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Y. Lia) Department of Materials Science & Engineering, National University of Singapore, 119260, Singapore (Received 12 June 2008; accepted 21 August 2008)

The glass formation in Fe-rich ternary Fe-B-Nd and quaternary (Fe,B,Nd)96Nb4 alloys has been studied and the best ternary and quaternary glass formers are located at Fe67B23Nd10 and (Fe68B25Nd7)Nb4 with critical diameters of 1 and 4 mm, respectively. For (Fe,B,Nd)96Nb4 alloys, the competing phases with glass were identified by monitoring the microstructure change. Fe14Nd2B was discovered to be one competing phase, which is the principle magnetic phase for Nd-Fe-B hard magnets. Composites with uniformly distributed Fe14Nd2B were formed for quaternary alloys with a diameter of 1.5 to 3 mm. Bulk hard magnets could be obtained by directly annealing the composites in a compositional area. A hard magnet with a coercivity of 1,100 kAm 1 and a maximum energy product, (BH)max, of 33 kJm–3 was obtained at (Fe67B23Nd10)96Nb4 by annealing. The combination of hard magnetic properties and the large critical sample size may make these alloys a commercially viable candidate for industrial applications.

I. INTRODUCTION

Cast Fe-B-Nd ingots do not show any hard magnetic properties due to the large grain size.1 Conventionally, the two ways to produce Fe-B-Nd permanent magnets are the traditional powder metallurgy (or sintering) approach2 and the melt rapid solidification technique of melt-spinning.1,3,4 It is only when the grain size is on a nanometer scale that the magnets show hard magnetic properties. Several steps are required to obtain a final product with optimal magnetic properties. The difficulty of these methods is a long production cycle accompanied with high manufacturing costs, not to mention contamination (by oxidation).5 It is of great significance therefore that if one can produce a bulk amorphous precursor, the final product can be simply obtained through a one step annealing process. This reduces the manufacturing cost, minimizes contamination, and shortens cycle time. Although the critical diameter of Fe-based bulk metallic glasses (BMGs) is larger than 10 mm6,7 and recently reached 16 mm,8 no BMG has yet been discovered in the Fe-B-Nd ternary system. The GFA for Fe-B-Nd alloys in the Fe-rich corner was studied extensively before9,10 and only ribbons of 50 mm were obtained. For the Fe-B-Nd based alloys, the largest reported critical diameter for amorphous formation was 1.2 mm in a six component

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0074 J. Mater. Res., Vol. 24, No. 2, Feb 2009

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system (Fe60.3Co9.2Nb2Nd3Dy0.5B25) and only its soft magnetic properties were reported.11 Bulk glass formation in the Fe-B-Nd system should be feasible as it generally satisfies the known rules.12 For example, the large negative heat of mixing: Fe-B 26 kJ/mol, B-Nd 49 kJ/mol, and Fe-Nd 1 kJ/mol; and the large atomic size mismatch: B-Fe 35%, B-Nd 55%, and Fe-Nd 31%. Furthermo