Atomic structure evolution related to the Invar effect in Fe-based bulk metallic glasses

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid State Chemistry

Background:

• The Invar effect, characterized by minimal thermal expansion, is a known phenomenon in iron-based bulk metallic glasses.
• Understanding the atomic-scale origins of the Invar effect in these materials remains a challenge.

Purpose of the Study:

• To investigate the atomic-scale mechanisms behind the Invar effect in specific Fe-B-Y-Nb and Fe-B-Y-Mo bulk metallic glasses.
• To correlate macroscopic thermal expansion with atomic structural changes around the Curie temperature.

Main Methods:

• Utilizing in-situ synchrotron-based high-energy X-ray diffraction.
• Analyzing structural transformations of (Fe71.2B24Y4.8)96Nb4 and (Fe73.2B22Y4.8)95Mo5 bulk metallic glasses near their Curie temperatures.

Main Results:

• Observed shifts in the first two diffraction peaks consistent with macroscopic thermal expansion, confirming the Invar effect.
• Established a strong correlation between the nearest-neighbor Fe-Fe pair distance and macroscopic thermal expansion.
• Identified atomic-scale manifestations of the Invar effect in average Fe-Fe distances and higher-order atomic shells.

Conclusions:

• In-situ X-ray diffraction effectively elucidates the Invar effect in Fe-based metallic glasses at the atomic scale.
• The Invar effect is not solely a macroscopic phenomenon but has a distinct atomistic equivalent.
• Atomic structural changes, particularly in Fe-Fe pair distances and multi-species shells, underpin the Invar effect.