Metals are one of the most essential classes of materials for modern life, prized for their strength, flexibility, conductivity, durability and aesthetic beauty. But you don’t always get all these properties in a single metal; in fact, those qualities are sometimes at odds with each other, says second-year mechanical engineering doctoral student Hossein Abbasi. One of the property tradeoffs engineers regularly encounter is between strength and ductility, the latter of which Abbasi says can be thought of as a metal’s elastic ability to temporarily deform and spring back to its original state. Typically, metals that are very strong aren’t very ductile, which makes them brittle. Conversely, metals that have high ductility usually sacrifice strength. The reason, Abbasi says, has to do with their crystalline structure — particularly the size of the “grains” that comprise the metal’s base architecture. Large grains, Abbasi says, make a metal more ductile, while smaller grains give it strength.
Materials scientists are trying to defy this basic rule of physics, however, with a new class of metals called gradient nano-grained metals. Unlike most metals, which have a singular grain size throughout their entire thickness, GNG metals have large grains toward the material’s interior that gradually taper into very small grains at the surface. This continuum of grain sizes gives GNG metals a rare combination of ductility and strength — and thus dozens of interesting possible applications. A turbine blade in an airplane engine, for example, must be both very strong and able to bounce back after impacts in order to avoid catastrophic failure. Similarly, Abbasi says, GNG metals could one day be an ideal material for vehicle suspension components, which have to be strong but also stand up to dynamic loads.
So if GNG metals have such desirable properties, why aren’t they in widespread use already? Abbasi says one reason is they’re still a relatively new class of materials, and as such, materials scientists and engineers don’t fully understand how they perform under a wide variety of conditions. Abbasi’s doctoral research, supervised by Associate Professor of Mechanical Engineering Lei Chen, is focused on helping engineers get a clearer picture of GNG metal behavior. Abbasi’s biggest contributions thus far are some advanced computer models, which map not only GNG metals’ ideal crystal structure but also defects within the metal. This makes his models much more representative of metals in the real world. “It’s almost impossible to create a perfect metal. In the real world, all metals have some defects, which we call geometrically necessary dislocations,” Abbasi explains, owing to the fact that the grain crystals don’t fit perfectly together. “So if we run our simulations without accounting for these voids in the underlying structure, we don’t get very accurate results compared to the real world.”
Using these highly detailed models, Abbasi can perform a battery of relatively quick, inexpensive tests. In one simulation, for example, he predicted how different grain sizes affect stress and strain behavior. In another, he modeled the effects of different strain rates on the metal’s tendency to “recrystallize,” which refers to the process in which large grains are subdivided into smaller grains as a result of applied stress. Simulations, of course, are only useful if they reflect what happens in the real world, so Abbasi then validated his simulated experiments with collaborators at Michigan State University, who put actual GNG materials through the same tests. This demonstrated that Abbasi’s models are able to provide highly accurate predictions of how the materials actually behave — making them an incredibly useful tool for engineers interested in GNG metal applications.
Abbasi’s work has won the UM-Dearborn doctoral student poster competition two years running — which is particularly impressive given he’s relatively new to the field of computer modeling and simulation. Abbasi came to UM-Dearborn from Iran, where he studied and then lectured at the University of Tabriz. His work focused on nanomaterials, but his research was focused on the manufacturing of experimental materials in the lab. When he was looking for a PhD program in the United States, he says he was attracted to Chen’s work because it combined manufacturing and modeling, giving him an opportunity to broaden his skillset.
Abbasi says his experience at an American university has been great so far, even if his studies will likely require him to be away from his family until his doctoral program is complete. With political relations strained between Iran and the U.S., he said he honestly didn’t know what to expect from Americans, and he was pleasantly surprised by how friendly most people are. “Right from the very start, in fact, when I arrived in Chicago, the border officer asked me to take everything out of my pockets, and I had some snacks I had brought with me from Iran,” he explains. “And he told me that I could not take them with me, that they had to go in the trash. But then a few moments later, he tapped me on the shoulder and brought me a package of nuts to replace the snacks. So here I am, my first minutes in the United States and a border officer is being very nice to me. That made me feel good about my new adventure in the United States.”
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Story by Lou Blouin
