When solutes like boron or carbon are added to a metal, their atoms tend to locate at the grain boundaries where the crystal structure is disjointed. This can alter the properties of the metal, for instance, strengthening the boundaries against cracking. Materials scientists who study grain boundaries had long wondered whether solutes substituted atoms in the host crystal structure or whether they occupy interstitial spaces.

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Small atoms appear as kite-shaped structures at the boundary. This reconstructed dark-field image represents the (110)//(010) atomic motif. The size marker is 200 pm. Credit: Nature Communications.

Xuyang Rhett Zhou of the Max Planck Institute for Iron Research and colleagues have used a two-pronged approach to study where solutes are located at grain boundaries within an iron crystal. They observed the second scenario: boron atoms fit into the spaces between iron atoms. Their findings were recently published in Nature Communications (https://doi.org/10.1038/s41467-023-39302-x).

The researchers’ first approach featured a relatively new imaging technique. The researchers placed a sample of iron alloyed with boron and carbon under a scanning transmission electron microscope and recorded a diffraction pattern at each pixel of the microscopic image. This allowed them to view the crystal structure in four effective dimensions: two real-world dimensions and two reciprocal dimensions.

This technique has only existed for around 10 years, but advances in hardware enabled the researchers to collect four-dimensional data relatively quickly. Zhou and colleagues could use those data to construct charge-density maps. “You can easily see the heavy atoms, like iron, and light atoms, like boron,” Zhou told MRS Bulletin, although discerning light boron atoms apart from similarly small carbon atoms is far more difficult.

The researchers then validated and interpreted their image results with the second approach where they performed three series of simulations and reconstructed the structures that may have formed those charge-density maps.

What they found was evidence that boron solute atoms fit interstitially into the gaps within iron crystal structure, rather than substituting and replacing the iron atoms. In particular, the research team found that repeating grain-boundary patterns—for instance, angled lines of atoms coming together into a kite shape—attracted solute atoms.

“This study is an important first step, but does not provide yet a complete picture on how solute atoms interact at grain boundaries,” Frederic Sansoz, professor of mechanical engineering and materials science at The University of Vermont who was not a study author, told MRS Bulletin.

In addition to conducting further simulations, Zhou said, the team would like to conduct in situ mechanical testing of boron-decorated grain boundaries in order to learn more about how boron actually impacts the boundaries’ mechanical properties. Additionally, they would like to learn if boron can cause grain boundaries to undergo a phase transformation.

Rahul Rao