INFLUENCE OF HEATING RATE ON MICROSTRUCTURE AND MAGNETIC PROPERTIES OF Fe3B/Nd2Fe14B NANOCOMPOSITE MAGNETS

The nanocomposite magnet, which is composed of exchange coupled nanocrystalline soft (Fe3B, α-Fe) and hard (Nd2Fe14B) magnetic phases, was first reported by Coehoorn et al. [1], and the theoretical basis of it was reported by Kneller and Hawig [2]. Nd-Fe-B based nanocomposite magnet has attracted considerable research interest due to its excellent magnetic properties with relatively low amount of rare-earth element. The grain size of the hard and soft phases in the alloy is the important microstructural parameter that determines the magnetic properties [2,3]. Microalloying and heat-treatments are two major factors to control the nanocomposite microstructures in these alloys. Several investigations reported that the microstructure and hard magnetic properties of αFe/Nd2Fe14B nanocomposites are sensitive to the heating rate of the post-annealing imposed on the melt-spun ribbon. Fang and Chin [4] studied rapid thermal annealing (RTA) on nanocrystalline magnets with various compositions and found that RTA is capable of yielding superior properties to those from conventional annealing in the alloys with higher α-Fe fraction. Gao et al. [5] reported that a higher heating rate inhibits the formation of intermediate phases and improves the magnetic properties of melt-spun Nd7Fe86Nb1B6 alloy. More recently, Kojima et al. [6] also showed improvement in magnetic properties of Fe-rich nanocomposite magnets by faster heating rate. Until recently, most studies on the influence of the heating rate were focused on αFe/Nd2Fe14B based nanocomposite magnets. The magnetic properties and microstructure of the Fe3B/Nd2Fe14B nanocomposite magnets are also influenced by the heating rate as well as the alloy composition. Recent report by Suzuki et al. [7] has shown that the final microstructure is sensitive to the heating rate in Nd5Fe77-xCrxB18 (x=0 and 3) alloys. They also showed that Cr addition inhibits the formation of metastable phases such as Nd2Fe23B3. However, no detailed microstructural investigation has been reported so far on the heating rate effect of Fe3B/Nd2Fe14B based nanocomposite magnet. The present paper aimed at studying the influence of heating rate on the microstructure and magnetic properties of the Fe3B/Nd2Fe14B nanocomposite permanent magnets during the crystallization of the Nd4.5Fe73B18.5Co2Cr2 amorphous alloy.