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What is a superalloy? Where is it used? What are its features?

Prehistoric ages were named as copper, bronze and iron ages according to the metals of the period. If we had to give such a name to our day, I think the "super alloy era" would be the most accurate one.

Compounds whose main components are nickel, cobalt or iron, containing high amounts of chromium and low amounts of wolfram (tungsten), molybdenum, aluminum and titanium are called superalloys.

Although the history of these alloys dates back to the early 1900s, the term superalloy was first used to describe high-performance materials used in turbo-supercharged engines just after the Second World War. Following the invention of austenitic stainless steel between 1910 and 1915, metallurgists experimented with many new alloys until the 1930s and found these materials sought after for aviation engines. Its developments still continue today.

SpaceX , the company that took an astronaut to the International Space Station (ISS) for the first time on May 31, 2020, owes this reusable rocket engine project mainly to this type of new material technologies. Elon Musk, the owner of the company, even tweeted about it in 2018.

The most distinctive feature of super alloys, or high performance alloys, is their good mechanical properties at high temperatures. Their high temperature creep, oxidation, corrosion and wear resistance are very good compared to other metals. The material group closest to the high temperature oxidation and corrosion resistance of these alloys is austenitic stainless, but their mechanical properties cannot meet expectations.

The crystal arrangements are mostly face-centered cubic, austenite. They can also be produced as single crystals.

Contrary to the fact that steels are referred to by their standard numbers in the market, these alloys stand out with their trade names. For example, almost everyone knows 2.4816, a nickel-based superalloy, by its trade name Inconel 600 ® .

Who uses these materials and where?

· In hot parts of aviation engines

· In nuclear reactors

· In submarines and ships

· In high temperature furnaces

They are divided into 3 classes: nickel, cobalt and iron based.

1. Nickel-based superalloys

They may contain 50-70% nickel and alloying elements such as carbon, chromium, molybdenum, niobium, iron, titanium, aluminum, vanadium and tantalum. They can operate for long periods of time at temperatures such as 950–1200 °C in rocket engines, gas turbines, aircraft engines and nuclear reactors.

For the sake of familiarity, I will share a few trade names and standard numbers here.

Inconel 718® ( UNS N07718 / 2.4668)

Monel 400® ( UNS N04400 / 2.4360)

Nimonic 901® ( UNS N09901 / 2.4662)

2. Cobalt-based superalloys

They contain 50-60% cobalt, 20-30% chromium, 5-10% Tungsten (Wolfram) and 0.1-1% carbon.

They have similar operating temperatures and areas to nickel-based superalloys. Thermal fatigue resistance of some special alloys is better than nickel and iron-based ones.

Example, UDIMET® R41 (UNS N07041)

3. Iron-based superalloys

They contain 15-60% iron, 25-45% nickel (Ni), and 15-28% chromium to increase oxidation resistance under high temperatures. Molybdenum, cobalt, boron, zirconium and carbon are among the alloying elements.

Maximum operating temperatures are 650 °C. They are widely used in low temperature applications because they are significantly cheaper due to their low nickel and cobalt content.

Example, NILO ® 42 (UNS K94100 / 1.3917)

All superalloys must be heat treated to meet the required high temperature mechanical properties.

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