Study shows what happens when crystalline grains in metals reform at nanometer scales, improving metal properties.
From: MIT News Office
By David L. Chandler
May 20, 2022 -- For the
first time, researchers have described how the tiny crystalline grains that
make up most solid metals actually 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.
Forming metal into the
shapes needed for various purposes can be done in many ways, including casting,
machining, rolling, and forging. These processes affect the sizes and shapes of
the tiny crystalline grains that make up the bulk metal, whether it be steel,
aluminum or other widely used metals and alloys.
Now researchers at MIT
have been able to study exactly what happens as these crystal grains form
during an extreme deformation process, at the tiniest scales, down to a few
nanometers across. The new findings could lead to improved ways of processing
to produce better, more consistent properties such as hardness and toughness.
The new findings, made
possible by detailed analysis of images from a suite of powerful imaging
systems, are reported today in the journal Nature Materials, in a
paper 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 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
applied a variety of empirically developed methods for reducing the sizes of
the grains in a piece of solid metal, generally by imparting various kinds of
strain through deforming it in one way or another. But it’s not easy to make
these grains smaller.
The primary method is
called recrystallization, in which the metal is deformed and heated. This
creates many small defects throughout the piece, which are “highly disordered
and all over the place,” says Schuh, who is the Danae and Vasilis Salapatas
Professor of Metallurgy.
When the metal is
deformed and heated, then all those defects can spontaneously form the nuclei
of new crystals. “You go from this messy soup of defects to freshly new
nucleated crystals. And because they're freshly nucleated, they start very
small,” leading to a structure with much smaller grains, Schuh explains.
What’s unique about the
new work, he says, is determining how this process takes place at very high
speed and the smallest scales. Whereas typical metal-forming processes like
forging or sheet rolling, may be quite fast, this new analysis looks at
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,” says Schuh.
Such a high-speed
process is not just a laboratory curiosity, he says. “There are industrial
processes where things do happen at that speed.” These include high-speed
machining; high-energy milling of metal powder; and a method called cold spray,
for forming coatings. In their experiments, “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 says.
Using a laser-based
system to shoot 10-micrometer particles at a surface, Tiamiyu, who carried out
the experiments, “could shoot these particles one at a time, and really measure
how fast they are going and how hard they hit,” Schuh says. Shooting the
particles at ever-faster speeds, he would then cut them open to see how the
grain structure evolved, down to the nanometer scale, using a variety of
sophisticated microscopy techniques at the MIT.nano facility, in collaboration
with microscopy specialists.
The result was the
discovery of what Schuh says is a “novel pathway” by which grains were forming
down to the nanometer scale. The new pathway, which they call nano-twinning
assisted recrystallization, is a variation of a known phenomenon in metals called
twinning, a particular kind of defect in which part 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 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 the experiments they
did using copper, the process of bombarding the surface with these tiny
particles at high speed could increase the metal’s strength about tenfold.
“This is not a small change in properties,” Schuh says, and that result is not
surprising since it’s an extension of the known effect of hardening that comes
from the hammer blows of ordinary forging. “This is sort of a hyper-forging
type of phenomenon that we’re talking about.”
In the experiments,
they were able to apply a wide range of imaging and measurements to the exact
same particles and impact sites, Schuh says: “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.”
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.
The research was
supported by the U.S. Department of Energy, the Office of Naval Research, and
the Natural Sciences and Engineering Research Council of Canada.
https://news.mit.edu/2022/crystalline-grains-metals-0520
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