Microstructures And Magnetic Properties Of Sm₂Co₁₇ Based Spark Plasma Sintered Magnets

Sm₂Co₁₇ based permanent magnets have been widely applied in the fields of national defense,aerospace,high-speed trains due to their high Curie temperature,superior comprehensive magnetic properties and good environmental stability.In this paper,Sm₂Co₁₇-based alloy ribbons and magnets were prepared by melt-spinning and spark plasma sintering(SPS)technology,respectively.The effects of element addition,sintering process,heat treatment process and hot deformation process on the microstructure,magnetic properties,high temperature properties and thermal stability of Sm₂Co₁₇-based alloys were studied.Firstly,The Sm_24.6Co₅₀Fe_17.4Cu_5.7Zr_2.3-xHf_x(x=0-2.0)ribbons were prepared by melt-spinning technique.The effect of Hf addition on the magnetic properties,microstructures of the alloy ribbons were systematically investigated.It is found that there is only Sm(Co,M)₇ phase in the alloy ribbons prepared at the wheel speed of 40m/s,the average grain size has been refined from 3.28?m at x=0 to 1.73?m at x=1.0.The optimal comprehensive magnetic properties J_r=0.61 T,H_cj=371 k A/m,(BH)_max=31 k J/m³ are obtained in Sm24.6Co50Fe17.4Cu5.7Zr1.3Hf1.0 alloy ribbons.Secondly,the SPSed magnets were prepared by powder of Sm_24.6Co₅₀Fe_17.4Cu_5.7Zr_2.3-xHf_x(x=0-2.0)ribbons.The full dense magnets were obtained at 950?under 50 MPa for 5 min.The micro-twin structure of 2:17R phase has been discovered in the spark plasma sintered magnets and Zr-Hf rich phases formed around grain boundaries as particulates because of their high melting point.The optimized experiments on the heat treatment process for the Sm_24.6Co₅₀Fe_17.4Cu_5.7Zr_1.3Hf_1.0 magnets were carried out by orthogonal experiments and the optimal comprehensive magnetic properties J_r=0.55 T,H_cj=887 k A/m,(BH)_max=45 k J/m³were obtained after heat treated.The grains grow up gradually and the Cu element distributed more uniformly during the heat treatment.The influence of Hf content on the magnetic properties and microstructure of annealed magnets were studied.The thickness of cell wall phase decreased from 9.52 nm at x=0 to 6.22 nm at x=1.0 and the density of the lamellar phase decreased from 0.03 nm^-1 at x=0 to 0.027 nm^-1 at x=1.0.In addition,the cell boundaries enriched with Cu while depleted with Fe and Co compared with the cellular phases.When x=0.5,the coercivity of the magnet is still 336 k A/m at 400?,the remanence was 0.41 T,and the coercivity temperature coefficient?was-0.17%/K in the temperature range of 300?673 K.Finally,the hot-deformed magnets were prepared from SPSed Sm_24.6Co₅₀Fe_17.4Cu_5.7Zr_2.3isotropic magnets.When the deformation temperature was 800?,deformation pressure was350 MPa,holding time was 5 min and holding pressure time was 1 min,the height reduction of the magnet reached 70%,and compared with the hot pressed magnet,the remanence of hot deformed magnet was enhanced from 0.39 T to 0.55 T.The composition of the alloy can be optimized by the addition of a certain amount of Sm.The remanence of the hot-deformed magnet with the addition of 10 wt.%Sm was 59%higher than that of the SPSed magnet.