Finding new alloys just became simpler

The discovery that iron became 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 Francesco Maresca, assistant professor at the Engineering and Technology institute Groningen (ENTEG), at the University of Groningen. As a civil engineer, he likes large structures such as bridges. But he is now studying metals at an atomic scale to find the best alloys for specific applications.

Dislocation

Maresca is particularly interested in high-entropy alloys (HEAs), which were first proposed some twenty 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 forty 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. It is impossible to test a large fraction of these by simply making them.

This is why Maresca wanted a good theory to describe important properties of HEAs. One of those properties is high-temperature strength, essential in various applications ranging from turbine engines to nuclear power plants. The strength of an alloy depends largely on defects in the crystal structure. 'Perfect crystals are the strongest, but these do not exist in real life materials.' 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,' explains Maresca.

Composition

Another type of defect is edge dislocation, where an extra atomic plane is inserted into part of the crystal structure. Maresca: '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 Maresca created an atomic-scale model for this dislocation in HEAs, which he then translated 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, 1300 Kelvin. This allows you to tweak the properties of such a high-temperature-resistant material.' The 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 dislocation in this process was confirmed using different experimental techniques.

Surprise

'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 dislocation was 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.