Phase Formation and Magnetic Properties of Nanocomposite Nd-Fe-B Adjusted by Small Amount of Dy and Co

摘要

In order to improve and stabilize the magnetic properties of nanocomposite Nd_2Fe_(14)B/α-Fe magnetic alloys by a compositional adjustment, small amount of Dy and/or Co was added to Nd_9Fe_(84)B_7 alloys. DTA analysis on the amorphous Nd_(9-x)Dy_xFe_(84)B_7(x = 0 ～ 1.5) and Nd_9Fe_(84-y)Co_yB_7(y = 0～3.0) both melt spun at 40 m·s~(-1) revealed that crystallization of the alloys took place as the soft magnetic phases were crystallized, and then the hard magnetic Nd_2Fe_(14)B was precipitated from them. While α-Fe and a metastable 1:7 ( TbCu_7-type) phase were formed simultaneously in Dy and Co-free alloys, they were crystallized separately at different temperatures after Dy or Co was added. This phase separation occurred more clearly in the Dy-treated alloys and the other soft magnetic phase Fe_3B was also stabilized by Dy and/or Co. The 1:7 phase that was stabilized by Dy and/or Co was not eliminated at 700°C, decreasing magnetic properties of the alloys. It was eventually disappeared above 725°C, but Fe_3B was not eliminated even at 750°C when Dy was added more than 0.5 at % or Co was added more than 2.0 at %. Amount of Nd_2Fe_(14)B in the alloys tended to increase as Dy addition increased, whereas Co addition did not lead to any appreciable change in the ratio of α-Fe and Nd_2Fe_(14)B. Moreover, Dy addition apparently increased coercivity of an alloy while Co addition had a beneficial effect on remanence. The grains in the Dy-treated alloys were usually finer than those in the Co-treated alloys. The grain size of both α-Fe and Nd_2Fe_(14)B in the alloys exhibiting mr ≥ 0.72 was in the range of 20 ～ 40 nm or even larger 50 nm, which is larger than the theoretical optimum size ( ～ 10 nm). Typical magnetic properties obtained from a Nd_(7.5)Dy_(1.5)Fe_(82.5)Co_(1.5)B_7 alloy annealed for 12 min at 725°C were ~iH_c = 4.85 kOe, B_r=11.32 kG, (BH)_(max) = 15.73 MGOe, and m_r = 0.73.