��ݮ����Ƶ����

It’s become one of the great challenges for the construction and transportation industries: how to make stronger metals.

Different processes have been developed and used to refine the process — everything from casting and machining to rolling — all with the goal of changing the size and shape of the tiny crystalline grains that compose steel, aluminum and other metals.

While metallurgists know the key is to make those grains smaller, it’s not an easy process.

Several researchers, including an assistant professor at the ��ݮ����Ƶ����, have been able to study what happens to those grains and, for the first time, have found a mechanism to make it happen.

, PhD, began the work while he was an MIT postdoc and recently had his findings published in the journal .

Invisible to the naked eye

The most common way of making grains smaller in metals is by deforming and heating them, otherwise known as recrystallization. However, determining how it takes place at faster speeds and on a smaller scale is difficult. Tiamiyu says:

I proposed we investigate the fate of grains by gradually increasing the particle impact speed, which, in turn, results in increased strain, strain rate and temperature of the participating materials. We discovered a rather new mechanism of grain refinement.

The resulting process is called “nanotwinning-assisted dynamic recrystallization.” The process occurs in far less than a second when a laser is used to launch particles invisible to the naked eye at very high velocities.

Creating the right environment

Tiamiyu, who is now an assistant professor in the Schulich School of Engineering’s , was tasked with carrying out the experiments with the laser-based system.

He would shoot very tiny (10-micrometre, or one-millionth of a metre in size) particles at a surface at different speeds, watch the particle-impact moment in real time, and then cut the metals open to see how the grain structure evolved, down to the nanometre scale (that’s one-billionth of a metre).

Not only was Tiamiyu’s team able to unravel the mystery nanograins formation, but a documented set of conditions to repeat the process was also created.

“We developed a user-friendly map in this work that can be used to predict the onset of nanograins for any metal,” says Tiamiyu, whose team included three MIT professors, along with an MIT alumnus and a current student.

Far-reaching implications

Tiamiyu believes the metal-production industry will be able to make use of the findings and graphs right away, as they provide guidance on the degree of deformation needed, how fast that deformation takes place, and the temperatures needed to get the best result for each metal.

“They’re not just hypothetical lines,” he said in an . “If you’re trying to determine if nanograins will form, if you have the parameters, just slot it into our formulas and the results will show.”

While the research results have an overarching structural application, Tiamiyu says the findings could also be useful at a smaller scale.

“They could be used for lightweight applications that are strategic to minimize energy consumption and greenhouse emissions in the energy and transportation sectors,” he says.

“The smaller the grains, the stronger the metals, the lower the thickness or weight requirement for structural design, and, in turn, the lower the energy consumption and greenhouse gas emissions for combating global climate change.”

The research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the U.S. Department of Energy, and the Office of Naval Research.