Speaker
Description
Nanocrystalline alloys represent an important class of soft magnetic materials. Their technological relevance stems from a combination of high saturation flux density, excellent permeability, and low core loss. Further improvements in magnetic performance can be achieved through careful compositional tuning and advanced processing techniques that optimize phase content and reduce grain size.
This work focuses on the ultra-rapid annealing technique, which involves compressing samples between pre-heated Cu blocks. The exceptionally high heating rates and short processing times promote the formation of finer nanocrystalline grains compared to conventional furnace annealing [1]. Two recent case studies on ultra-rapid annealing will be presented.
The first study examines the impact of elevated temperatures on the soft magnetic properties of ultra-rapidly annealed high-Bs Fe-Co-B-(Cu) and Fe-Co-Si-B-(P) nanocrystalline alloys with reduced metalloid content. Hysteresis loops were measured from room temperature (RT) to $300$°C using a Förster-type B-H loop tracer with a built-in furnace inside a magnetically shielded room. Understanding the thermal stability of ultra-rapidly annealed nanocrystalline alloys is essential for assessing their application potential, particularly for components exposed to high-temperature environments. Our experiments demonstrate that high-Bs Fe-Co-B-(Cu) alloys exhibit very good thermal stability, maintaining excellent soft magnetic characteristics between $30–250$°C, making them promising candidates for elevated-temperature applications.
The second study investigates the effects of conventional and ultra-rapid thermal processing on the amorphous-to-crystalline transformation and soft magnetic properties of Ni-rich Fe-Ni-Nb-B rapidly quenched ribbons. A sample with the composition (Fe$_{0.25}$Ni$_{0.75}$)$_{81}$Nb$_7$B$_{12}$ exhibits a significant degradation in soft magnetic properties after conventional annealing for $60$ min at $435$°C, with coercivity increasing to $550$ A/m due to the formation of large fcc-type (Fe-Ni)$_{23}$B$_6$ grains coexisting with fcc-FeNi nanograins. In contrast, ultra-rapid annealing for one second at $520$°C effectively suppresses the formation of unwanted (Fe-Ni)$_{23}$B$_6$ grains, resulting only in ultrafine fcc-FeNi grains ($5–8$ nm). This process reduces the coercivity to just $2.3$ A/m achieving a superior $H_c$ value for FeNi-based nanocrystalline alloys with permalloy-like ferromagnetic grains.
Acknowledgements
This research was supported by the projects APVV 23-0281, VEGA 2/0148/23 and JRP SAS-TUBITAK NOMAGRAD.
References
[1] B. Kunca et al., “Soft magnetic performance of ultra-rapidly annealed high-Bs Fe-(Co)-B nanocrystalline alloys at elevated temperatures,” Journal of Alloys and Compounds, vol. 911. Elsevier BV, p. 165033, Aug. 2022. https://doi.org/10.1016/j.jallcom.2022.165033
