Monday, October 14, 2013

How a microstructure can self-heal by pulling it apart

Surprising finding could lead to self-healing materials that repair incipient damage before it has a chance to spread
October 14, 2013
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A computer simulation of the molecular structure of palladium, showing the boundaries between microcystalline grains (white lines forming hexagons). Note the small crack (dark horizontal bar just right of bottom center of upper image) that mends itself as the grain boundary migrates (moves). This simulation was one of several the MIT researchers used to uncover this new self-healing phenomenon. (Credit: G. Q. Xu/MIT)
Under certain conditions, putting a cracked piece of metal under tension — that is, exerting a force that would be expected to pull it apart — has the reverse effect, causing the crack to close and its edges to fuse together, MIT researchers  have discovered.
The surprising finding could lead to self-healing materials that repair incipient damage before it has a chance to spread.
The results were published in the journal Physical Review Letters in a paper by graduate student Guoqiang Xu and professor of materials science and engineering Michael Demkowicz.
Why is this happening?’ It’ based on how grain boundaries interact with cracks in the crystalline microstructure of a metal — in this case nickel, which is the basis for “superalloys” used in extreme environments, such as in deep-sea oil wells.
By creating a computer model of that nickel microstructure and studying its response to various conditions, “We found that there is a mechanism that can, in principle, close cracks under any applied stress,” Demkowicz says.
How self-healing occurs in “disclinations”

Most metals are made of tiny crystalline grains whose sizes and orientations can affect strength and other characteristics. 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.
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 had been 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.”
When stress heals
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.
Having discovered this mechanism, the researchers plan to study how to design metal alloys so cracks would 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. Engineering metals’ microstructure to generate disclinations could slow the progression of this type of failure, Demkowicz says.
Such failures can be “life-limiting situations for a lot of materials,” Demkowicz says, including materials used in aircraft, oil wells, and other critical industrial applications. 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.”
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. Xu and Demkowicz, he says, “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.
A computer simulation of the molecular structure of a metal alloy, showing the boundaries between microcystalline grains (white lines forming hexagons). It shows a small crack (dark horizontal bar just right of bottom center) that mends itself as the metal is put under stress. This simulation was one of several the MIT researchers used to uncover this new self-healing phenomenon. (Credit for simulation: Guoqiang Xu And Michael Demkowicz)

Abstract of Physical Review Letters paper
We present a new mechanism — discovered using molecular dynamics simulations — that leads to complete healing of nanocracks. This mechanism relies on the generation of crystal defects known as disclinations by migrating grain boundaries. Crack healing by disclinations does not require any compressive loads applied normal to the crack faces and even occurs under monotonic tensile loading. By closing small cracks and suppressing the propagation of others, this mechanism may provide a novel way of mitigating internal damage that influences ductility in nanocrystalline metals.


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