The interfaces between individual crystals in a material, known as grain boundaries (GBs), play a critical role in dictating the strength, durability and overall performance of a material. For this reason, GB phase transitions - abrupt changes at a material's interface resulting in distinct structures and properties - are becoming increasingly recognized as a new frontier in materials science.
Researchers from Lawrence Livermore National Laboratory (LLNL), Ruhr University Bochum and other international collaborators have provided the first demonstration of how iron atoms, when introduced into titanium, undergo a GB transition. During their study, the researchers observed that the iron atoms segregate (concentrate) to form quasicrystalline-like structures (those with patterns that are ordered but not periodic) at the interface. This work is described in a recent issue of Science.
"This segregated alloy structure is nothing like we have seen before; the icosahedral [having 20 faces, 12 vertices and 30 edges] units cluster together at the interface forming agglomerates of different sizes and shapes depending on the amount of iron," said Timofey Frolov, the LLNL scientist who led the modeling part of the study.
Using atomic-resolution scanning transmission electron microscopy, the researchers directly observed how iron atoms alter the structure of GBs in titanium, correlating their observations with advanced computer simulations to be able to understand the unusual segregation behavior.
"The iron atoms do not simply segregate to the interface, but they form entirely new and unexpected cage-like structures," said Christian Liebscher from Ruhr University Bochum.
This newly discovered segregation phenomenon not only deviates from the classic picture of segregation, but it also deepens scientific understanding of interface complexity.
"As iron segregation at the boundary increases, more icosahedral units appear," said Vivek Devulapalli who conducted the microscopy experiments at the Max Planck Institute for Sustainable Materials. "We have identified more than five distinct structures or GB phases within the same boundary, all composed of different arrangements of the same icosahedral structural units," said Enze Chen from Stanford University.
The unusual segregation behavior originates from the fivefold symmetry (where a shape can be mapped onto itself by rotating it 72 degrees around a central point) of the icosahedral units, which is impossible to achieve in traditional periodic crystals.
Constrained by this symmetry, the icosahedral cages form complex, aperiodic clusters as iron content increases, creating distinct aperiodic GB phases that lack regular repetition despite being ordered. The resulting interface states are reminiscent of building blocks found in quasicrystals or topologically close pack phases, but they only appear at the grain boundaries.
This discovery opens exciting new avenues for materials design. The observation of icosahedral GB phases not only provides deeper insights into the complexities of interfaces but also empowers scientists with new tools and strategies to design advanced materials with superior properties.
By understanding and controlling the segregation and formation of different GB phases, scientists can potentially tailor the properties of materials for specific applications. Looking ahead, GB structure engineering presents a powerful tool for materials engineering, with significant implications for enhancing properties such as strength, ductility, corrosion resistance and more.
