Why are aircraft turbine blades made of nickel alloys? What is the advantage of using them? Why did they replace steel alloys for turbine blades?
How to overcome failure cases of turbine blades?
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2$\begingroup$ Welcome to aviation.SE! I've edited your question a little to make it clearer, but I don't know what you mean by "overcoming failure cases"? Can you explain that part? It might be better as a separate question, depending on what you're asking. You might also want to check out some existing questions that cover this topic. $\endgroup$– PondlifeCommented Apr 29, 2015 at 19:18
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$\begingroup$ The answers here do a good job of addressing the aerospace requirements. What makes nickel especially useful would be an interesting question for the Physics or Chemistry sites. $\endgroup$– foootCommented Apr 29, 2015 at 22:27
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$\begingroup$ There is a very nice video by Rolls Royce about their engines here on youtube: youtube.com/watch?v=VfomloUg2Gw $\endgroup$– PavelCommented Apr 30, 2015 at 9:12
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2 Answers
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Not all turbine blades are made of nickel alloy, some blades are made of such family of alloys but clearly not all. Some blades are actually made of ceramic materials.
The turbine is always located after combustion chamber and the temperature which the first stage of turbine blades is able to resist is a proof of high technology engine. Those blades are exposed over to 2000ºC; the melting point of steel is around 1100ºC, and nickel can go much further up to 1700ºC. Ceramic can go even further. Note that there is also a significant difference between external flow temperature and surface temperature.
Using a high melting point material is one way to solve the problem, there are also ways to reduce the effective temperature over the surface of the metal like using film cooling.
In the figure I introduced above you can see holes in the blade, where "cooling air" (maybe at 400ºC) flows out of the blade creating effectively a film that goes around the blade protecting (cooling) it from external temperature.
However a metal exposed at a high temperature (below the melting point) is not behaving in the same way as at normal temperature. There are some phenomena affecting the materials like creep. When a material is exposed during long term to high temperatures close to the melting point and high stress at the same time (this is important) it starts suffering deformations that lead to a failure of the metal.
So nickel alloys, apart for having a higher melting point are also having one of the best behaviours in creep environments.
The last stages of the turbine rotors, and more easily stators (less demanded in stress) are less exposed to creep conditions and are not needed to be manufactured in a nickel base alloy.
Just to add as well, in order to reduce creep another approach followed (combined with previous) is to solidify the metal ensuring having a single crystal metal (instead of being a polycrystalline metal). This is utmost art of material engineering (from my point of view).
answered Apr 29, 2015 at 19:42
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2$\begingroup$ Ah yes, those cool, refreshing 400°C breezes. $\endgroup$– corsiKaCommented Apr 30, 2015 at 19:47
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Their main advantage is higher strength at elevated temperatures. With elevated I mean temperatures up to 1200°C, which is what you will find in modern, film-cooled high-performance turbines where the gas temperature is even a few hundred centigrades higher than that. Steel would melt and other materials like titanium would quickly oxidize, and only nickel alloys or ceramics are up to the job. Being still too brittle for everyday use, ceramics have not managed to escape the laboratory, so nickel alloys is all we have for now.
Failures are overcome by shutting the engine down and scheduling it for repair. When a turbine blade fails, the engine will develop an imbalance that might cause secondary damage due to vibrations. A single blade loss can be tolerated for a while, especially when power setting is reduced, but should be repaired as soon as possible.
answered Apr 29, 2015 at 19:29