Reviewed by Alex SmithMay 23 2022
Metal can be formed into various designs in a variety of ways, such as casting, machining, rolling, and forging. These mechanisms have an impact on the sizes and shapes of the small crystalline grains that comprise the bulk metal, whether it is steel, aluminum, or another commonly used metal or alloy.
For the first time, researchers have described how the tiny crystalline grains that make up most solid metals form. Understanding this process, they say, could theoretically lead to ways of producing stronger, lighter versions of widely used metals such as aluminum, steel, and titanium. Image Credit: Courtesy of the researchers.
MIT scientists have now been able to examine what happens when these crystal grains form during an intense deformation at the smallest scales, down to very few nanometers across. The recent findings could lead to better processing methods that produce good, more sustained properties like hardness and strength.
Former MIT postdoc Ahmed Tiamiyu (currently an assistant professor at the University of Calgary), Christopher Schuh, Keith Nelson, and James LeBeau (MIT Professors), former student Edward Pang, and current student Xi Chen published the discoveries, enabled by a thorough analysis of images from a suite of powerful imaging systems, in Nature Materials.
In the process of making a metal, you are endowing it with a certain structure, and that structure will dictate its properties in service.
Christopher Schuh, Danae and Vasilis Salapatas Professor, Metallurgy, Massachusetts Institute of Technology
The tinier the grain size, in fact, the stronger the resulting metal. Striving to enhance strength and durability by reducing grain size “has been an overarching theme in all of the metallurgy, in all metals, for the past 80 years,” he says.
Metallurgists have long-used empirically evolved methods to minimize the sizes of the grains in a portion of solid metal, usually by imparting various types of strain by disintegrating it in some way. However, it is difficult to make these grains narrower.
The basic mechanism is recrystallization, which involves deforming and heating the metal. This results in numerous minor flaws throughout the piece, which is “highly disordered and all over the place,” says Schuh.
All of those flaws can spontaneously produce 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,” resulting in a structure with much smaller grains, Schuh explains.
He claims that the current work is exceptional in that it identifies how this process occurs at extremely high speeds and on the tiniest scales. While traditional metal-forming operations such as forging or sheet rolling can be relatively quick, this new study examines processes that are “several orders of magnitude faster,” Schuh says.
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.
Christopher Schuh, Danae and Vasilis Salapatas Professor, Metallurgy, Massachusetts Institute of Technology
He claims that such a fast process is not merely a laboratory fascination. “There are industrial processes where things do happen at that speed.”
These comprise high-speed machining, high-energy metal powder milling, and a cold spray coating process. In the research, “we’ve tried to understand that recrystallization process under those very extreme rates, and because the rates are so high, no one has been able to dig in there and look systematically at that process before,” he says.
Using a laser-based system to shoot 10-micrometer particles at a surface, Tiamiyu, who conducted the experiments, “could shoot these particles one at a time, and measure how fast they are going and how hard they hit,” Schuh says.
Using a range of advanced microscopy techniques at the MIT.nano facility in partnership with microscopy experts, he would shoot the particles at ever-faster rates and then cut them apart to study how the grain structure altered, down to the nanometer scale.
The outcome was the identification of a “novel pathway” for grain formation down to the nanoscale scale, according to Schuh. The new approach, termed nano-twinning assisted recrystallization, is a version of twinning, a well-known occurrence in metals in which a portion of the crystalline structure switches its orientation.
It is 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 these collisions occurred, the more this process occurred, resulting in smaller grains as the nanoscale “twins” split up into fresh crystal grains.
The technique of blasting the surface with these small particles at high speed could boost the metal’s strength by tenfold in their copper tests. “This is not a small change in properties,” Schuh says, and this is not surprising because it is an expansion of the known effect of hardening caused by conventional forging hammer strikes. “This is sort of a hyper-forging type of phenomenon that we’re talking about.”
Researchers were able to use a variety of imaging and measurement techniques on the identical particles and impact locations in the trials.
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.
Christopher Schuh, Danae and Vasilis Salapatas Professor, Metallurgy, Massachusetts Institute of Technology
The latest research can be immediately applied to real-world metals manufacturing since they assist the extent of deformation required, how quickly that deformation occurs, and the temperatures to use for maximum effect for any given particular metal or processing technique, according to Tiamiyu. Researchers should be able to use the graphs they created from their experiments in other situations.
“They’re not just hypothetical lines,” Tiamiyu says. For any given metal or alloy, “if you’re trying to determine if nanograins will form, if you have the parameters, just slot it in there” into the formulas they established, and the results should illustrate what kind of grain structure can be predicted at different impact speeds and temperatures.
The US Department of Energy, the Office of Naval Research, and the Natural Sciences and Engineering Research Council of Canada all contributed to the study.
Journal Reference:
Tiamiyu, A. A., et al. (2022) Nanotwinning-assisted dynamic recrystallization at high strains and strain rates. Nature Materials. doi.org/10.1038/s41563-022-01250-0.