Long-range anisotropy in a metallic glass
When we hear the word ‘glass’, most of us think of drinking glasses, windowpanes and smartphone screens. However, this type of glass, made mostly of silicon dioxide, is just the tip of the iceberg when it comes to a materials scientist’s view of glasses. For example, there are many types of metallic glasses, which are metal alloys that are disordered, not crystalline like most metals. Their superior mechanical properties have led to their use in applications including spacecraft shielding, biomedical implants and golf club heads. Despite the widespread use and utility of glasses, scientists know very little about their structure and dynamics. With the support of the Marie Skłodowska-Curie Actions (MSCA) programme and under the supervision of Giulio Monaco of the University of Padua, the GlassX project took advantage of the recent availability of a new generation of X-ray sources to perform detailed studies of the structure and dynamics of glasses at nanometre resolution.
The glass transition: more than meets the naked eye
Glasses are formed by cooling a liquid, where the flow is the bulk manifestation of the continuous microscopic rearrangement of the liquid’s atoms. As the liquid is cooled, the flow becomes slower and slower, until it begins to behave like a solid. This is called the glass transition. However, even in its solid form, the material is not at equilibrium. In fact, the apparent ‘solidity’ hides a continuing rearrangement of atoms ‘relaxing’ towards the ‘ideal glass’ state – the lowest energy state – albeit over very long periods of time. Thus, glasses are non-equilibrium states of matter that are thought to be disordered like a liquid. At least, that was the theory until now.
The revelations of high-tech X-ray nanodiffraction
GlassX leveraged the highly focused, very intense X-rays of the Extremely Brilliant Source (EBS) at the European Synchrotron Radiation Facility to shed light on the structure and behaviour of glasses. According to MCSA fellow Peihao Sun at the University of Padua: “The very bright and narrow X-ray beam of the EBS allows the study of materials’ structure on the 100 nm scale. This enabled us to obtain information about metallic glasses that was previously inaccessible. Remarkably, we found that these glasses are not disordered even on the 100 nm scale. They are anisotropic, meaning they are not the same when viewed from different directions – these differences can span more than 100 nm, meaning thousands of layers of atoms.”
A potentially new state of glasses
He adds: “The direct observation of long-range anisotropy in a metallic glass goes against our common understanding of glasses as completely disordered systems. In fact, the anisotropy in our sample disappeared after it was heated again and allowed to slowly cool. Therefore, the original glass seems to have been in a different, possibly new, state. It will thus likely have different physical properties as well, which remain to be studied. The properties of this anisotropic glass continue to surprise us,” concludes Sun. GlassX’s outcomes and the researchers’ further studies should support exploitation of the technological promise of novel glasses in improved existing and innovative new applications.
Keywords
GlassX, glasses, glass, anisotropy, anisotropic, , glass transition, metallic glasses, X-ray nanodiffraction, Extremely Brilliant source, synchrotron
