Understanding the deformation of conventional crystalline metals is well-established, with atoms arranged in orderly patterns. However, the deformation of metallic glasses and other amorphous metals, which lack a regular crystal structure, has been more challenging to explain, especially at the nanoscale.
In a recent study, Professor Jan Schroers delves into the unique physical characteristics of how these metals behave at very small sizes. This exploration of nanoscale behavior could pave the way for innovative approaches to creating metallic glasses.
Metallic glasses, combining metal strength with plastic flexibility, are under development for diverse applications such as aerospace, space, robotics, electronics, sports equipment, and biomedical uses. These materials derive their unique properties from their atomic structures—when metallic glasses solidify from a liquid state, their atoms arrange randomly without forming a crystalline structure seen in traditional metals.
Achieving this non-crystalline state is challenging, and understanding the intricate details of their behavior could significantly enhance the efficient production of metallic glasses.
The study emphasizes the need for a comprehensive understanding of size- and temperature-dependent deformation in amorphous metals to advance their fabrication and utilization. Over the past decades, it has been established that, on a macroscopic scale, atoms move collectively during deformation at temperatures allowing flow.
Prof. Jan Schroers, the Robert Higgin Professor of Mechanical Engineering and Materials Science, said, “They deform collectively, almost like honey. You see all of these atoms kind of moving collectively together.”
The Schroers lab conducted experiments to understand the deformation behavior of nanoscale-sized metallic glass samples using materials like zirconium copper. Graduate student Naijia Liu created progressively smaller samples and discovered that, at sizes of 100 nanometers or smaller, the standard deformation rules began to deviate.
The key observation was that, at this reduced size, the chemical composition of the samples would remain the same if atoms continued to move collectively. Instead, the atoms started moving individually, leading to a rapid deformation of the metal.
Indeed, as the metallic glass samples get smaller, particularly at sizes around 100 nanometers or less, the conventional flow of atoms collectively ceases. Instead, the atoms start moving individually across the surface.
The significance lies in that atoms exhibit faster movement on the surface of crystalline materials. As samples become smaller, a larger proportion of the material is either on or close to the surface. In the deformation process, atoms opt for an expedited route by utilizing the fast surface path, enabling overall quicker deformation. This insight provides a glimpse into a realm of physics still rife with unanswered questions, particularly concerning the intricate dynamics at play during deformation at the nanoscale.
Schroers said, “We know essentially everything about crystals, and we know essentially everything about gasses. However, the scientific community does not know the liquid state well. Things move around too quickly, so observation methods are challenged, and as the order in a liquid is non-periodic, we can’t reduce the problem to a smaller unit.”
Schroers’ lab focuses on identifying which alloys hold the most promise for creating metallic glasses through their experimental method. The alloy composition is a crucial factor in this process, requiring sufficiently similar but not too identical elements. This specific balance is essential to form a glass structure on the template, contributing to the successful production of metallic glasses.
Journal Reference:
- Liu, N., Sohn, S., Na, M.Y. et al. Size-dependent deformation behavior in nanosized amorphous metals suggesting transition from collective to individual atomic transport. Nat Commun 14, 5987 (2023). DOI: 10.1038/s41467-023-41582-2