Introduction: Incoloy 903, Incoloy 907 and Incoloy 909 are three low-expansion superalloys with excellent high-temperature strength and low expansion properties. This article will detail the origins, similarities and differences between these three alloys and discuss their applications in engineering.
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At the end of the 19th century, it was found that when the nickel content of a nickel-iron alloy was about 36 per cent, the alloy had a very low coefficient of linear expansion. Based on this property, a series of alloys with low linear expansion coefficients were developed. Among them, Invar alloy is the most famous one, which plays an irreplaceable role in some specific applications.
However, Invar alloy has significant limitations in its applications. Its low expansion characteristics exist only in the temperature range below 200°C. The temperature range of Invar alloys is not very high. When the temperature rises above 200°C, the material reaches the Curie temperature and the magnetic properties of the alloy are transformed, thus losing its low expansion properties.
In order to solve the above problems, cobalt has been added to nickel-iron alloys. The addition of cobalt increases the Curie temperature of the alloy, thus reducing the high-temperature linear expansion coefficient of the alloy. This nickel-iron cobalt alloy is called Kovar alloy and has low expansion characteristics in the range of 20~450°C.
Incoloy 903
In the 1970s, international oil prices rose dramatically, and people began to develop more efficient engines. The spacing between engine components has a large impact on the operating efficiency of the engine. Control of component spacing can be achieved if the component materials are low expansion alloys.
Although Kovar alloys can achieve low expansion properties at high temperatures, the alloys are not very strong. In engines, components are subjected to cyclic stresses at high temperatures for long periods of time. And Kovar alloys are clearly not up to the task. Therefore, it became critical to increase the strength of the low expansion alloy.
Precipitation strengthening is an effective method to increase the strength of alloys. Therefore, attempts have been made to add aluminium, titanium, niobium and tantalum to nickel-iron cobalt alloys. After aging treatment, the alloy reaches a precipitation strengthened state. This greatly increased the strength of low expansion alloys at elevated temperatures.Incoloy 903 was developed against this background. It is known as a first-generation low-expansion superalloy. the high-temperature strength of Incoloy 903 can reach the level of Inconel 718, while the coefficient of linear expansion is only half of Inconel 718.
Incoloy 903 has excellent high-temperature strength and a low coefficient of linear expansion. However, due to the absence of chromium, Incoloy 903 has poor oxidation resistance. And oxidation reactions can reduce the material’s durability and increase notch sensitivity. To avoid this problem, Incoloy 903 must be specially processed to reduce the chance of oxidation diffusion. However, it still doesn’t fully solve the durability performance problem under long-term high-temperature use.
Alloy researchers are constantly looking for ways to solve this problem. In the meantime, they developed Incoloy 904, Incoloy 905 and Incoloy 906 alloys. Eventually, Incoloy 907 was born. It solved the durability performance problems of Incoloy 903.
It was found that the aluminium content adversely affected the durability properties of Incoloy 903 alloy. As a result, Incoloy 907 alloy has a strictly controlled aluminium content (less than 0.2%). Aluminium also plays a role in precipitation strengthening. To balance the reduced aluminium content, Incoloy 907 is strengthened by the addition of more niobium than aluminium. Incoloy 907 is also known as a second-generation low-expansion superalloy.
By optimising the chemical composition, Incoloy 907 no longer requires special treatment like Incoloy 903. However, in order to ensure durability, it requires a longer aging treatment than normal precipitation strengthened alloys. This over-aging treatment reduces the transient properties of the alloy.
Shortly after the introduction of Incoloy 907, researchers stumbled upon the discovery that increasing the silicon content could further improve the durability of the low-expansion alloy. This became the key to solving the problem. Taking advantage of this property, researchers added more silicon to Incoloy 907 and better controlled the aluminium content to develop a third-generation low-expansion superalloy: Incoloy 909.
Incoloy 909 controls the silicon content to approximately 0.4% and the aluminium content to less than 0.15%. This maximises the durability properties of Incoloy 909. At the same time, by controlling grain size, Incoloy 909 achieves better overall performance.
Post time: Jul-22-2023
