A team of U.S. academic researchers has taken the first three-dimensional (3-D) image of a microscopic crack propagating through a metal damaged by hydrogen.
Previously, the only way to analyze such a metal failure was to look at the separated pieces of a completely fractured component, which entails significant guesswork. The new research shows what is happening at the crack tip as a part begins to fracture.
“It’s much better than arriving at the crime scene after the fact,” says Michael J. Demkowicz, a professor at the materials science and engineering school at Texas A&M University (College Station, Texas, USA) and a project research lead. In their work, the team identified microscopic structures that make metals stronger and less susceptible to hydrogen, present mostly from water, which can damage metals through hydrogen embrittlement (HE).
One prominent example involves the Bay Bridge in San Francisco, California, USA. As the bridge was being constructed in 2013, engineers discovered that 32 of the 96 huge bolts key to the structure had cracked due to HE. The problem was caught early, so there was no disaster, but it delayed the bridge’s opening by a few years.
“As a result, engineers have to overdesign with additional material to cover any sudden failure, and that costs a lot,” says Peter Kenesei, the study’s co-author and a researcher at the U.S. Department of Energy’s Argonne National Laboratory (Lamont, Illinois, USA). At Argonne, the researchers used two different synchrotron tools, high-energy diffraction microscopy and x-ray absorption tomography, to analyze the microscopic structure of a crack in a superalloy of nickel. The study represents the first time the microscopy technique was used by researchers not involved in its development.
Metals are composed of microscopic crystals, or grains. In nickel superalloys, the fractures brought on by HE travel along the boundaries between those grains. The researchers say Argonne’s unique 1-ID beamline tools allowed them to not only look at the grain orientations around a crack in progress, but also the grain boundaries. From those observations, the team identified 10 grain boundaries that are more resistant to cracks.
“We were able to show not only which grain boundaries are stronger, but exactly what it is about them that improves their performance,” says John Hanson, a reactor engineer at Oklo (Sunnyvale, California, USA) and first author of the paper. This could ultimately allow engineers to build stronger metals by designing them with those characteristics.
The study took eight years to complete, primarily because it involved huge amounts of data that were difficult to analyze. “It’s highly encrypted in the form of streaks and dots, or diffraction patterns, that must be analyzed by a supercomputer,” says analysis expert Robert M. Suter from Carnegie Mellon University (Pittsburgh, Pennsylvania, USA).
Going forward, the Argonne tools could be used to image the microstructure of existing metal components to better predict their susceptibility to failure. Kenesei notes that the tools are already used this way to study other engineering materials, like those related to airplanes, batteries, and nuclear reactors.
Source: Texas A&M University College of Engineering, engineering.tamu.edu.