researchers stumbled upon the fact that increasing the silicon content could further improve the durable properties of the low expansion alloy. This became the key to solving the problem. The researchers used this feature to add more silicon content and better control of the aluminum content to Incoloy 907, and developed a third-generation low-expansion superalloy: Incoloy 909.

Incoloy 909 controls the silicon content to around 0.4% and the aluminum content to less than 0.15%. This maximizes the durable performance of Incoloy 909. At the same time, by controlling the grain size, Incoloy 909 obtains better comprehensive properties.

Incoloy 903 has excellent high temperature strength and low coefficient of linear expansion. It is specially machined to meet the needs of controlling the spacing of parts in the engine.

Incoloy 907 has better control of aluminum content and added more niobium, which solves the problem of insufficient durability of Incoloy 903.

Incoloy 909 adds more silicon on the basis of Incoloy 907, which improves the overall performance of the alloy.

Incoloy 903, Incoloy 907 and Incoloy 909 are all low expansion superalloys. They have excellent high temperature strength and low expansion properties.

Incoloy 907 improves the durability of Incoloy 903. Incoloy 909 improves the overall performance of Incoloy 907.

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