With Grant, Researchers Take Atomic-Scale Look at Metallic Glasses

Bulk metallic glasses (BMGs) have unusual and potentially very valuable properties. But so much is still unknown about how they form and behave that researchers have not been able to take full advantage of this relatively new class of materials. 

The research groups of Corey O’Hern and Udo Schwarz, both professors in the Department of Mechanical Engineering & Materials Science, have teamed up to get an atomic-scale look at bulk metallic glasses to better understand their structure – in particular, the mechanisms that cause them to break. The three-year project, which links experiments conducted in Schwarz’s lab to computer simulations conducted by O’Hern, is funded by a $657,243 grant from the National Science Foundation (NSF) as part of its Civil, Mechanical and Manufacturing Innovation Division within the Engineering Directorate.  

“BMGs hold great promise as a new class of materials for a variety of applications, but one of the problems is that they can be very brittle,” O’Hern said. “Put a small load on them and suddenly, they can catastrophically break.”

Predicting when these breaks will occur is currently impossible because the material’s atomic structure – and the ‘flaws’ associated with any structural irregularities – can presently not be seen. 

Metallic glasses are a class of materials made from complex, multicomponent alloys. They have the moldable pliability of plastics, but can have strengths higher than that of steel. When metallic glasses cool from a liquid to a solid, their atoms settle into random arrangements and do not crystallize the way traditional metals do. Because there’s no pattern, finding flaws in the material that lead to brittle failure is difficult. 

“All of the atoms are arranged randomly,” said Amit Datye, a postdoctoral associate in Schwarz’s lab. “So there’s no obvious structural signature of a defect in the material.” 

The researchers are specifically looking for “shear transformation zones” (STZs), a random group of atoms that moves together in response to a load. Once these STZs form, they assemble into “shear bands” that move through the material and cause it to break. The researchers want to track single shear bands and correlate their formation and evolution to the composition and strength of the material. 

Isolating a single shear band is tricky because each one consists of only small numbers of atoms, and typically samples form multiple shear bands under external load. Schwarz’s lab, however, is the first to characterize the surfaces of BMGs at an atomic level. Schwarz and his co-workers have developed sample-preparation methods that result in what’s known as atomically flat materialsthat have far fewer structural flaws that could affect the mechanicalproperties than regularly cast samples. His lab has also developed special “nano-indentation” methods - a process for testing the hardness and deformations of small samples of materials. 

“Clarifying the fundamental mechanisms of an individual shear band will be incredibly helpful,” Schwarz said. “Then you know better what to do to get by with the minimal amount of material, which makes the structure you’re building as light and cheap as possible.”

O’Hern’s lab will then use atomic-scale computer simulations to observe what’s happening during deformation, atom by atom. 

“The experiments can’t clearly see into the bulk of metallic glasses on an atomic scale,” O’Hern said. “Using simulations we will be able to watch the initial formation as STZs cluster into shear bands, which is the initial process involved in material failure.”

The researchers noted that the work builds on previous research in BMGs led by Jan Schroers, professor of mechanical engineering & materials science, and the support of Yale’s Materials Research Science and Engineering Center (MRSEC) program.