Scientists at Clemson, Lehigh discover possible key to a mysterious metal failure

CLEMSON, SC – September 28, 2011 – Studying images of the very boundaries where atoms meet, a team of Clemson University and Lehigh University researchers has gained an atomic-level understanding of a spectacular form of metal failure, yielding clues to its causes that have eluded science for a century.

Jian Luo, an associate professor of materials science and engineering and member of the Center for Optical Materials Science and Engineering Technologies (COMSET) at Clemson, said the work reveals a possible key to why certain metals become brittle and fail catastrophically after they are brought in contact with other liquid metals; for example, during welding or when applying anti-corrosion coatings.

 Photo at left: This aberration-corrected scanning transmission electron micrograph of shows a bilayer interfacial phase that is the underlying cause for liquid metal embrittlement in nickel infused with bismuth. image by: Clemson University

This research yields a perspective that is different from the traditional view, Luo said. Researchers knew for a long time that adsorption of the liquid metal caused problems. We’ve shown that it can actually change the atomic-level interfacial structure drastically and abruptly, which in turn leads to a sudden reduction of the strength of the solid metal.

Exactly what has caused certain metals to lose their strength under these circumstances — known as liquid metal embrittlement, or LME — has puzzled scientists in the materials and physics communities for more than a hundred years.

The research, conducted in a collaboration with a Lehigh University team led by Martin Harmer, shed light on the mystery by identifying the exact atomic-level interfacial structure that is the cause of the sudden loss of strength.

Using a scanning transmission electron microscope at Lehigh to observe a nickel sample infused with bismuth atoms, the researchers showed unequivocally that a bilayer interfacial phase is the underlying cause of embrittlement. 

The research, published in the current issue of the journal Science, could prove important in many manufacturing processes.

It is an interesting study because this is a very old problem, Luo said. It gives scientists a new perspective to develop an atomic-level model for this phenomenon.

Luo cautions that, because the study was conducted on only one system, more work remains to be done.

As is the case in most engineering research, the long-term goal is to solve the problem. The first step is to understand what happened, he said. The beauty of this study is to demonstrate that an external stimulus — impurity or temperature increases — can change the atomic-level interfacial structure abruptly, which will in turn induce a drastic change in material properties. In this case, a strong and ductile metal suddenly becomes very brittle and fails catastrophically.

That understanding could shed further light on the mechanisms of an even broader issue: how the strength of metals is reduced by the presence of minor impurities, a phenomenon known as grain boundary embrittlement, which affects many industries that rely on the strength of metallic structural materials.

It also is a significant finding for the nuclear power industry, which has considered the use of liquid metals for a new generation of safer and more efficient reactors.

Luo’s research at Clemson was supported by a grant from the Office of Basic Energy Sciences at the U.S. Department of Energy. He was joined in the research by Clemson graduate student Kaveh Meshinchi Asl; at Lehigh, materials science and engineering professor Harmer, nanocharacterization lab director professor Christopher J. Kiely and former research scientist Huikai Cheng collaborated on the project.

This material is based upon work supported by the U.S. Department of Energy under Grant No. DE-FG02-o8ER46511. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the U.S. Department of Energy.