Another important factor is how the material behaves when it starts to wear and fatigue. A material that is fractured in a brittle manner with a sharp cross section like glass there is no built in warning; it breaks quickly, resulting in a costly breakdown. On the other hand if a material stretches, deforms plastically, performance is affected but the fault can often be remedied without a disaster.
Creating twinsThe materials researchers are now studying a nickel-based alloy, CMSX-4, which is called a Superalloy because of its extraordinary characteristics. It is relatively brittle at room temperature but becomes extremely strong at high temperatures – it withstands high load and does not corrode. Also it can be cast so that an entire turbine blade consists of a single crystal, which means there are not any grain boundaries where cracks tend to appear when the material is loaded. However Superalloys do fail, and until now researchers have not understood the mechanisms behind.
Almost by chance, Sten Johansson, professor of Engineering materials at Linköping University’s Department of Management and Engineering, discovered that an existing technology – the scanning electron microscope – provided excellent opportunities for looking inside of the material. By studying small test bars subjected to high heat and repeated load until they failed, the researchers were able to get inside the Superalloy and find the solution to the mystery.
They could demonstrate that when the Superalloy is loaded repeatedly, eventually parts of the structure are changed. This appears in the electron microscope as light bands. If the load continues, a mirror image – a twin – of the molecular structure is created along the bands. Where two or more twins cross, new crystals form. The material becomes polycrystalline, which means it quickly weakens and eventually fractures abruptly.
ChannelingThis technology, which is used to get a clear image of how the material is transformed below the surface, is called channeling, or more correctly, electron channeling contrast imaging. In brief it works like this: high energy electrons are sent into the surface of the material. Depending on how each electron travels inside the crystalline structure, they either pass straight through, in the channels formed by the atomic plane inside the material, or they collide with an atom and bounce back at some angle. Using the contrast from the bouncing electrons, the researchers can create images of the material’s inside.
So by comparing images of the intact material with images from material that is starting to fatigue and finally fail, Prof Johansson and his group can demonstrate exactly what takes place inside the material, and during operation.
Also, using electron electron backscattering diffraction, they can study fracture surfaces and see how the crystals are oriented.
“When you don’t know what is happening inside the material, you have to measure how the material reacts to external loads and temperature variations. But if you know what is happening, at the atomic level, you can work in a more focused way to improve its qualities,” Prof Johansson says.
The next step was to heat a test bar of the material to 1000 degrees Celsius for more than 500 hours, and then to subject it to the same load as the previous test bar. They found that when the material was fatigued this time it did not break sharply; it deformed plastically, a behaviour the researchers could explain using the information provided by the channeling technology.
“We found that the deformation mechanisms that in normal cases apply don’t always describe what happens in plastic deformation of a Superalloy. Rather there are other types of mechanisms based on the formation of twins and a stress-induced atomic diffusion – where the atoms diffuse somewhat randomly in the material, that is, superrafting,” explains Prof Johansson. (picture below)
The results were published in Acta Materiala 2009 and other journals.
Important knowledgeKnowledge of these phenomena is increasingly important, because we need to be able to predict fatigue and damage to materials using modeling and computer simulation.
Sten Johansson and his group collaborate with companies such as Siemens, Sapa, Gränges and Scania, all of which have a keen interest in materials development. It is not simply about studies of nickel-based alloys, it is also about stress in cast iron, surface coatings that make materials more durable or studies of other high-temperature alloys.
“We have a solid reputation and our research has strong links to the requirements of industry. But while industry is often satisfied to go halfway, we go all the way, so we can really understand the mechanisms at play. Then we can get back to them with new knowledge that they can really benefit from. It’s an honor to work in such a talented group, where we train highly skilled engineers who have so much to contribute when they enter the workforce,” he explains.
Monica Westman Svenselius 2013-11-28
Sten Johansson, professor of engineering materials
Foto: Monica Westman
Prof Johansson researches the characterization and development of materials and processes with a distinct industrial relevance, such as lightweight materials and alloys, composites and various compounds. Other research fields include high-temperature materials, ceramic and metal coatings. His group also studies casting processes, heat treatment and high-speed machining of materials.
His group is constantly growing, currently numbering fifteen, including three senior researchers and eight doctoral students.