A team of ceramists pinpoints a hitherto unknown internal phase transition.
Why does a solid metal that is engineered for ductility become brittle in the presence of certain liquid metal impurities?
The phenomenon, known as liquid metal embrittlement, or LME, has baffled metallurgists for a century.
Now, ceramics researchers from Lehigh and Clemson University have 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 at Lehigh, 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 gives us a much clearer understanding of the atomic mechanism of LME,” says Harmer, who directs Lehigh’s Center for Advanced Materials and Nanotechnology. “It promises to improve our ability to control and fine-tune the properties of metals and other materials during fabrication.”
The researchers reported their findings Sept. 23 in Science. Their study was funded by the U.S. Navy. The group is continuing its work, with a focus on rectifying LME-related problems in metals.
The critical grain boundary
Harmer became interested in LME after his group in 2006 identified six grain-boundary “complexions,” each with a distinct rate of grain growth, in alumina. The discovery prompted him to seek insight into the embrittlement of metals.
Using Lehigh’s JEOL 2200FS aberration-corrected scanning transmission electron microscope (STEM), which has unparalleled imaging capabilities, the group examined a nickel-bismuth alloy. They employed 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. Harmer’s group found two more – 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,” says Harmer, “so it had never been clear why the alloy is prone to embrittlement. But the bonds between bismuth atoms are weak. We are the first group to see the formation of the bismuth bilayer that weakens this material.”
A comprehensive study
Harmer’s group examined 12 independent interfaces and excluded “imaging artifacts” introduced by experimental error or by technology. To avoid distortions that result from projecting a 3-D image onto a 2-D film, they took images at different depths on the sample.
“By looking sequentially at these images and their structural thickness,” says Harmer, “we were able to rule out artifacts that give the illusion of a bilayer.”
In contrast with previous studies that looked at synthetic bi-crystals, the group examined polycrystalline nickel which resembles industrial materials.
“Real grain boundaries are typically less symmetrical and have higher energy than synthetic bicrystals,” says Harmer.
The group plans next to experiment with the chemistry of nickel-bismuth GBs to try to produce a more ductile behavior.
”Perhaps combining the bismuth with other elements that bond at the interface will prove effective,” says Harmer.