Researchers at the Massachusetts Institute of Technology (MIT) (Cambridge, Massachusetts, USA) have engaged in a study to determine what happens to the tiny crystalline grains that make up bulk metals such as steel and aluminum as these grains form during an extreme deformation process, at the tiniest sale, down to a few nanometers across. The new findings from researchers could lead to improved ways of metal processing—including casting, machining, rolling, and forging—to produce better, more consistent properties such as hardness and toughness.
These findings, made possible by detailed analysis of images from a suite of powerful imaging systems, were reported in the journal Nature Materials in a paper written by 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.
“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 says.
In general, the smaller the grain size, the tougher the resulting metal. Striving to improve strength and toughness by making grain sizes smaller “has been an overarching theme in all of metallurgy, in all metals, for the past 80 years,” says Schuh, who is the Danae and Vasilis Salapatas Professor of Metallurgy.
Although metallurgists have long applied a variety of empirically developed methods for reducing the sizes of grains in a piece of solid metal, it has not been easy to do so. The primary method used by the researchers is called recrystallization, in which metal is deformed and heated, creates small defects that Schuh characterizes as “highly disordered and all over the place.” When the metal is deformed and heated, these defects spontaneously form the nuclei of new crystals, according to Schuh.
“You go from this messy soup of defects to freshly new nucleated crystals,” he explains. “And because they’re freshly nucleated, they start very small,” which in turn leads to a structure with much smaller grains.
What’s unique about the new work, Schuh says, is determining how this process takes place at very high speeds and the smallest scales. Whereas typical metal-forming processes such as forging or sheet rolling may be fast, this new analysis looks at processes that he says are “several orders of magnitude faster.”
“We use a laser to launch metal particles at supersonic speeds,” says Schuh. “To say it happens in a blink of an eye would be an incredible understatement, because you could do thousands of these in the blink of an eye.”
The researchers used high-speed industrial processes that included high-speed machining, high-energy milling of metal powder, and a method called cold spray for forming coatings in their experiments. According to Schuh, the researchers “tried to understand that recrystallization process under those very extreme rates,” which is something that no one had done before in a systematic manner.
Tiamiyu carried out the research team’s experiments using a laser-based system that shot 10 mm particles at a surface. Shooting these particles one at a time at ever-faster speeds, he would first measure their speed and impact and then cut them open to see how the grain structure evolved down to the nanometer scale. The result was the discovery of what Schuh termed a “novel pathway,” or nano-twinning assisted recrystallization—this is a variation of a known phenomenon in metals known as twining, a particular kind of defect in which part of the crystalline structure flips its orientation. The team found that the higher the rate of these impacts, the more this process took place, leading to ever smaller grains as those nanoscale “twins” broke up into new crystal grains.
In its experiments, the MIT team applied a wide range of imaging and measurements to the exact same particles and impact sites. “So, we end up getting a multimodal view,” Schuh says. “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.”
Because the new findings provide guidance about the degree of deformation needed, how fast that deformation takes place, and the temperatures to use for maximum effect for any given specific metals or processing methods, they can be directly applied right away to real-world metals production, Tiamiyu says. The graphs they produced from the experimental work should be generally applicable. “They’re not just hypothetical lines,” Tiamiyu says. 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 developed, and the results should show what kind of grain structure can be expected from given rates of impact and given temperatures.
Source: MIT News, https://news.mit.edu.