Boundary structure modification and magnetic properties enhancement of Nd–Fe–B sintered magnets by diffusing (PrDy)–Cu alloy Minghui Tang, Xiaoqian Bao ⁎, Kechao Lu, Lu Sun, Jiheng Li, Xuexu Gao State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30 Xue Yuan Road, Beijing 100083, People's Republic of China abstract article info Article history: Received 16 December 2015 Received in revised form 14 February 2016 Accepted 14 February 2016 Available online xxxx The grain boundary diffusion process was applied to the commercial sintered Nd–Fe–B magnets using Pr 68 Cu 32 , Dy 70 Cu 30 and Pr 35 Dy 35 Cu 30 ribbons as direct diffusion source. The coercivities increased from 1114.4 kA/m for the original magnet to 1642.8 kA/m for the processed magnet by Pr 68 Cu 32 , higher than 1402.7 kA/m by Dy 70 Cu 30 . Microstructural investigations showed that the evident coercivity enhancement for the sample by Pr 68 Cu 32 was mainly attributed to continuous intergranular layers isolating Nd 2 Fe 14 B grains. Nevertheless, the coercivity enhancement of the sample by Dy 70 Cu 30 was mainly resulted from the magnetic strengthening of the extensive layers in Nd 2 Fe 14 B grains. © 2016 Elsevier B.V. All rights reserved. Keywords: Permanent magnet Grain boundary diffusion (PrDy)–Cu alloy Coercivity It is very essential to achieve Nd–Fe–B magnets with high coercivity at room temperature. The nucleation of magnetic domains from grain boundaries is a regular phenomenon, which causes magnetic reversal [1]. So the microstructure and chemistry of the grain boundary play an important role in coercivity of sintered Nd–Fe–B magnets. The introduction of Dy element is a beneficial procedure to increase the magnetocrystalline anisotropy of the Nd 2 Fe 14 B compound in the ex- tensive layers of the Nd 2 Fe 14 B grains where magnetization reversal starts. Recently, pure Dy metal [2] or its nonmetal compounds like Dy 2 O 3 [3], DyF 3 [4], or DyH 2 [5] have been selected to form the so- called core–shell structure. Metal nanoparticles like Al [6], Cu [7], and Al 85 Cu 15 [8] were also chosen as the intergranular additions to enhance corrosion resistance of sintered Nd–Fe–B magnets since they have higher standard electrode potentials than that of Nd. These metal or compounds with low melting points can also modify boundary struc- ture by improving the wettability between intergranular phase and ma- trix phase, which isolates the Nd 2 Fe 14 B grains in the magnet. Some investigations focused on Dy–metal compounds like Dy 73 Ni 9.5 Al 17 [9] and Dy 32.5 Fe 62 Cu 5.5 [10] by intergranular addition or grain boundary diffusion. The core–shell structure and more continuous grain boundary were achieved in the magnet by suitable annealing since these alloys have relatively low melting points. However, the anti-ferromagnetic coupling between Dy and Fe atoms causes reduction in overall rema- nence and magnetic energy product. The low reserves in the earth and high cost of Dy metal are also the obstacles. Recently, Nd–Cu and Pr– Cu alloys with low melting points were used for improving boundary microstructure and coercivity of HDDR [11], and hot deformed [12,13] and sintered [14,15] Nd–Fe–B magnets. In this present work, the grain boundary diffusion process was ap- plied to the commercial sintered Nd–Fe–B magnets using Pr 68 Cu 32 , Dy 70 Cu 30 and Pr 35 Dy 35 Cu 30 alloys, respectively. Especially, the alloy ribbons prepared by melt-spinning technique were directly flatted on the upper and lower surfaces of the commercial magnet as direct diffu- sion source. The coercivity of the magnet significantly increased after boundary diffusing and annealing treatment. The influence of boundary microstructure modification on coercivity enhancement of the magnet was discussed. The commercial Nd–Fe–B sintered magnets were used as the origi- nal magnets. The magnet was cut into cylinder shape with a dimension- al size of Φ 8 × 5 mm 3 by wire-electrode cutting. The ingots used as diffusion source with the composition of Pr 68 Cu 32 , Dy 70 Cu 30 and Pr 35 Dy 35 Cu 30 (at.%) were prepared by vacuum induction melting. Then the alloy ribbons were prepared by melt-spinning technique using the high vacuum quenching system with the copper roller speed about 8 m/s. The thickness of the ribbons was about 30 μm. The magnets and ribbons were polished by abrasive papers and cleaned by an ultra- sonic cleaner in alcohol. The original magnets, covered by pieces of rib- bons as diffusion source on the upper and lower surfaces, were set in the ceramic crucible. Then, these magnets were performed the diffusion treatment at 900 °C for 4 h following subsequent annealing at 500 °C for 2 h with the protection of high vacuum following high-purity argon. The alloy ribbons melted into liquid during diffusion treatment, which was beneficial to flow and diffuse, since the melting points of the Pr 68 Cu 32 (472 °C), Dy 70 Cu 30 (790 °C) and Pr 35 Dy 35 Cu 30 alloys were lower than the diffusing temperature. The room-temperature magnetic properties of the processed magnets by mechanical polishing were Scripta Materialia 117 (2016) 60–63 ⁎ Corresponding author. E-mail address: bxq118@ustb.edu.cn (X. Bao). http://dx.doi.org/10.1016/j.scriptamat.2016.02.019 1359-6462/© 2016 Elsevier B.V. All rights reserved. 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