Three years ago, Richard James and his coworkers at the University of Minnesota discovered a metallic film—an alloy of zinc, gold, and copper—that seemed to flout the rules of materials science. When chilled to about −40 °C, it collapsed from a high-symmetry crystalline phase, austenite, to a low-symmetry one, martensite. The phase change reversed itself just as abruptly when the film was reheated. (The microscope image shows martensite, left, advancing into austenite, right, as the alloy is cooled.)

Such phase transformations normally take a mechanical toll; typical metals show wear and tear after just a few cycles across the phase transition. But the Minnesota group’s alloy, Zn 45 Au 30 Cu 25 , remained pristine through tens of thousands of cycles. Aided by one of the world’s brightest x-ray sources, James and collaborator Sherry Chen (Hong Kong University of Science and Technology) now think they’ve figured out why.