Materials > Superalloys
Check out our science articles about superalloys.
Starting from the most basic ingredients we have pure metals, then alloys, and then they somehow become superalloys. Pure metals are where there is only one type of atom, usually one from the middle bit of the periodic table (being a metal has got to do with how many electrons they have and things like being shiny, malleable, ductile and fusible). Once you have more than one type atom, then a material becomes an alloy. The reason for using alloys is simply because all pure metals have strengths and weaknesses, so if you add another element you can balance out the weaknesses and sometimes improve the properties you want (including the price). One of the most well known alloys is steel, which is basically iron (Fe) and a bit of carbon (C). By changing the ratio of these two elements, we can change how the atoms arrange themselves aka ‘the crystal structure’ resulting in materials with vastly different properties. If we then add a few more elements like chromium (Cr) we come up with other variations such as stainless steel. Indeed, a lot of modern alloys have lots and lots of different elements in precise ratios.
So what makes an alloy super?
Alloys mean we can enhance the properties we want, but what happens when you start needing materials that perform at really high temperatures and stresses? Simple, you need new alloys with super powers i.e. superalloys. Superalloys by definition are a range of alloys designed specifically for high temperature conditions like what turbines experience in a jet engine.
Superalloys are specifically designed to have high strength at high temperatures, have good creep properties, and be resistant to corrosion and oxidation. To do this superalloys go one step further than alloys. Alloys are a combination of elements in a crystal structure or atomic arrangement which is often complex but still repeated regularly over and over again. Superalloys use the power of different phases. This means instead of having one atomic arrangement repeated throughout the material, superalloys have multiple arrangements known as phases. Think of it like ice cubes in a glass of water, they are both made of H2O but because of how their atoms are arranged one is a liquid and the other one a solid. Two phases of the same material with different properties. By having more than one phase, superalloys can have the best of both worlds. If one phase is ductile or stretchy and the other one is stiff and strong, then superalloys can be made with the best combination of both.
And how do different phases make it super?
What happens at high temperatures is that atoms start jiggling about and so it becomes easier for them to break their inter-atomic bonds, or the ties that keep them in place. Once an atom breaks its bonds or rips a hole in the crystal structure, it moves about or propagates through the material. If these holes start to join together the material starts to tear apart. Superalloys use their different phases to stop these holes from forming and joining together. There are two ways superalloys do this, solid solution strengthening and precipitation hardening.
A solid solution is where other atoms become part of the crystal structure and start taking up positions in the atomic arrangement, but when they do the material ‘hardens’ and we get solid solution hardening. What happens is that when the new atoms join the material their bonds cause the other atoms to arrange themselves in specific ways. This is deliberate because we can make atoms arrange themselves in ways that make it really hard for holes (dislocations) to form, move about and join together. Long range order, short range order and increasing friction stress on dislocations, are how scientists describe the specific types of arrangements they try to trigger by solid solution hardening.
The other type superalloy uses precipitation hardening. Precipitates are essentially little bits of a different phases that grow in a material and are bonded together by ordered atomic bonds (not the like the random bonds you see at a grain boundary). Precipitates don’t have to have the same crystal structure or chemistry as the larger material, but do directly bond to the atoms and often grow when heated.
In general superalloys, phases and precipitates can be thought of like sailing a boat in the Arctic. If the boat is a hole in a material, the sea is the original material and once the engine is going, sailing is pretty easy. Once you get to the ice (i.e. another phase or precipitate) you need a lot more energy to break through it. Or, if there is an iceberg you can’t sail through it and so it takes time and energy to sail around it. This is how superalloys prevent, trap and divert the little holes in a material, thus keeping it in tact for hotter and longer.
What are superalloys made of?
Currently there are only a few types of superlloys as it takes a very special combination of elements and chemistry for a superalloy to form. The most common are nickel-based, cobalt-based, and iron-based superalloys. All these come in both solid solution strengthening and precipitation hardening forms. Here is a link to an article that talks about superalloys a bit more and lists a heap of superalloys and what they are made of.
So the ISM researches superalloys?
Superalloys are used more and more in jet engines, especially for the hot bits. At the ISM we take new superalloys and mechanically test them, or simply break them in very controlled ways. Before we use a material in a jet engine we need to know when and how it will break, so we make sure it doesn’t break when you are on the plane. We do a bunch of tests that tell us basic things about the material like tensile tests, hardness, etc. We then do more complicated tests with different stresses and temperatures which try and copy what the material would experience in the engine.
Once we break a bit of material we have some information, but by doing analysis we can get more. Testing tells us how hot and stressed a material can get in a certain situation. The destroyed bit of material can tell us how it broke i.e. what is going on on a very small scale. Earlier we talked about how little holes form, join together and eventually the material breaks. By looking at the broken bits of material we can use science to figure out how these holes happen and how they behave. Once we know this, we then get our ‘game on’ and start trying to control these holes by adding elements, heating it, hitting it, etc. This is where we optimise the material to get the best out of it.
In the past the ISM has focused mainly on precipitate hardened nickel based superalloys for use in turbines. Some of the materials we have worked are Inconel 100 & 718, Nimonic 80A & 105, and Udimet 720 to name a few. The more modern nickel superalloys we investigate are RR1000, and in single crystals CMSX-4 and RR3010. Although we have previously focused on nickel superalloys, we are developing new iron based superalloys to build jet engine shafts out of too.
Institute of Structural Materials 