Alloy discovered that barely changes with temperature

Alloy discovered that barely changes with temperature
by Robert Schreiber
Berlin, Germany (SPX) Feb 04, 2025

Most metals tend to lengthen as temperatures climb. The Eiffel Tower, for instance, can stand roughly 10 to 15 centimeters taller in summer than in winter because of this thermal expansion. While such elongation is a normal phenomenon in metals, it poses challenges in precision applications, prompting researchers to look for substances that remain dimensionally stable under heat. One well-known candidate is invar, an iron-nickel blend that shows extremely low thermal expansion. However, its detailed physical explanation had long been uncertain.

A joint effort by theoretical specialists at TU Wien in Vienna and experimentalists from the University of Science and Technology Beijing has now shed new light on invar's behavior. With advanced computational modeling, the scientists uncovered the precise mechanism behind invar's tiny expansion and applied that insight to create a pyrochlore magnet alloy with even more impressive zero expansion qualities. Over a wide range of more than 400 Kelvins, this new compound's length varies by only about one ten-thousandth of one percent per Kelvin.

"The higher the temperature in a material, the more the atoms tend to move - and when the atoms move more, they need more space. The average distance between them increases," explains Dr Sergii Khmelevskyi from the Vienna Scientific Cluster (VSC) Research Centre at TU Wien. "This effect is the basis of thermal expansion and cannot be prevented. But it is possible to produce materials in which it is almost exactly balanced out by another, compensating effect."

His team generated computer simulations describing how magnetic materials behave at finite temperatures on an atomic scale. "This enabled us to better understand the reason why invar hardly expands at all,' says Khmelevskyi. "The effect is due to certain electrons changing their state as the temperature rises. The magnetic order in the material decreases, causing the material to contract. This effect almost exactly cancels the usual thermal expansion."

It was already recognized that magnetic ordering played a decisive part in the invar phenomenon, but the computing breakthroughs at TU Wien made it possible to precisely identify each detail of this process and predict how it might appear in other substances. "For the first time, a theory is available that can make concrete predictions for the development of new materials with vanishing thermal expansion," says Sergii Khmelevskyi.

The pyrochlore magnet with Kagome planes emerged from these predictions, thanks to a close collaboration with Prof. Xianran Xing and Ass. Prof. Yili Cao at the University of Science and Technology Beijing. This latest alloy uses four different elements: zirconium, niobium, iron, and cobalt. "It is a material with an extremely low coefficient of thermal expansion over an unprecedentedly wide temperature range," says Yili Cao.

One key to the alloy's capabilities is that it does not have a perfectly uniform crystal arrangement. Instead, it displays a subtle chemical variability across different regions, with some pockets containing slightly more cobalt than others. Each local configuration reacts in its own way to shifts in temperature. By offsetting these changes at the micro level, the overall expansion is almost zero.

Such a robust material could prove vital in situations prone to large temperature swings or in contexts that demand extreme accuracy, such as aerospace systems or advanced electronic instruments.

Research Report:Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets