Atomic-scale features of phase boundaries in hot deformed Nd–Fe–Co–B–Ga magnets infiltrated with a Nd–Cu eutectic liquid

• Published in: Acta materialia 2014-09, Vol.77, p.111-124
• Main Authors: Woodcock, T.G., Ramasse, Q.M., Hrkac, G., Shoji, T., Yano, M., Kato, A., Gutfleisch, O.
• Format: Article
• Language: English
• Subjects: Applied sciences; Coercive force; Coercivity; EELS; Enrichment; Exact sciences and technology; Hard magnets; Interfacial segregation; Liquids; Magnets; Metals. Metallurgy; Neodymium; Phase boundaries; TEM; Unit cell
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2014.05.045

Hot deformed Nd–Fe–Co–B–Ga magnets were infiltrated with a Nd–Cu eutectic liquid, resulting in a 71% increase in coercivity to μ0Hc=2.4T without the use of Dy, and a 22% decrease in remanence, attributed to the dilution effect. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy have been used to reveal the structure and chemical composition of phase boundaries in the magnets on the atomic scale. The results showed that the Nd–Cu liquid penetrated the ≈1nm thick intergranular regions. The coercivity increase following infiltration was therefore attributed to improved volume fraction and distribution of the intergranular phases. Co enrichment in the outermost 1–2 unit cells at several {001} and {110} surfaces of the Nd2(Fe,Co)14B crystals was shown for the infiltrated sample. The as-deformed sample did not appear to show this Co enrichment. Molecular dynamics simulations indicated that the distorted layer at an {001} surface of a Nd2(Fe,Co)14B grain was significantly thicker with higher Co surface enrichment. The magnetocrystalline anisotropy may be reduced in such distorted regions, which could have a detrimental effect on coercivity. Such features may therefore play a role in limiting coercivity to a fraction of the anisotropy field. Interfacial segregation of Cu between Nd2(Fe,Co)14B and the Nd-rich intergranular phase occurred in the infiltrated sample. Step defects in Nd2(Fe,Co)14B {001} surfaces, a half or a whole unit cell in height, were also observed.