Alloys Become Firm And Malleable

The scientists started with a type of material that has been investigated in materials science for a few years, but has so far been too brittle for many applications: alloys in which metallurgists mix similar amounts of typically five or more different metals.

Atomic mess fixes alloys

Since the atoms of the various elements are distributed without any recognizable order in the positions in the crystal lattices of these substances and the entropy is a measure of the disorder, the materials are called high entropy alloys. Such materials can be particularly strong because the confusion of the many different atoms in a structure makes it difficult to move dislocations. Dislocations are defects in the crystal lattice that travel through a crystal when a material is deformed. However, the high strength of the alloys with the atomic disorder has so far also had a disadvantage: If such a material gives way under a load, it usually deforms very abruptly and breaks quickly: it behaves brittle.

By changing the crystal structure, the material becomes ductile

In contrast, steels that mainly contain iron, usually another main component and small amounts of other components such as carbon, vanadium or chromium, are often ductile. So they are just not brittle, but so far they have not been strong enough to enable the construction of thin-walled car bodies, for example. In the crystals of steels, the atoms are arranged more or less regularly. However, steels become particularly ductile when they can change from one structure to another. Because this process swallows energy, which can then no longer cause damage in the material. In a body or other steel components, tiny areas alternate with the two different atomic orders.

Exactly the juxtaposition of different crystal structures was considered harmful in high-entropy alloys - until now. The scientists have researched a material that is solid on the one hand like a high-entropy alloy and on the other hand has two crystal structures side by side like particularly ductile steels. The search resulted in an alloy of 50% iron, 30% manganese and 10% cobalt and chromium. Now the researchers are working on improving the microstructure and composition. The metallic materials from the Düsseldorf-based material smithy can then be processed as easily and inexpensively as a particularly ductile steel and, as a body, can absorb just as much energy from the impact in an accident. At the same time, the material should be so strongthat even thin and therefore inexpensive and resource-saving sheets do not yield even with a weak impact. (qui)