Febrication And Magnetic Properties Of TbCu₇-Type Sm-Co Based High Temperature Permanent Magnetic Alloys

The permanent magnets require good magnetic properties and high working temperature of the order of 150³00? or even 400⁵00? for using in the fields of engines and aerospace.Due to the large anisotropic fields,high curie temperatures and low temperature coefficients,the TbCu₇-type SmCo₇ permanent magnetic alloys have attracted increasing attention in high temperature permanent magnetic alloys.In this thesis,TbCu₇-type SmCo₇alloy ribbons and magnets were prepared by melt-spinning and spark plasma sintering?SPS?,respectively.The effects of element addition and sintering process on the microstructure and magnetic properties of TbCu₇-type SmCo₇ high temperature permanent magnetic alloys were studied.Firstly,the SmCo₇ metastable phase was stabilized with Hf element substituted for Co element.The results showed that SmCo_7-xHf_x?x=0.1,0.15,0.2,0.3?alloy ribbons with different Hf substitution amounts can obtain stable TbCu₇ type SmCo₇ phase when fabricated at the speed of 40 m/s.As the Hf substitution amount increased,the lattice constants a and c increased,and the c/a ratio was 0.82⁰.83.The grains changed from long-axis crystals to equiaxed crystals and the average grain size decreased from 3.00?m to 1.41?m.SmCo_6.8Hf_0.2alloy ribbons exhibited best magnetic properties with intrinsic coercivity=1084 kA/m,remanence=0.55 T.The saturation magnetization was decreased from 0.76 T to 0.51 T.The initial magnetization process was suppressed.In high and low temperature cycling working environment,the properties of ribbons were stable and the intrinsic coercivity was 264 kA/m and remanence was 0.42 T at 400?.The temperature coefficients of remanence was-0.066%/? and temperature coefficient of coercivity was-0.20%/? in the temperature range of 27⁴00?.Secondly,the SmCo_6.8Hf_0.2B_0.4 alloy ribbons with high coercivity and low temperature coefficient was obtained by adding B element.The addition of B element has a grain refinement effect on the SmCo_6.8Hf_0.2B_x?x=0,0.1,0.2,0.3,04,0.5?alloy.With the increase of B element,HfB₂ phase gradually appeared,and acted as pinning site in the alloys.The coercivity mechanism changed from nucleation type to pinning type as the content of B increased in the alloys.The coercivity first increased and then decreased and the remanence gradually decreased as a function of B-content.SmCo_6.8Hf_0.2B_0.4 alloy ribbons exhibited best magnetic properties with intrinsic coercivity=1630 kA/m,remanence=0.52 T and remanence ratio=0.78.A high coercivity of 497 kA/m was obtained at 500?,while temperature coefficient of coercivity of-0.15%/? was achieved in the temperature range27⁴00?.With the increase of annealing temperature from 400 to 900?,the magnetic properties first increased and then decreased,and the optimal values?intrinsic coercivity=1721 kA/m,remanence=0.54 T?were obtained after annealing at 600?.The exchange coupling among the grains was weakened after annealing at 600?.After annealing at 700?,the magnetic properties decreased,and the coercivity mechanism changes from pinning to nucleation.The coercivity of the alloys decreased to 275 kA/m after annealing at 900?.Finally,the effects of SPS process on the magnetic properties and microstructure of SmCo_6.8Hf_0.2B_0.4 magnets prepared by crystalline ribbons powder and amorphous powder were studied.The SmCo_6.8Hf_0.2B_0.4 SPSed magnet by crystalline ribbons powder formed TbCu₇-type SmCo₇ structures at different sintering temperatures.The magnet exhibited best magnetic properties with intrinsic coercivity=1291 kA/m,remanence=0.5 T after sintering at 650?.With the increase of the sintering temperature,the grains grew in size,the magnetic properties and the remanence ratio decreased significantly.The elements of the ribbons sintered magnet were uniformly distributed in the SPSed magnets.The ingot of SmCo_6.8Hf_0.2B_0.4 alloy was subjected to ball milling for 40 h to obtain amorphous powder,subsequently magnet was obtained by SPS process.The magnet was composed of Co phase,CoO phase,SmO phase and HfB₂ phase.Remanence of 0.93 T was obtained in the amorphous powder SPS magnet.Due to long time milling and smaller grains,the amorphous powders were susceptible to oxidation.