Scientists finally explain why invar alloys don’t expand

Thermal expansion, a property seen in most materials, causes them to expand with rising temperature and contract when it cools down. This phenomenon has diverse applications, from hot air balloons to thermostats, and is considered while designing structures like railroads and buildings. The increased temperature makes atoms in the material vibrate more, pushing away from neighboring atoms, resulting in increased overall size due to decreased density.

However, there is a special class of metal alloys known as Invars, which defy this principle and maintain their size and density over a wide temperature range. Combining iron and nickel in a specific proportion yields these unique alloys. They find use in precision-demanding applications, such as clocks and telescopes. Despite their historical significance, the reason behind Invars’ behavior remained unknown until recently.

Researchers from the lab of Brent Fultz discovered the secret behind Invars’ steadiness. In their paper titled “Thermodynamic explanation of the Invar effect,” published in Nature Physics, they found that while thermal expansion is usually related to entropy, the concept central to thermodynamics, Invars display counteractive factors that prevent their expansion at higher temperatures. This explanation sheds light on the anomalous behavior of Invar alloys.

Phonon DOS of Invar at different pressures. Experimental 57Fe partial DOS curves measured by NRIXS (black curves) are compared with the calculated DOS (orange curves), and both are normalized to 1. The error bars show the standard deviation of sequential NRIXS scans (on average, a total of 39,480 counts were recorded for each inelastic spectrum). At ambient conditions, the NRIXS measurement is in excellent agreement with the DOS measured by INS (blue markers). Credit: Nature Physics (2023). DOI: 10.1038/s41567-023-02142-z

Lohaus and the Caltech research group explored the link between magnetism and the unusual behavior of Invars. These alloys, which maintain their size and density despite temperature changes, have long been suspected to have a connection with magnetism. To investigate this, the researchers used an experimental setup with diamond anvil cells to measure both magnetism and atomic vibrations of small samples of Invar under high pressures.

The experiments revealed a delicate balance between thermal expansion from atom vibrations and magnetism, both of which changed with temperature and pressure. Using a new theoretical approach, the team identified how the interactions between vibrations and magnetism contributed to this balance. Such coupling between these factors has implications for understanding thermal expansion in other magnetic materials and could aid in developing materials for magnetic refrigeration.

The experimental setup involved passing powerful X-rays through the Invar sample within the diamond anvil cell to detect atomic vibrations and monitor the spin state of the electrons. As the temperature increased, the electrons’ spin state changed, affecting their distance from neighboring electrons and parent atoms. Simultaneously, the atoms vibrated more, taking up more space. The counteracting effects of spin state changes and atomic vibrations kept the Invar’s size constant, providing a new explanation for the Invar effect that had been a scientific mystery for over a century.

The co-authors of the paper include graduate students Pedro Guzman and Camille M. Bernal-Choban, visitor Claire N. Saunders, as well as researchers from the Argonne National Laboratory, Weizmann Institute of Science, and Boston College.

Source: California Institute of Technology

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