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At MIT, Metal Heals Itself Under Force

Monday, October 14, 2013

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The researchers were so surprised that they “had to go back and check" the result: Exerting force on the cracked piece of metal was causing the crack to mend—not pull apart.

But, says the MIT team, that was indeed the case: Under certain conditions, putting a cracked piece of metal under tension had the opposite effect that one would expect, causing the crack to close and its edges to fuse together.

Nickel-base superalloy crack
Southwest Research Institute

This crack in a gas turbine follows the grain boundaries of the cast nickel-base superalloy. MIT's research suggests that such materials could be designed to self-mend small cracks.

The surprising finding could lead to self-healing materials that repair incipient damage before it has a chance to spread, according to an MIT research announcement.

The results were published in the journal Physical Review Letters in an article by graduate student Guoqiang Xu and professor of materials science and engineering Michael Demkowicz.

The article says the new mechanism "leads to complete healing of nanocracks," closing some and suppressing the propagation of others.

“We had to go back and check,” Demkowicz says, when “instead of extending, [the crack] was closing up. First, we figured out that, indeed, nothing was wrong. The next question was: ‘Why is this happening?’”

Microstructures and Superalloys

The answer turned out to lie in how grain boundaries interact with cracks in the crystalline microstructure of a metal—nickel, in MIT's case, which is the basis for “superalloys” used in extreme environments, such as in deep-sea oil wells, the university said.

The team created a computer model of that microstructure and studied its response to various conditions.

“We found that there is a mechanism that can, in principle, close cracks under any applied stress,” Demkowicz says.

Most metals are made of tiny crystalline grains whose sizes and orientations can affect strength and other characteristics.

Simulation courtesy of Guoqiang Xu and Michael Demkowicz / MIT

A simulation of the molecular stucture of a metal alloy shows the boundaries between microcrystalline grains (white lines forming hexagons) and a small crack (dark horizontal bar at bottom) that mends itself as stress is applied.

But under certain conditions, Demkowicz and Xu found, stress “causes the microstructure to change: It can make grain boundaries migrate. This grain boundary migration is the key to healing the crack,” Demkowicz says.

Migration in Solid Metal

The very idea that crystal grain boundaries could migrate within a solid metal has been extensively studied within the last decade, Demkowicz says. Self-healing, however, occurs only across a certain kind of boundary, he explains—one that extends partway into a grain, but not all the way through it. This creates a type of defect is known as a “disclination.”

Disclinations were first noticed a century ago but were considered “just a curiosity,” Demkowicz says. When he and Xu found the crack-healing behavior, he says, “it took us a while to convince ourselves that what we’re seeing are actually disclinations.”

These defects have intense stress fields, which “can be so strong, they actually reverse what an applied load would do,” Demkowicz says. In other words, when the two sides of a cracked material are pulled apart, instead of cracking further, it can heal. “The stress from the disclinations is leading to this unexpected behavior,” he says.

Designing for Disclination

The team's next step is to study how to design metal alloys so cracks close and heal under loads typical of particular applications. Techniques for controlling the microstructure of alloys already exist, Demkowicz says, so it’s just a matter of figuring out how to achieve a desired result.

“That’s a field we’re just opening up,” he says. “How do you design a microstructure to self-heal? This is very new.”

The technique might also apply to other kinds of failure mechanisms that affect metals, such as plastic flow instability—akin to stretching a piece of taffy until it breaks, MIT explains.

MIT computer simulation
Simulation courtesy of Guoqiang Xu and Michael Demkowicz / MIT

By engineering metal microstructures to generate defects known as disclinations, researchers hope to slow the progression of metal fatigue, which shortens material life.

Engineering metal microstructures to generate disclinations could slow the progression of this type of failure, Demkowicz says.

Fighting Metal Fatigue

Such failures can limit the lives of many materials, including those used in aircraft, oil wells, and other critical industrial applications, Demkowicz notes.

Metal fatigue, for example—which can result from an accumulation of nanoscale cracks over time—“is probably the most common failure mode” for structural metals in general, he says.

“If you can figure out how to prevent those nanocracks, or heal them once they form, or prevent them from propagating,” Demkowicz says, “this would be the kind of thing you would use to improve the lifetime or safety of a component.”

'Plausible and Exciting'

William Gerberich, a professor of chemical engineering and materials science at the University of Minnesota who was not involved in this research, says that the significance of disclinations in materials was initially reassessed a few years ago, MIT reports.

The new findings “have taken this one step further and suggested that wedge dislocations, in conjunction with stress-driven grain boundary migration, could actually heal cracks. This is indeed provocative [and] may be a plausible and exciting pursuit.”

The work was funded by the BP-MIT Materials and Corrosion Center.

   

Tagged categories: Cracks; Engineers; Failure analysis; Metals; Oil and Gas; Research

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