Rapid Solidification of Sm-Co Permanent Magnets
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TRODUCTION
PERMANENT magnets based on intermetallic compounds between Sm and Co represent the first and second generation of rare earth permanent magnets, dating back to the 1960s.[1,2] The early Sm-Co magnets were centered on the SmCo5 compound. The SmCo5 compound has tremendous intrinsic magnetic properties, including a magnetocrystalline anisotropy and high Curie temperature. Later, permanent magnets based on the Sm2Co17 compound were developed. The SmCo5 and Sm2Co17 compounds are structurally related; the Sm2Co17 structure is obtained by replacing a Sm atom with a pair of Co atoms. The pair of Co atoms is commonly referred to as ‘‘dumbbells.’’ The Co dumbbells are arranged on the lattice either randomly to form the TbCu7-type structure[3] or in an ordered fashion in one of two ways to form either the Th2Ni17-type structure or the Th2Zn17-type structure.[4] The disordered TbCu7-type structure is metastable but can be stabilized through the addition of alloying elements such as Ti or Zr[5,6,7] or rapid cooling.[8,9] The disordered structure is critically important in obtaining the excellent magnetic properties in powder-processed Sm-Co permanent magnets. Proper heat treatment results on the formation of the ordered Sm2Co17 phase and SmCo5 from the disordered TbCu7-type SmCo7 phase. Appropriate alloying additions aid in controlling the scale of the structure and the microchemistry of the phases, resulting in very high coercive forces, and recently resulted in excellent properties at elevated temperatures.[10,11,12] However, the heat treatment required to develop the appropriate microstructure is complicated. V.K. RAVINDRAN, Graduate Student and J.E. SHIELD, Associate Professor, are with the Department of Mechanical Engineering, Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588, USA. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Phase Transformations in Magnetic Materials’’, which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS/MPMD and ASMI-MSCTS Phase Transformations Committee. Article published online May 10, 2007. 732—VOLUME 38A, APRIL 2007
Recently, bonded magnets have gained a significant share of the permanent magnet market, especially finding applications in consumer electronics. While Nd-Fe-B-based permanent magnets are the most widely used magnetic material in bonded magnet applications, their applications are confined to lower temperatures because of the low Curie temperature of Nd2Fe14B. SmCo–based materials are attractive candidates for elevated temperature applications because of their high Curie temperatures. However, the magnetization is lower in Co-based materials. An increase in magnetization can be accomplished by increasing the concentration of the magnetic element, which in this case is Co. Nanocomposite permanent magnets have proved to be an effective means to increase the magnetization and energy products of high-energy permanent magn
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