Cyclic healing removes defects in metals while maintaining strength

October 19, 2015 by Jocelyn Duffy, Carnegie Mellon University
Cyclic healing removes defects in metals while maintaining strength
Abrupt de-pinning and de-stabilization of a dislocation line due to repeated cyclic loading. (a) Evolution of nominal tensile stress during two selected cycles: 10 and 11 in loading sequence G4, described in Table 1. Figures (b) to (f) are still frames extracted from the recorded movie that corresponds to the stress state marked in (a). The direction of tensile straining is indicated by the arrows in Figs. (c) and (e). The scale bars in (b) to (f) represent 200 nm.

When designing a new material, whether for an airplane, car, bridge, mobile device, or biological implant, engineers strive to make the material strong and defect-free. However, methods conventionally used to control the amount of defects in a material, such as applying heat or mechanical stress, can also have undesirable consequences in terms of the material's strength, structure and performance.

An international team of researchers, including Carnegie Mellon University President Subra Suresh, Zhiwei Shan and colleagues from Xi'an Jiaotong University in China, Ming Dao and Ju Li from MIT, and Evan Ma from Johns Hopkins University, has developed a new technique called cyclic healing that uses repetitive, gentle stretching to eliminate pre-existing defects in metal . Their results have been published online today (Monday, Oct. 19) in the Proceedings of the National Academy of Sciences.

Most materials are made of crystals. When materials fail, it is usually the result of defects in the crystal or in the arrangement of multiple crystals in a polycrystalline structure. While much research has been done on metal fatigue at larger scales, new technologies are just now allowing researchers to see how atomic-scale defects nucleate, multiply and interact in materials subjected to monotonic or fatigue loading inside a high-resolution microscope.

In this study, the researchers used to look inside sub-micrometer-sized specimens of aluminum crystals as they subjected the samples to stressors like repeated, small-amplitude deformation or fatigue loading. They found that gentle cyclic deformation, a process that repetitively stretches the crystal, helps to unpin or shakedown rows of known as dislocations in the metal and move these dislocations closer to free surfaces in the sample. Image forces, which act to minimize the energy of the defects, attract the dislocations closer to the free surfaces and force them out of the crystal. As a result, the crystal "heals," becoming essentially free of pre-existing dislocations, thereby significantly increasing its strength.

This finding was surprising to researchers because cyclic deformation has an opposite effect in micro- and macro-scale metal crystals. In these larger samples, repeated stretching generally leads to the creation, accumulation and interaction of defects, which can lead to cracking and failure.

"This work demonstrates how cyclic deformation, under certain controlled conditions, can lead to the removal of from crystals of small volume," says Suresh, who holds the Henry L. Hillman President's Chair at CMU. "It also points to potential new pathways for engineering the defect structure of metal components in a variety of sub-micro-scale systems."

Explore further: 2-D materials' crystalline defects key to new properties

More information: Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1518200112

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12 comments

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RealScience
4 / 5 (4) Oct 19, 2015
This finding was surprising to researchers because cyclic deformation has an opposite effect in micro- and macro-scale metal crystals. In these larger samples, repeated stretching generally leads to the creation, accumulation and interaction of defects, which can lead to cracking and failure.

Makes sense - cyclic deformation allow defects to move around within the material. In a large sample most defects are very far from an edge and so are more likely to encounter and merge with other defects, producing larger defects weakening the material. In contrast in a very small sample all defects are near an edge of the material, and 'merging' with an edge eliminates the defect and so strengthens the material.
docile
Oct 19, 2015
This comment has been removed by a moderator.
Nik_2213
4.3 / 5 (3) Oct 19, 2015
Why do I get the persistent image of a flaming forge, and a Smith patiently shaping a hot work-piece by hammering ??
SciTechdude
3.7 / 5 (3) Oct 19, 2015
Did they heat and quench it 1000 times and then cut down a samurai with it?

On a less sarcastic note, it's cool that we have the technology to examine the individual crystals deforming and reforming in real time.
24volts
5 / 5 (1) Oct 20, 2015
Why do I get the persistent image of a flaming forge, and a Smith patiently shaping a hot work-piece by hammering ??


Because blacksmiths had used the technique for 100's of years at least. The scientists just finally figured out what was happening at atomic level... That's all.
RealScience
5 / 5 (4) Oct 20, 2015
Blacksmiths were my first thought as well, but as I continued reading I realized that the process used 'cyclic healing' and not 'cyclic heating'.

While this is somewhat related to hot-forging by blacksmiths, hot forging uses much higher temperatures and plastic deformation of the material being worked. In contrast, the process in the article is a cold process that uses elastic deformation of single crystals.

The PNAS link at the bottom of the article leads to a PDF of the original paper, which is full of details on the deformations used an their impact on defects.
RealScience
5 / 5 (3) Oct 20, 2015
Ah - I see lajib in the ratings again.
Let's see if lajib and hewewa continue their usual pattern of never making comments themselves while giving docile's comments 5s and everyone else's comments 1s.
Captain Stumpy
5 / 5 (3) Oct 20, 2015
Ah - I see lajib in the ratings again.
Let's see if lajib and hewewa continue their usual pattern of never making comments themselves while giving docile's comments 5s and everyone else's comments 1s.
@RealScience

it is part of zephir's sock army...
.... although he previously denied that he created the sock armies, did you notice that he is now posting from a previously reported sock puppet he used to use to downvote others with?

it would actually be funny if it wasn't so pathetic

RealScience
5 / 5 (2) Oct 20, 2015
@Captain - yes, docile has said that he is not lajib or hewewa, and until there is more than circumstantial evidence to the contrary, he gets the benefit of the doubt.

Getting a '1' from a true troll is a good thing because it generally means that a point was scored against the troll and the troll is showing his/her/its lack of guts in not responding face to face.

But whoever lajib and hewewa are, it's pathetic - mass-down-rating everyone else's posts without even reading them just to make docile's ratings better. That's even worse than people who auto-'1' posts because they don't like a given poster (which in some cases at least has a very high chance of being an accurate rating).
docile
Oct 20, 2015
This comment has been removed by a moderator.
24volts
not rated yet Oct 21, 2015
Blacksmiths were my first thought as well, but as I continued reading I realized that the process used 'cyclic healing' and not 'cyclic heating'.

While this is somewhat related to hot-forging by blacksmiths, hot forging uses much higher temperatures and plastic deformation of the material being worked. In contrast, the process in the article is a cold process that uses elastic deformation of single crystals.

The PNAS link at the bottom of the article leads to a PDF of the original paper, which is full of details on the deformations used an their impact on defects.


I just read it. As far as I can tell what they did was mechanically anneal crystals of aluminum. I want to see them anneal a piece maybe 6 inches long 3 wide and 1 inch thick. That will impress me otherwise I don't see much use in doing individual small crystals unless they are just doing basic research on how metals react to that process.
Vietvet
not rated yet Oct 21, 2015
"I just read it. As far as I can tell what they did was mechanically anneal crystals of aluminum. I want to see them anneal a piece maybe 6 inches long 3 wide and 1 inch thick. That will impress me otherwise I don't see much use in doing individual small crystals unless they are just doing basic research on how metals react to that process."

You must have missed the last paragraph in the article.

"This work demonstrates how cyclic deformation, under certain controlled conditions, can lead to the removal of defects from crystals of small volume," says Suresh, who holds the Henry L. Hillman President's Chair at CMU. "It also points to potential new pathways for engineering the defect structure of metal components in a variety of sub-micro-scale systems."

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