The bizarre behavior of metals that can shift shape, only to metamorphose back to their original form, is by turns intriguing and, perhaps, slightly intimidating to the untrained eye. But understanding such alloys, often called “intelligent” or “shape memory” materials, poses a singular challenge to Ryan Elliott, a scientist who himself deftly shifts from discussions of materials science to physics to multi-scale simulation techniques.
“As a graduate student at the University of Michigan I was struck by the properties of these exotic materials,” says Elliott. “My advisors were interested in modeling these materials by simulating them from the atomistic level, and when I got the (DOE Computational Science Graduate) Fellowship, we were able to pursue that.”
What started as a graduate research project became a full-fledged research program in 2005, when Elliott received a faculty appointment in the department of aerospace engineering and mechanics at the University of Minnesota.
“Fundamentally, my simulations try to describe the way that atoms exert forces on each other when they are brought together in a crystal,” he says.
“What I am predicting are the different crystal structures or phases that are possible, for a particular material, based on a model of how its atoms apply forces to each other.”
Essentially, Elliott’s computer models describe the physical properties that allow a metal – an alloy of titanium and nickel for example – to shift from one crystal shape to another at low temperatures and revert faithfully to the original shape when heated. His work is stretching the boundaries of elasticity theory to incorporate behaviors that current theory does not sufficiently support.
While the research is basic in the sense that Elliott predicts atomic-level forces, companies ranging from aerospace and automobile manufacturers to surgical and medical device makers are eager to develop applications for shape memory materials.
One of the most successful applications for shape memory materials has been in aerospace, where a “patch” was developed to repair leaks that appear in airplane hydraulic and fuel lines. When a line springs a leak, a cold shape-memory alloy sheath is installed over the line. As it warms to room temperature, the sheath contracts, forming a seal that efficiently repairs the leak.
“They work incredibly well,” Elliott says. “To my knowledge, these patches have never failed.”
Some shape memory alloys are biologically inert and have been used successfully for orthodontia, reducing the need for frequent adjustment of braces, and for self-expanding coronary artery stents – hollow mesh tubes that are used to keep arteries open after a balloon angioplasty procedure.
Despite their promise, the cost and time-consuming nature of devising reliable new materials is hampering development of shape memory alloys. Elliott’s simulations are providing a theoretical underpinning to shape memory behavior and with it the potential to discover entirely new materials through computer simulation. The research is so promising that Elliott was awarded a five-year National Science Foundation (NSF) Faculty Development (CAREER) grant, the foundation’s most prestigious early career award.
“Currently, materials scientists are good at predicting how these materials react when they make a small change,” he says. “The goal of the computational work I am doing is to allow these scientists to make bigger changes and be able to accurately predict what will happen to the properties of the alloy.”
Indeed, Elliott’s work may one day reshape our understanding of these mysterious materials.