When materials scientists want to create steel with specific properties—say, a certain combination of strength, hardness, and fracture resistance—they know how to approach the problem. They can add carbon and other elements to the iron in just the right quantities, and they can adjust the heat-treating process. Those interventions will have predictable effects because most steels are crystalline solids, with molecules organized in regular patterns.
But materials scientists know much less about how to predict and alter the mechanical properties of metallic glasses and other amorphous solids, whose molecules are arranged irregularly.
Michael Falk, A&S ’90, MS ’91, a professor of materials science and engineering, has been working on that problem for two decades—and with a new paper published last summer, he has taken a significant step forward. In the late 1990s, when he was a doctoral student at the University of California, Santa Barbara, Falk and his colleague James Langer developed mathematical models of shear transformation zones. According to the two scientists’ theory, these tiny, pre-existing weak zones are the first areas in an amorphous solid to undergo plastic deformation in response to an external stress.
Falk’s recent paper, published in Physical Review Letters, offers new support to the STZ theory. For this study, Falk and two French colleagues simulated the behavior of 50 glass samples placed under stress. These simulated glass models were tiny—only 10,000 atoms each—and two- dimensional. The simulations found that external stresses do, in fact, tend to act first on discrete, long-lived, highly plastic zones, just as the STZ theory predicts.
“These plastic zones are not only localized, but they also have a preferred orientation along which they will act,” Falk says. “They can’t be caused to flip or rearrange in just any direction.”