Ever pulled your chewing gum into a long, narrow string? In fact, most solids will stretch some 1 or 2 percent without breaking. Superplastic alloys, on the other hand, will elongate more than 200 percent under the right kinds of physical stress. Superplasticity makes these alloys especially useful for building the thin, strong metal parts found in jet engines and high-performance cars, like the Bugatti EB110 pictured at right. But material scientists have never really been able to explain the property. Well, until now, maybe.
In this week's issue of the journal Physical Review Letters, Miguel Lagos of the University of Chile in Santiago put forth a new theory on superplasticity--one that so far fits all of the data on six different alloys tested. His idea involves the spaces, or grain boundaries, between the tiny crystals that make up superplastic alloys.
When superplastic alloys are stretched, the crystals jockey with one another for space while staying intact. At their boundaries, though, vacancies--or missing atoms in the crystal structure--sometimes open up. Lagos believes that such vacancies move in closed loops as individual grains push against each other: a vacancy opens between two grains and an atom fills it, forcing open another vacancy, and so on. Thus, the vacancies move through the material like a conveyor belt, facilitating the motion of the grains without damaging them