Why does a solid metal that is engineered for ductility become brittle, often suddenly and with dramatic consequences, in the presence of certain liquid metal impurities?
The phenomenon, known as liquid metal embrittlement, or LME, has baffled metallurgists for a century.
Now, a team of ceramics researchers has shed light on LME by obtaining atomic-scale images of unprecedented resolution of the grain boundaries, or internal interfaces, where LME occurs.
In doing so, says Martin Harmer, professor of materials science and engineering, the researchers have achieved the first direct observation in a metal system of a bilayer grain boundary phase transition.
The study suggests that interior interfaces can undergo transitions similar to the solid-to-liquid and liquid-to-gas phase transitions that occur in larger, “bulk” materials.
It also paves the way for scientists to prevent LME by strengthening the chemical bonds of the materials present at grain boundaries.
“This is a very exciting discovery,” says Harmer, who directs Lehigh’s Center for Advanced Materials and Nanotechnology. “It gives us a much clearer understanding of the atomic mechanism of LME and it promises to improve our ability to control and fine-tune the properties of metals and other materials during fabrication.”
Harmer and his colleagues reported their findings Sept. 23 in Science magazine. Their article, titled “The Role of a Bilayer Interfacial Phase on Liquid Metal Embrittlement,” was written by Harmer; Jian Luo and Kaveh Meshinchi Asl of Clemson University’s School of Materials Science and Engineering; Huikai Cheng, a former research scientist at Lehigh; and Christopher Kiely, director of Lehigh’s Nanocharacterization Laboratory.
Their study was funded by the U.S. Navy. The group is now focusing on rectifying LME-related problems in metals, with help from a five-year, $7.5 million grant through the Department of Defense’s Multidisciplinary University Research Initiative program. That project involves researchers from Lehigh, Carnegie-Mellon, Clemson, Illinois and Kutztown universities.
The common ground of ceramics and metals
Many of the consequences of LME affect everyday life, says Harmer.
A steel highway signpost can crack because it was weakened by LME when the molten zinc alloy was applied to the steel during fabrication. Mercury and gallium, both liquid at room temperature, cause normally corrosion-resistant aluminum to become brittle. And concerns over LME make nuclear power plant operators hesitate to switch from water to liquid metal coolant, whose higher boiling point and ability to absorb radiation give it superior and more reliable cooling properties.
Harmer became interested in LME after he and his students in 2006 identified six grain-boundary “complexions,” each with a distinct rate of grain growth, in the ceramic alumina. That discovery prompted him to seek insight into the embrittlement of metals.
Harmer’s group examined a nickel-bismuth alloy using Lehigh’s JEOL 2200FS aberration-corrected scanning transmission electron microscope (STEM), which has unparalleled imaging capabilities. The group employed a technique called high-angle annular dark-field imaging (HAADF), which focuses a beam of electrons only 1 angstrom (0.1 nm) wide on a sample.
Previous studies had revealed the existence of four interfacial phases at grain boundaries (GB) in metals.
The aberration-corrected STEM revealed two additional phases – a bilayer and a trilayer.
“A bilayer had been seen before in a ceramic system,” says Harmer, “but no one had seen such examples of bi- and trilayers in metals.”
The aberration-corrected STEM pinpointed a bilayer of bismuth atoms at the grain boundary as the source of a weak atomic-scale bond in the nickel-bismuth alloy.
“There is a very strong bond between bismuth and nickel, so it had never been clear why the alloy is prone to embrittlement,” says Harmer. “But the bonds between bismuth atoms are weak. We are the first group to see the formation of a bismuth bilayer that weakens this material.”