MIT Scientists Unveil a Secret of Stronger Metals

New research reveals what happens when metal crystalline grains reassemble at nanoscale dimensions, enhancing metal characteristics.
Casting, machining, forging, and rolling are all methods for forming metal into the specific forms required for diverse uses. These operations have an impact on the sizes and shapes of the microscopic crystalline grains that make up the bulk metal, whether it be steel, aluminum, titanium, or other commonly used metals and alloys.

At the lowest scales, down to a few nanometers wide, researchers at MIT have been able to investigate exactly what happens as these crystal grains develop during an intensive deformation process. The new findings could lead to better processing methods that result in qualities like hardness and toughness that are more constant.
Former MIT postdoc Ahmed Tiamiyu (now assistant professor at the University of Calgary), MIT professors Christopher Schuh, Keith Nelson, and James LeBeau, former student Edward Pang, and current student Xi Chen describe their findings today in the journal Nature Materials in a publication.

“In the process of making a metal, you are endowing it with a certain structure, and that structure will dictate its properties in service,” Schuh adds. In general, the smaller the grain size, the stronger the resulting metal. Striving to improve strength and toughness by making the grain sizes smaller “has been an overarching theme in all of metallurgy, in all metals, for the past 80 years,” he says.

Metallurgists have long used a number of experimentally discovered procedures to reduce the size of the grains in a solid metal object, usually by applying various types of strain by deforming it in some way. However, making these grains smaller is difficult.

Recrystallization is the most common procedure, which involves deforming and heating the metal. This results in numerous minor flaws that are “highly disordered and all over the place,” according to Danae and Vasilis Salapatas Professor of Metallurgy Schuh.

All of those flaws can spontaneously form the nuclei of new crystals when the metal is bent and heated. “You go from this messy soup of defects to freshly new nucleated crystals. And because they’re freshly nucleated, they start very small,” Schuh continues, resulting in a structure with significantly smaller grains.

He claims that the current work is unique in that it identifies how this process occurs at extremely high speeds and on the tiniest scales. Typical metal-forming operations, such as forging or sheet rolling, can be relatively rapid, but this new study examines techniques that are “several orders of magnitude faster,” according to Schuh.

“We use a laser to launch metal particles at supersonic speeds. To say it happens in the blink of an eye would be an incredible understatement, because you could do thousands of these in the blink of an eye,” says Schuh.

He claims that such a fast process isn't merely a laboratory curiosity. “There are industrial processes where things do happen at that speed.” High-speed machining, high-energy milling of metal powder, and a coating process known as cold spray are among them.
“We’ve tried to understand that recrystallization process under those very extreme rates, and because the rates are so high, no one has really been able to dig in there and look systematically at that process before,” he adds.

Tiamiyu, who carried out the research, "could shoot these particles one at a time, and really measure how fast they are going and how hard they hit," Schuh adds. Using a range of sophisticated microscopy techniques at the MIT.nano facility in partnership with microscopy specialists, he would shoot the particles at ever-faster rates before cutting them open to study how the grain structure altered, down to the nanometer scale. 

As a result, a "new pathway" for grain formation down to the nanoscale scale was discovered, according to Schuh. The new method, dubbed nano-twinning assisted recrystallization, is a version of twinning, a well-known metal defect in which a portion of the crystalline structure flips its orientation. It’s a “mirror symmetry flip, and you end up getting these stripey patterns where the metal flips its orientation and flips back again, like a herringbone pattern,” he says. The researchers discovered that the faster the hits occurred, the more this process occurred, resulting in increasingly smaller grains as the nanoscale "twins" split up into new crystal grains. 

The technique of blasting the surface with these tiny particles at high speed could boost the metal's strength by tenfold in their copper experiments.  “This is not a small change in properties,”  Schuh adds, which is not surprising given that it's an extension of the recognized effect of hardening caused by typical forging hammer strikes.  “This is sort of a hyper-forging type of phenomenon that we’re talking about.”

They were able to apply a wide variety of imaging and measurements to the very same particles and hit locations in the trials, according to Schuh:  “So, we end up getting a multimodal view. We get different lenses on the same exact region and material, and when you put all that together, you have just a richness of quantitative detail about what’s going on that a single technique alone wouldn’t provide.”

The new findings can be immediately applied to real-world metals production because they provide guidance on the degree of deformation required, how quickly that deformation occurs, and the temperatures to use for maximum effect for any given specific metals or processing methods, according to Tiamiyu. They should be able to use the graphs they created from their experiments in other situations. 

“They’re not just hypothetical lines,” Tiamiyu explains. For any given metals or alloys, “if you’re trying to determine if nanograins will form, if you have the parameters, just slot it in there”into the formulas they devised for any particular metals or alloys, and the results should show what kind of grain structure can be predicted from given rates of impact and temperatures.
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