Modern calculation answers decades-old question

Materials scientists from The Grainger College of Engineering have provided the first quantitative explanation for a phenomenon first observed in iron in the 1970s. 

Researchers from the Department of Materials Science and Engineering at The Grainger College of Engineering have presented the first physical explanation of how a magnetic field slows the movement of carbon atoms through iron. Their findings, published in Physical Review Letters, will improve scientists’ understanding of how carbon influences grain structure in steel. 

An alloy of iron and carbon, steel is one of the most-used building materials on the planet. Engineering its microstructure requires high temperatures; as a result, most steel processing consumes significant energy. In the 1970s, scientists noted that some steels exhibited better properties when heat treated under a magnetic field — but their ideas explaining this behavior were only conceptual. Understanding the mechanism behind this phenomenon could improve engineers’ ability to control heat treatment, improving material processing and potentially lowering energy costs.

“The previous explanations for this behavior were phenomenological at best,” said Dallas Trinkle, the Ivan Racheff Professor of Materials Science and Engineering and the senior author of the paper. “When you’re designing a material, you need to be able to say, ‘If I add this element, this is how (the material) will change.’ And we had no understanding of how this was happening; there was nothing predictive about it.”

Trinkle used his expertise in diffusion modeling to tackle the persistent question as part of a larger group funded by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. In iron-carbon alloys like steel, carbon atoms sit in octahedral “cages” surrounded by iron atoms. By modeling the diffusion of carbon through iron, Trinkle and his colleagues could better understand the mechanism causing this unique behavior. 

Using a technique called spin-space averaging, Trinkle generated computer simulations replicating the effects of temperature and magnetic fields on the spin alignments of iron atoms. When the north and south poles of an iron atom align, they are considered ferromagnetic: highly likely to magnetize. When the poles are unaligned, they are paramagnetic, or weakly magnetized. Trinkle’s simulation revealed a change in energy barrier when the atom spins were aligned, suggesting that increased magnetic order hinders the movement of carbon atoms between cages.

“It takes an extremely strong field to switch magnetic moments,” Trinkle said. “If you’re near the Curie temperature, the magnetic field has a strong effect… When the spins are more random, the octahedron (cage) actually gets more isotropic: the whole thing kind of opens up and has more space to move.” 

Trinkle hopes the recent findings can be used to reduce the energy required to process steel, lowering both its cost and CO₂ emissions. He also believes this knowledge can be transferred to other materials to quantitatively predict diffusion under magnetic fields. 

“We wanted to be able to do real calculations; to show not just qualitatively but quantitatively the effective field and temperature. Now that we have this information, we can start thinking more about engineering alloys. It may be choosing alloys that already exist or even thinking about alloy chemistries that we’re not yet using that could be extremely advantageous.” 

Illinois Grainger Engineering Affiliations

Dallas Trinkle is an Illinois Grainger Engineering professor in the Department of Materials Science and Engineering. He is also affiliated with the Materials Research Laboratory. Trinkle holds the Ivan Racheff Professorship appointment.