This phase-transforming metal never gets old

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, Zn₄₅Au₃₀Cu₂₅, 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.

Diffraction experiments that Chen performed at Lawrence Berkeley National Laboratory’s Advanced Light Source show that the alloy’s austenite and martensite lattices, despite having vastly different symmetries, can arrange themselves to match up almost perfectly at shared edges. As a result, austenite can grow within martensite, and vice versa, without introducing strain at the interfaces. And it’s interfacial strain that causes ordinary metals to crack and form dislocations during phase transformations. If Zn₄₅Au₃₀Cu₂₅’s lattice attributes can be replicated in other alloys—and James and his coworkers suspect they can—they could potentially guide the design of efficient multiferroic switches, microelectromechanical actuators, sensors, and other devices. (X. Chen et al., App. Phys. Lett. 108, 211902, 2016. 10.1063/1.49510010003-6951)