Featured Research

from universities, journals, and other organizations

Self-healing materials could arise from finding that tension can fuse metal

Date:
October 9, 2013
Source:
Massachusetts Institute of Technology
Summary:
An unexpected result shows that in some cases, pulling apart makes cracks in metal fuse together.

A screen capture from the computer simulation of the molecular structure of a metal alloy. The boundaries between microcystalline grains are the white lines forming hexagons. A small crack (dark horizontal bar just right of bottom center) mends itself as the metal is put under stress.
Credit: Guoqiang Xu and Michael Demkowicz

It was a result so unexpected that MIT researchers initially thought it must be a mistake: 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.

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.

"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?'"

The answer turned out to lie in 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 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.

A computer simulation of the molecular stucture of a metal alloy, showing the boundaries between microcystalline grains (white lines forming hexagons), 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. Simulation courtesy of Guoqiang Xu and Michael Demkowicz

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."

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."

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


Story Source:

The above story is based on materials provided by Massachusetts Institute of Technology. The original article was written by David L. Chandler. Note: Materials may be edited for content and length.


Journal Reference:

  1. G. Q. Xu, M. J. Demkowicz. Healing of Nanocracks by Disclinations. Physical Review Letters, 2013; 111 (14) DOI: 10.1103/PhysRevLett.111.145501

Cite This Page:

Massachusetts Institute of Technology. "Self-healing materials could arise from finding that tension can fuse metal." ScienceDaily. ScienceDaily, 9 October 2013. <www.sciencedaily.com/releases/2013/10/131009130124.htm>.
Massachusetts Institute of Technology. (2013, October 9). Self-healing materials could arise from finding that tension can fuse metal. ScienceDaily. Retrieved July 23, 2014 from www.sciencedaily.com/releases/2013/10/131009130124.htm
Massachusetts Institute of Technology. "Self-healing materials could arise from finding that tension can fuse metal." ScienceDaily. www.sciencedaily.com/releases/2013/10/131009130124.htm (accessed July 23, 2014).

Share This




More Matter & Energy News

Wednesday, July 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Robot Parking Valet Creates Stress-Free Travel

Robot Parking Valet Creates Stress-Free Travel

AP (July 23, 2014) 'Ray' the robotic parking valet at Dusseldorf Airport in Germany lets travelers to avoid the hassle of finding a parking spot before heading to the check-in desk. (July 23) Video provided by AP
Powered by NewsLook.com
Boeing Ups Outlook on 52% Profit Jump

Boeing Ups Outlook on 52% Profit Jump

Reuters - Business Video Online (July 23, 2014) Commercial aircraft deliveries rose seven percent at Boeing, prompting the aerospace company to boost full-year profit guidance- though quarterly revenues missed analyst estimates. Bobbi Rebell reports. Video provided by Reuters
Powered by NewsLook.com
Europe's Car Market on the Rebound?

Europe's Car Market on the Rebound?

Reuters - Business Video Online (July 23, 2014) Daimler kicks off a round of second-quarter earnings results from Europe's top carmakers with a healthy set of numbers - prompting hopes that stronger sales in Europe will counter weakness in emerging markets. Hayley Platt reports. Video provided by Reuters
Powered by NewsLook.com
9/11 Commission Members Warn of Terror "fatigue" Among American Public

9/11 Commission Members Warn of Terror "fatigue" Among American Public

Reuters - US Online Video (July 22, 2014) Ten years after releasing its initial report, members of the 9/11 Commission warn of the "waning sense of urgency" in combating terrorists attacks. Mana Rabiee reports. Video provided by Reuters
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins