Analytical electron microscopy of corrosion-resistant Nd-(Fe, Co, Ni, Ti)-B magnets

Abstract The microstructures of cast ingot, annealed ingots and sintered bodies of Nd 15 (Fe 0.585 Co 0.3 Ni 0.1 Ti 0.015 ) 77 B 8 alloy were investigated with analytical electron microscopes to clarify the changes of the phases with successive processing. Five phases were identified in the specimens, namely Nd 2 (Fe 0.6 Co 0.3 Ni 0.1 ) 14 B, Nd(Fe 1− x−y Co x Ni y ) 4 B, Nd(Ni 1− x Co x ), Nd(Fe 1− x−y Co x Ni y ) 2 and TiB 2 . In addition, (Nd 1− y TM y ) 2 O 3 (TM  Fe, Ni, Co) phase was also identified in the sintered specimen. An Nd-rich phase was not observed in the present specimens. The phases, except the main magnetic phase (Nd 2 TM 14 B), were different in crystal structure from those in ternary Nd-Fe-B magnets. The majority of NdTM 4 B dissolved to supply B atoms to TiB 2 during the sintering. Some of the Nd(Ni 1− x Co x ) phase reacted with O atoms absorbed during the milling and/or sintering process, and formed Nd 1− y TM y ) 2 O 3 precipitate during the sintering. The remaining Nd(Ni 1− x Co x ) phase precipitated as thin precipitates at grain boundaries. It is suggested that the phases enriched with Ni and the absence of the Nd-rich phase give rise to the high corrosion resistance of Nd-(Fe, Co, Ni, Ti)-B magnets.