Presentation Information
[O3-5]Rapid sintering of microcrystalline Nd-Fe-B systems: Challenges and opportunities
*Tomaž Tomše1, Mihaela Rebernik1, Fabian Burkhardt1, Lindrit Krasniqi1, Sašo Šturm1, Kristina Žužek1 (1. Jožef Stefan Institute, Department for Nanostructured Materials (Slovenia))
Keywords:
Nd-Fe-B,Rapid sintering,SPS,Microstructure refinement
Sintered Nd-Fe-B permanent magnets are typically fabricated using conventional powder metallurgy, which relies on high-temperature sintering without applied pressure. This approach leads to grain growth and offers limited control over the magnets’ microstructure. Rapid sintering techniques, such as spark plasma sintering (SPS), offer an alternative route by enabling lower processing temperatures and shorter sintering cycles. However, the nonequilibrium conditions introduced by SPS can influence phase stability and microstructural homogeneity, presenting both challenges and opportunities for the final magnetic performance.
We investigated the impact of SPS-specific conditions on the microstructure and magnetic properties of samples prepared from anisotropic, microcrystalline powders. Key challenges identified were i) decomposition of the Nd2Fe14B matrix phase due to Joule overheating at the particle-particle contacts and ii) incomplete formation of a thin Nd-rich grain-boundary film due to hydrogen-related effects1. Reducing the electrical currents in the powder bed during consolidation mitigated the decomposition of the hard-magnetic matrix, thereby improving the phase composition2. Additionally, by implementing a controlled degassing of hydrogen before sintering, significant improvements in grain-boundary phase distribution were achieved.
Our findings indicate that rapid powder consolidation performed at ≈880 °C can refine the final microstructure, yielding a ≈33% reduction in grain size compared to conventionally sintered samples. This microstructural refinement contributed to an increase in coercivity of over 15%, while the temperature coefficient of coercivity was reduced from −0.65 to −0.58%/°C. These results underscore the potential of SPS for improved processing of standard Nd-Fe-B powders while highlighting critical process parameters that must be controlled to maximize performance. Ongoing work focuses on further optimization of the microstructure of rapidly sintered magnets and improvement of their crystallographic texture.
1 Tomše T., Microstructure refinement of Nd-Fe-B permanent magnets prepared via spark-plasma sintering, IEEE Tans. Magn., vol. 60, 2024
2 Tomše T. et al., Unravelling the intricacies of micro-nonuniform heating in field-assisted sintering of multiphase metallic microstructures, J. Mater. Process. Technol., vol. 328, 2024
We investigated the impact of SPS-specific conditions on the microstructure and magnetic properties of samples prepared from anisotropic, microcrystalline powders. Key challenges identified were i) decomposition of the Nd2Fe14B matrix phase due to Joule overheating at the particle-particle contacts and ii) incomplete formation of a thin Nd-rich grain-boundary film due to hydrogen-related effects1. Reducing the electrical currents in the powder bed during consolidation mitigated the decomposition of the hard-magnetic matrix, thereby improving the phase composition2. Additionally, by implementing a controlled degassing of hydrogen before sintering, significant improvements in grain-boundary phase distribution were achieved.
Our findings indicate that rapid powder consolidation performed at ≈880 °C can refine the final microstructure, yielding a ≈33% reduction in grain size compared to conventionally sintered samples. This microstructural refinement contributed to an increase in coercivity of over 15%, while the temperature coefficient of coercivity was reduced from −0.65 to −0.58%/°C. These results underscore the potential of SPS for improved processing of standard Nd-Fe-B powders while highlighting critical process parameters that must be controlled to maximize performance. Ongoing work focuses on further optimization of the microstructure of rapidly sintered magnets and improvement of their crystallographic texture.
1 Tomše T., Microstructure refinement of Nd-Fe-B permanent magnets prepared via spark-plasma sintering, IEEE Tans. Magn., vol. 60, 2024
2 Tomše T. et al., Unravelling the intricacies of micro-nonuniform heating in field-assisted sintering of multiphase metallic microstructures, J. Mater. Process. Technol., vol. 328, 2024
