Annealing phenomena in electrodeposited Invar foils

초록

The demand for ultrahigh-resolution organic light-emitting diodes (OLEDs) necessitates the extremely thin Fe–36 wt%Ni Invar foils, yet conventional pyrometallurgical rolling reaches a fundamental limit in producing foils below 10 μm due to microstructural heterogeneity. Here, we report the fabrication of ultrathin (<8 μm) Invar foils via electrodeposition, validated through a multi-institutional study across five independent laboratories. We demonstrate that the electrodeposition yields a high-energy, metastable body-centered cubic (α-BCC) phase with a nanocrystalline grain size of approximately 110 nm. Upon annealing at 600 °C, this nonequilibrium phase undergoes a reconstructive phase transformation into a defect-minimized face-centered cubic (γ-FCC) structure. Atom probe tomography (APT) and transmission electron microscopy (TEM) reveal that this bottom-up approach achieves atomic-scale compositional homogeneity with no detectable segregation, effectively bypassing the macro-segregation inherent to casting. We elucidate that this transformation is fundamentally driven by a thermally triggered electron configuration rearrangement, where a ∼2% spontaneous collapse of Fe atomic volume stabilizes the Invar phase. This microstructural engineering successfully recovers the magnetovolume effect, achieving a coefficient of thermal expansion (CTE) of 0.98 × 10−6 K−1, comparable to that of bulk standards. Furthermore, the optimized foils exhibit superior mechanical reliability, with a yield strength exceeding 850 MPa and uniform ductility. Our findings provide a scalable bottom-up metallurgical paradigm to overcome the thickness and quality trade-offs in traditional manufacturing.

키워드