Atomistic models can shed light on the strengthening mechanisms of dislocations in metal alloys (panel a). Based on easily accessible inputs (composition, lattice parameters, elastic constants), the new analytical model can efficiently screen millions of alloys (panel b). This screening can generate predictions of the high-temperature yield strength of millions of high-entropy alloys (panel c). Image: Francesco Maresca/University of Groningen.Together with an international team of colleagues, Francesco Maresca, an engineer at the University of Groningen in the Netherlands, has developed a theoretical model that can rapidly determine the strength of millions of different alloys at high temperatures. Some of the predictions of this model have already been confirmed by experiments. Maresca and his colleagues describe the model and its predictions in a paper in Nature Communications.
The discovery that iron becomes much stronger with the addition of a little bit of carbon was one of the discoveries that heralded the industrial revolution. "Tweaking the composition of a base metal by adding different elements, thus creating an alloy, has been important in human history," says Maresca, assistant professor in the Engineering and Technology institute Groningen (ENTEG) at the University of Groningen.
As a civil engineer, Maresca likes large structures such as bridges. But he is now studying metals at the atomic scale to find the best alloys for specific applications. He is particularly interested in high-entropy alloys (HEAs), which were first proposed some 20 years ago. These are complex alloys with five or more elements that can have all kinds of useful properties. But how to find the best one?
"There are around 40 metallic elements that are not radioactive or toxic and are therefore suitable for use in alloys. This gives us roughly 1078 different compositions," he explains. This makes it impractical to test more than a tiny fraction of these by actually synthesizing them.
This is why Maresca wanted to develop a good theory to describe important properties of HEAs. One of those properties is high-temperature strength, which is essential for various applications ranging from turbine engines to nuclear power plants.
The strength of an alloy depends largely on defects in its crystal structure. "Perfect crystals are the strongest, but these do not exist in real-life materials," says Maresca. A major determinant of strength at high temperatures in body-centred cubic alloys is thought to be a screw dislocation, a dislocation in the lattice structure of a crystal in which the atoms are rearranged into a helical pattern. "These dislocations are very hard to model at the atomic scale."
Another type of defect is an edge dislocation, where an extra atomic plane is inserted into part of the crystal structure. "It was believed that these dislocations have no effect on strength at high temperatures, because that was shown experimentally in pure metals. However, we found that they can determine strength in complex alloys."
Edge dislocations are much easier to model and so Maresca created an atomic-scale model for this dislocation in HEAs. He then translated this model into a MATLAB script that could predict the engineering-scale strength of millions of different alloys at high temperatures in a matter of minutes.
The result is a strength-versus-temperature relationship for these different alloys. "Using our results, you can find which compositions will give you a specific strength at, for example, 1300K. This allows you to tweak the properties of such a high-temperature-resistant material."
These theoretical results can be used to create alloys with new properties, or to find alternative compositions when one element in an alloy becomes scarce. The model was validated by creating two different alloys and testing their predicted ‘yield strength’, the amount of stress they can withstand at high temperatures without irreversible deformation. The importance of edge dislocations in this process was confirmed using different experimental techniques.
"We also made an atomic model for screw dislocations, which was too complicated for the high-throughput analysis used for the edge dislocation," says Maresca. This confirmed that screw dislocations were not the most important determinant of yield strength in these alloys. The finding that edge dislocation actually determines a large part of the yield strength of complex HEAs was a major surprise and one that has made a simple, theory-driven discovery of new complex alloys possible.
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.