Presentation Information
[P2-11]Phase equilibria in Nd-based sintered magnets with Carbon
*Taichi ABE1, Tadakatsu OHKUBO1 (1. NIMS (Japan))
Keywords:
CALPHAD,grain boundary phase,Rare-earth oxides,computational thermodynamics
Carbon is a contaminant from organic lubricants and is considered harmful to the coercivity of the Nd-based sintered magnets [1]. To examine the effect of the carbon on the microstructure formation and phase equilibria of the grain boundary phases, in the present work, the CALPHAD-type thermodynamic database was constructed based on the previously created database for the Nd-based magnets with oxygen [1] where the two-sublattice ionic solution model was applied for the liquid phase with oxygen. Solid-solution phases were modelled as a substitutional solution and the Nd2Fe14B phase and intermetallic compounds were modelled as sublattice models. Thermodynamic calculation software packages (Thermo-Calc and PANDAT) were used to estimate phase equilibria and to optimize model parameters. Although it is a multi-component system, the alloy composition is simplified/reduced to the Fe-Nd-O-C-B quinary system, where binary subsystems (Fe-Nd, C-Nd, C-O, B-O, B-C, Fe-C), and key ternary subsystems (Fe-Nd-O, Fe-Nd-C, Fe-Nd-B) were examined and assessed. Carbon distributions in the sintered magnets (High carbon: Fe-14.17Nd-5.84B-1.29O-0.41C and Low carbon: Fe-14.52Nd-5.86B-1.70O-0.103C in at.%) were measured using SEM-EDS and TEM-EDS. It was observed that in the high-carbon specimen, most of the carbon was solved in the NdO phase, while no obvious segregation of carbon was detected in the low-carbon specimen. From the view of the Nd-O binary phase diagrams, oxygen can be classified as a liquid phase destabilizer. Since the lanthanide elements are consumed for the formation of oxides, the amount of remaining liquid decreases with increasing oxygen content. On the other hand, carbon can be a liquid phase stabilizer, it is expected that carbon addition may increase the amount of residual liquid in equilibrium with other solid phases. In the thermodynamic calculations using our database, it is accordingly that the amount of liquid phase increases with increasing carbon addition when no oxygen is included in the alloys. In the case when oxygen and carbon co-exist, carbon mainly goes to the NdO phase in the experimental observations. The present thermodynamic calculations suggest that the NdO phase is more stabilized when carbon co-exists in the specimens because of carbon segregation to the NdO phase. Consequently, in the alloys with carbon and oxygen together, the equilibrium amount of the NdO phase increases with carbon concentration and the amount of Nd-rich liquid decreases. Therefore, as it may affect the microstructure formations during sintering and heat treatments, it results in lower coercivity found in the previous work Acknowledgements: This work was partly supported by MEXT Program: Data Creation and Utilization-Type Material Research and Development Project (Digital Transformation Initiative Center for Magnetic Materials) Grant Number JPMXP1122715503 and NIMS materials open platform for magnet. References: [1] T.T. Sasaki, et al., Acta Mater., 84 (2015) 506-514. [2] T. Abe, et al., STAM, 2 (2021) 557-570. [3] H.L. Lukas, et al., “Computational thermodynamics”, Cambridge, (2007).