In-situ modeling of an FeBYNb metallic glass revealing the Invar effect

Fe-based metallic glasses universally show an exceptionally low coefficient of thermal expansion due to their ferromagnetic interactions. This effect is called the Invar effect and is rarely seen in crystalline materials. With the loss of ferromagnetic ordering at the Curie temperature, the coefficient of thermal expansion increases by up to one order of magnitude. It has been shown that only Fe is responsible for generating and controlling the Invar effect in Fe-based metallic glasses. However, there is no atomistic explanation of how the disordered atomic arrangement enables it. By combining experimental data of synchrotron X-ray diffraction and extended X-ray absorption fine structure experiments via reverse Monte Carlo modeling, we obtain a series of 3D structural models of an (Fe71.2B24Y4.8)96Nb4 bulk metallic glass. Comparison with related ternary metallic glasses shows remarkably similar short- and medium-range order. The models reveal that the Invar effect is not caused by structural rearrangements or changes in coordination number. Instead, the magnetic contributions to the interatomic forces stabilize the structure in the ferromagnetic state.