Using advanced microscopy and simulation techniques, an international team of researchers has closely studied how iron atoms influence the structure of grain boundaries in titanium. To their surprise, they discovered that “Iron atoms not only migrate to the interface, but also create entirely unexpected cage-like formations,” says Prof. Dr. Christian Liebscher from the Research Center Future Energy Materials and Systems at the University Alliance Ruhr. This unexpected behavior was not anticipated by the researchers. Their results were published in the journal Science on October 25, 2024.
A new type of segregation behavior
Many technological materials feature a polycrystalline structure, meaning they consist of various crystals where atoms are arranged in a systematic lattice. These crystals exhibit different orientations throughout, and the boundaries separating them are referred to as grain boundaries. “These grain boundaries significantly impact a material’s strength and overall performance,” notes Dr. Vivek Devulapalli, who conducted the microscopy work in this study. He adds: “However, our understanding of what happens when elements concentrate at grain boundaries and how they impact material properties is quite limited.”
The breakthrough was made by observing and modeling the structures at an atomic level. The researchers linked their findings from atomic-resolution scanning transmission electron microscopy with sophisticated computer simulations. A new predictive algorithm for grain boundary structure was capable of creating the structures observed experimentally, allowing for further investigation. “Our simulations reveal that regardless of the varying amounts of iron, we consistently identify the cage structures as fundamental components of different grain boundary phases. As the iron concentration at the grain boundary increases, more icosahedral units form and eventually cluster together,” explains Dr. Enze Chen from Stanford University. An icosahedron is a geometric shape with 12 corners (or vertices), which in this scenario are occupied by atoms, and 20 faces.
“We have discovered more than five unique structures or grain boundary phases for the same boundary, all featuring various configurations of the same icosahedral cage units,” adds Dr. Timofey Frolov, who led the computational aspect of the study.
Quasicrystalline-like grain boundary phases
A detailed examination of the cage structures showed that the atoms arrange in an icosahedral formation, with iron atoms positioned at the center and titanium atoms situated at the corners. “The icosahedral cages allow for a denser packing of iron atoms, and since they can form aperiodic clusters, they can hold more than two to three times the normal amount of iron at the grain boundary,” explains Vivek Devulapalli. “It seems that iron is effectively trapped within quasicrystalline-like grain boundary phases,” adds Chen. “This phenomenon is attributable to the unique properties of the icosahedral cages,” states Liebscher, “and now we need to explore how they impact the properties of the interface and, consequently, the overall material behavior.”
New pathways for materials design
By grasping and managing the formation of icosahedral grain boundary phases with varying structures and characteristics, the researchers believe it could be possible to customize material properties. They aim to systematically explore how these novel grain boundary states can be utilized to adjust material behavior, tailor specific functionalities, and enhance materials’ resilience against degradation.
