Ji Ma Takes Fresh Look at Diffusion in Shape Memory Alloys

Ji Maknows a lot about shape memory alloys, a versatile metal material typically used in medical implants. Prior to joining the University of Virginia School of Engineering and Applied Science, while working toward his Ph.D. at Texas A&M University, Ma conducted research on this unique class of materials that can be deformed and recover their previous shape by raising or lowering temperature. Ma stumbled upon a mystery about a behavior in these alloys that continues to confound him nearly a decade later. But now, thanks to a grant from the National Science Foundation, he'll be able to pick up where he left off — and get down to the truth. Ma's dissertation research focused on beta titanium shape memory alloys, an alternative to standard titanium shape memory alloys, which contain nickel — an element that can be harmful to humans and therefore not appropriate for medical implants. While conducting his experiments, Ma observed something odd about the alloy's behavior. Because the material has a very high melting point of around 2,200 degrees Celsius, he expected his samples to remain stable at room temperature. “I would put a sample aside on my work bench to discover a few weeks later that it had become stronger but less ductile at the same time,” said Ma, now an assistant professor of materials science and engineering at UVA. “If I retested a sample, I got different results depending on how long they have been sitting around. At first I thought I might have done something wrong, but then realized the results are repeatable. I have always been very curious about why this happens.” The alloy gained strength through a process called precipitation hardening, in which atoms group together within the material, forming tiny clusters with different composition from the rest of the material. When these small precipitates — each just a few nanometers — appeared inside the alloy, they initiated strengthening of the material at the cost of ductility. When Ma calculated the atomic motion required to get precipitates to form, he discovered that the atoms were moving billions of times faster than they should under standard diffusion theory — the common-sense notion that things move slower at lower temperatures, like sugar taking more time to dissolve in iced tea than hot tea. “That's still a mystery, why the atoms are moving so quickly, relatively speaking, in a metal at room temperature,” Ma said. “We didn't have time to dig down and find a cause.”