“Showing that this material has order at the nanoscale will likely lead to new work on how to expand these ordered domains, and potentially manipulate the material’s mechanical properties,” Irving says. And, because there is a high concentration of chromium atoms in the material, this creates nanoscale domains of order with the overall “chaos” of the high entropy alloy. In short, chromium’s spin properties force the chromium atoms to be as far apart as possible in the NiFeCrCo structure. But if two chromium atoms are next to each other they can’t both align their spins differently from all of their neighbors – because they themselves are neighbors. They can all spin up, and chromium can spin down. In NiFeCrCo, chromium can align its spin against its neighbors if it is surrounded by iron, nickel or cobalt. But the electrons in antiferromagnetic materials – like chromium – tend to align so that their spin is the opposite of their neighbors. The electrons in ferromagnetic materials – like iron, nickel and cobalt – tend to align so that their spin is oriented in the same direction. Specifically, the researchers learned that chromium – and spin – play key roles.Īll atoms have electrons, and all electrons have a property called spin. “But now we have determined that there is some order in the composition of this alloy,” Irving says. That impression of chaos is why they’re called high entropy alloys. But which atoms fill which spaces is seemingly random – it seems impossible to predict which element might be in any given box. “If you look at NiFeCrCo, it has a fixed structure,” Irving explains. “For example, NiFeCrCo-based high entropy alloys have a good combination of hardness, tensile strength, ductility, and fracture resistance at extremely low temperatures,” Irving says. This is a diagram of the NiFeCrCo high entropy alloy’s structure with ordered chromium.
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