Kai Nip, Staff Photographer

A new Yale study from the School of Engineering and Applied Science developed a method to increase the ductility of bulk metallic glasses, or BMGs. 

Researchers at SEAS, in professor of mechanical engineering and materials science Jan Schroers’ lab, work with new materials such as bulk metallic glasses. Their recent study presents a method for improving the ductility of the material, which is a significant issue in engineering materials. The discovery addresses the important problem of brittility in engineering materials. The study was published in the journal Nature Communications on Feb. 26. 

“I strongly believe BMGs are one big scientific breakthrough away from wide-spread commercial adoption, and I wanted to work on finding that solution,” Ethen Lund, a graduate student in Schroers Lab and a coauthor of the paper, wrote in an email to the News. The study found that “the ductility of a BMG can be dramatically improved (3x greater) by mechanically straining the alloy as it cools to form a solid glass” Lund wrote in the email. 

The inspiration for the study came from the unique properties of BMGs, which are fairly new materials in the field of material science. Their mechanical properties have been extensively studied, but when it comes to their short range atomic structure and the physics of the glass itself, these materials are still misunderstood. 

“BMGs as a class of materials are unique in that they can be stronger than steel but still moldable like plastic,” Lund wrote.  

According to Lund, this feature of BMGs makes them a promising option in the generation of new engineering materials. 

Metallic glasses differ from the traditionally crystalline metals due to their ability to form different structures — from complex nanostructures to strong, robust ones. There is, however, a shortcoming to this property of BMGs. 

“They also exhibit a wide range of toughness values and are often brittle, so when they do fail, they often do so quickly and completely, rather than exhibiting necking or strain hardening like other metals (steels, titanium alloys, etc.),” Lund wrote.  

The main problem with most commercial materials today is their inability to exhibit both strength and ductility simultaneously, according to the paper. This study addresses that problem by manipulating the atomic structure of metallic glass during the cooling phase of its synthesis process. 

The importance of the study is two fold according to Rodrigo Miguel Ojeda Mota GRD ’21, one of the authors of the paper. 

“We provide for the first time solid experimental evidence that strain rate, when appropriately used, can ductilize metallic glasses, as we were able to separate the effect of cooling and straining,” Mota wrote in an email to the News. 

According to Mota, the second step of the study involved providing a theoretical explanation of how to carry out this ductilization process, in noting the specific conditions required for such process. 

The technique the team developed to successfully achieve this ductility involved challenges. The increased ductility is achieved through three competing processes, which all take place on different millisecond time scales. These mechanisms control the atomic structure of the eventual solid glass. The three time scales are the cooling rate, the mechanical strain rate and the intrinsic relaxation rate of the atoms that depends on the temperature of the glass. 

The team had to come up with a method to be able to have the strain rate as the dominant process, as that is the process determining the ductility of the material. In order to achieve that goal, the mechanical strain itself had to be applied very quickly as the material cooled from 430 degrees celsius to 350 degrees celsius. The main challenge the team faced in the process was controlling the important factors such as temperature and strain rate in such a dynamic system, considering the whole process only takes approximately one second. 

“The research discovered that if the stretching is very fast — the rate of energy input is so high that the embrittling effects of slow cooling can be counteracted and overcome,” David Browne, professor of  materials science and engineering at University College Dublin, wrote in an email to the News. “It had previously been thought (at least by me and I suspect many others) that one could only significantly alter the mechanical properties of metallic glass alloys by straining or deforming them in their solid state.”

Browne added that this study showed that some metallic alloys can be made more ductile through stretching, even when in a liquid state. 

Magnesium-based, or Mg-based, metallic glasses are another type of metallic glass that can be used for similar applications to BMGs. However, there are some key differences, according to Mota. 

“Mg-based metallic glasses are highly biocompatible as medical implants, but they are very brittle, so their full potential for applications has not been exploited,” Mota wrote. 

According to Lund, this study is one of the first methods developed to produce large quantities of ductile BMGs. Even though the brittility in BMGs is still the main flaw of these engineering materials, the findings in this study bring scientists one step closer to overcoming the issue. 

The team believes that the future directions of the study include the application of the same technique to two and three-dimensional set ups. This development could potentially lead to bulk net shaping techniques, which are useful for producing BMG-based devices of larger sizes. 

The group is currently investigating “how the direction of straining affects the properties of the material in different orientations,” according to Lund. 

The first known bulk metallic glass was produced in 1960. 

Elifnaz Onder | elifnaz.onder@yale.edu