Magnetostriction in Invar-type alloys: the role of co-substitution for Ni and dimensional stability under magnetic field

Invar alloys, renowned for their low thermal expansion, are critical in high-precision optical instruments and other applications requiring exceptional dimensional stability. However, their magnetostrictive behavior under magnetic fields poses challenges for this stability, particularly in environments with fluctuating magnetic fields. This study investigates the effects of Co substitution for Ni, and in some cases, for both Ni and Fe in Fe-Ni-Co-based low thermal expansion alloys, focusing on their magnetostrictive and magnetic properties. Alloys with varying Co content were synthesized and analyzed for magnetostriction under controlled magnetic fields, alongside thermal expansion and magnetic behavior. Results reveal that Co substitution enhances magnetic anisotropy, suppressing magnetostriction over a broader range of magnetic field intensities. Alloys with low Co content exhibit minimal magnetostriction in weak magnetic fields, while higher Co content alloys maintain stability even under stronger fields, making them suitable for environments with fluctuating magnetic fields. Notably, alloys with 15.3% Co content show unique negative magnetostriction at low fields, attributed to increased magnetic anisotropy and strain-induced effects. The close correlation between saturation magnetization and the onset of linear magnetostriction highlights the interplay between magnetization and magnetostrictive mechanisms. In contrast, Cr-based non-ferromagnetic Invar alloys demonstrate minimal magnetostriction due to antiferromagnetic ordering, offering an alternative for non-magnetic applications. These findings underscore the potential of Co substitution to optimize magnetostrictive properties, providing a pathway for developing advanced low thermal expansion materials with enhanced dimensional stability.