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MIT Unveils Grain Size Study for Stronger Metals

There are many ways to create metals in shapes needed for different purposes. However, these processes affect the sizes and shapes of the tiny crystalline grains that make up the bulk metal. A new Massachusetts Institute of Technology (MIT) study determines what happens as these crystal grains form during an extreme deformation process, down to a few nanometers across at the tiniest scales.

The properties of a particular metal depend on its structure—the smaller the grain size, the stronger the resulting metal. For the past 80 years, an overarching trend in all metallurgy, in all metals, has been to reduce grain sizes to improve strength and toughness.

Using various empirically developed methods, scientists tried to reduce the sizes of grains in a piece of solid metal. But, making these grains smaller is an arduous task. Recrystallization is often used method for this purpose. The process involves deforming and heating the meal, creating small defects throughout the piece. These defects can spontaneously form the nuclei of new crystals.

The characteristic of the new work is determining how this process takes place at very high speed and on the smallest scales. For the study, scientists analyzed the images from a suite of robust imaging systems.

MIT professor Christopher Schuh said, “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.”

“Such a high-speed process is not just a laboratory curiosity. 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 excessive rates, and because the rates are so high, no one has been able to dig in there and look systematically at that process before.”

Scientists used a laser-based system to shoot 10-micrometer particles at a surface. The system could shoot these particles one at a time and measure how fast they are going and how hard they hit. They then used various sophisticated microscopy techniques to cut them open to see the evolution in grain structure at a nanometer scale.

Schuh said, “The discovery of this 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 get these stripey patterns where the metal flips its orientation and flips back again, like a herringbone pattern.”

In the experiments, they did use copper, the process of bombarding the surface with these tiny particles at high speed could increase the metal’s strength about tenfold. Schuh said, “This is not a small change in properties, and that result is not surprising since it’s an extension of the known hardening effect that comes from the hammer blows of ordinary forging. This is a hyper-forging type of phenomenon that we’re talking about.”

“In the experiments, we applied a wide range of imaging and measurements to the same particles and impact sites. So, we end up getting a multimodal view. We get different lenses on the same 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.”

MIT postdoc Ahmed Tiamiyu said, “Because the new findings guide 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. They’re not just hypothetical lines. For any given metals or alloys, if you’re trying to determine if nanograins will form, if you have the parameters, slot it 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.”

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