Mechanism Research Of Crystallization Of Nd-Fe Amorphous Alloys

Nanocomposite magnets consisting of a mixture of magnetically hard and soft phases are potential permanent magnets with excellent comprehensive performance. This nanocrystalline material could be prepared by crystallization of amorphous alloys. However, since there are lots of metastable phase in the ternary system of Nd-Fe-B and the phase transition is complex, the crystallization behavior of this system is not clear yet. So that it is significant to study the crystallization process of this system.In this paper, we mainly researched the crystallization processes of two types of nanocomposite magnets, Nd2Fe14B/α-Fe, Nd2Fe14B/Fe3B, and the effect of boron contents on the crystallization process.In this study, we analyzed the DSC curves deeply Furthermore, we studied the nucleation and growth of grains in the Nd2Fe14B/α-Fe system and the effect of wheel speed on crystallization process and optimize the crystallization process. In this paper, the matrix alloy compositions of Nd2Fe14B/α-Fe and Nd2Fe14B/Fe3B nanocomposite are Nd9Fe85Nb0.sB5.5 and Nd9Fe78.5Nb0.5B15. The main conclusions in this paper are as follows:(1) The crystallization of as-quenched alloys Nd9Fe85Nb0.5B5.5 prepared by the wheel speed of 18-24m/s follows the two steps:(Ⅰ)TbCu7-type metastable phase and a little amount of α-Fe precipitate from the matrix firstly.(Ⅱ)TbCu7-type metastable phase is decomposed into α-Fe and Nd2Fe14B phase, the two phases distribute uniformly.(2) In the matrix of Nd9Feg5Nb0.5B5.5, the crystallization mechanisms of α-Fe and Nd2Fe14B phase are different. α-Fe satisfies the JMA(Johnson-Mehl-Avrami) generalized model while Nd2Fe14B phase satisfies the SB(Sestak-Berggren) model. In addition, the crystallization of Nd2Fe14B phase is an autocatalysis reaction.(3) In the matrix of Nd9Fe85Nb0.5B5.55 the crystallization of as-quenched alloys prepared by relatively low speed is a two-step reaction that TbCu7-type metastable phase and a little amount of α-Fe precipitate from the matrix firstly; then TbCu7-type metastable phase is decomposed into α-Fe and Nd2Fe14B phase, the two phases distribute uniformly. However, the crystallization of as-quenched alloys prepared by relatively high speed (over 30m/s) is only one step that the Nd2Fe14B phase and α-Fe precipitate from the matrix directly. The crystallization processes are primarily influenced by initial phase structures.(4) The optimum technological condition of NdFeB/α-Fe preparation is determined. The matrix with the composition of Nd9Fe85Nb0.5B5.5 is prepared by the wheel speed of 24m/s, Then the as-quenched alloy is annealed at 730℃ for 4min, which could get the optimum magnetic performance. (BH)max=10.96MGOe, Br=8.36kGs, Hcj=7.75kOe. However, the as-quenched alloys prepared by over 30m/s could not get high performance in any conditions of annealing, for Nd2Fe14B phase and α-Fe are precipitated from the matrix directly and distribute nonuniformly.(5) The crystallization processes are different with the increasing of boron contents. Nd2Fe14B and α-Fe precipitated from the matrix at the beginning of crystallization, and then iron-rich clusters are formed in the amorphous matrix. The grains of a-Fe which are precipitated from the matrix of Nd9Fe78.5Nb0.5B12 satisfy the Oswald Ripening mechanism. Fe23B6 and Nd2Fe23B3 metastable phases are precipitated from the matrix of Nd9Fe75.5Nb0.5B15. Nd2Fe23B3 metastable phase could be decomposed into α-Fe,Fe3B and Nd1.1Fe4B4.