Correlating atomic structure with metallic glass forming ability

NON-TECHNICAL SUMMARY

When a liquid is cooled to become a solid, like water solidifying into ice when cooled in a freezer, the atoms could form a crystalline solid, where the atoms are arranged periodically, or a glass, where the atoms are arranged randomly. Metal alloys can form metallic glasses when cooled rapidly. Metallic glasses have properties that suggest improvements in applications such as casings for electronics, batteries, and hydrogen, for medical devices and implants, and applications in space for robotics gears and satellite tanks.

Whether the liquid becomes crystalline or a glass depends on the cooling rate; the faster the liquid is cooled, the more likely it will form a glass. The slowest cooling rate that still allows for glass formation is the critical cooling rate, which is a key characteristic of any material. Determining critical cooling rates and hence, quantifying the glass forming ability (GFA), has been a very difficult and elaborate task in the past.

Recent work by the PI revealed that the GFA can be determined from the structure of the glassy solid through x-ray diffraction measurements that gives information on the arrangement of the atoms. Correlating these x-ray diffraction measurements to the GFA provides a cutting-edge tool that will advance the science of glasses and liquids significantly.

Thereby, this research has the potential to develop new metallic glasses faster to be used in a wide range of applications and impact US manufacturing. The PI will also train the next generation of scientist through an open, interactive approach in the spirit of the Materials Genome Initiative (MGI) by broadly sharing all experimental data with the scientific community and through outreach programs where high-school students learn and teach about cutting edge materials science research by making YouTube videos.

TECHNICAL SUMMARY

Vitrification, the formation of a glass upon solidification of a liquid metal, which is quantified in the critical cooling rate and referred to as the glass forming ability, GFA, is a key characteristic of paramount importance for the physics, formation and processing of glasses. However, quantifying glass forming ability has been experimentally elaborate and its complexity has prevented use of computational methods.

These challenges have impeded progress in metallic glass science, discovery and technology which takes place in a vast potential compositional space. Preliminary results by the PI suggest that this grant can overcome previous limitations of identifying GFA of alloys. The PI provides evidence that the GFA is reflected in the structure the glass assumes and this structure can be readily measured with standard x-ray diffraction measurements.

The alloy’s atomic structure is quantified in the full-width-half-max of the first diffraction peak in the XRD spectrum, Dq, and the PI’s preliminary results reveals a direct correlation between Dq and GFA; Best glass formers exhibit the most disordered amorphous structure, reflected in their highest Dq in their as-sputtered state which is furthest away from equilibrium. The Dq-GFA relationship suggested that current models to describe the atomic structure of metallic glasses are limited to the ideal glass state but do not describe the, vastly larger in number, real glasses that occupy the higher potential energy states in the potential energy landscape.

To describe such real glasses, contributions of entropy, and possibly kinetics have to be considered as well. Separately, by exploring the Dq-GFA relationship over large compositional spaces and temperatures or cooling rates, will provide insight into formation motifs of metallic glasses.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.