An alloy of several metals that does not expand when heated

Amazing stuff! This could be a breakthrough!

"... Now, a collaboration between theoretical researchers ... has led to a decisive breakthrough: using complex computer simulations, it has been possible to understand the invar effect in detail and thus develop a so-called pyrochlore magnet – an alloy that has even better thermal expansion properties than invar. Over an extremely wide temperature range of over 400 Kelvins, its length only changes by around one ten-thousandth of one per cent per Kelvin. ...

It had already been known that the magnetic order in the material is responsible for the invar effect. But only with the computer simulations from Vienna, it became possible to understand the details of this process so precisely that predictions for other materials could be made. “For the first time, a theory is available that can make concrete predictions for the development of new materials with vanishing thermal expansion,” ...

The so-called pyrochlore magnet.

In contrast to previous invar alloys, which only consist of two different metals, the pyrochlore magnet has four components: Zirconium, niobium, iron and cobalt. “It is a material with an extremely low coefficient of thermal expansion over an unprecedentedly wide temperature range,” ...

This remarkable temperature behaviour has to do with the fact that the pyrochlore magnet does not have a perfect lattice structure that always repeats itself in exactly the same way. The composition of the material is not the same at every point, it is heterogeneous. Some areas contain a little more cobalt, some a little less. Both subsystems react differently to temperature changes. This allows the details of the material composition to be balanced point by point in such a way that the overall temperature expansion is almost exactly zero. ..."

From the abstract:

"Design of zero thermal expansion (ZTE) materials is urgently required as dimension stable component in widespread modern high-precision technologies. Local chemical order has been of great importance in engineering advanced inorganic materials, but its role in optimizing the ZTE is often overlooked.

Herein, we propose local composition heterogeneity for developing superior ZTE via a nonstoichiometric strategy.

A remarkably low coefficient of thermal expansion of αa = +1.07 × 10−6 K−1 is achieved from 3 to 440 K in a quaternary Zr-Nb-Fe-Co pyrochlore magnet, which is the widest temperature range among known cubic ZTE metals.

High-resolution synchrotron X-ray diffraction and magnetisation measurements reveal that all the Bragg peaks split as resulting from two cubic phases with different magnetic orders.

Scanning transmission electron microscopy, Mössbauer spectroscopy and theoretical calculations indicate that such phase separation intimately derive from excess Co dopant preferentially clustering on the Fe pyrochlore-lattice (16d) and simultaneously yielding an antisite Fe on Zr/Nb sublattice (8a).

The Co content in pyrochlore-lattice has weaker exchange interactions than that of Fe, but the antisite Fe introduces extra positive exchange interactions between 8a-16d site.

Local composition fluctuation of Co and Fe thus affects interplanar ferromagnetic order of pyrochlore-lattice and balances the normal phonon effect successively on heating. Superior corrosion resistance to both acid and alkaline conditions merits potential applications of the present ZTE metal."

The metal that does not expand | TU Wien "Breakthrough in materials research: an alloy of several metals has been developed that shows practically no thermal expansion over an extremely large temperature interval."

Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets (open access)

Credits: Keine Ausdehnung: Neues Metall hält den Temperaturen stand