Preparation And Properties Of Co-Zr Based Permanent Magnetic Materials

Permanent magnetic materials,especially rare-earth permanent magnetic materials,have wide applications in various areas.However,with the rapidly increasing demand of rare-earth permanent materials,the supply-demand relationship of critical rare-earth resources tends to be significantly stressed across the world.Besides,the increasing use of Nd-Fe-B magnets is limited due to their low temperature stability and corrosion resistance.Among numerous non-rare earth magnetic materials,the Co-based hard magnetic materials have the merits of high temperature stability,high corrosion resistance and high anisotropic field,which can not only fill the performance gap of Nd-Fe-B magnets,but also make it possible to relieve the pressure of rare-earth resources.In this work,aiming to reduce the use of rare earth resources and improve the hard magnetic properties,the optimizations of composition and process of non-rare earth Co-Zr based alloys have been conducted.In addition,the multi-component Nd-Fe-B-Co-Zr alloys were prepared by combining Nd₂Fe₁₄B with Co₈₂Zr₁₈ alloys.The main research contents and results are as follows:Firstly,the phase composition,magnetic properties,temperature stability and corrosion resistance of melt-spun Co_100-xZr_x（x=14-20）and Co_82-yZr₁₈Fe_y（y=0,2,4,6,and 8）alloys were studied.The results show that the high magnetic properties can be obtained at x=16-18 on account of high content of hard magnetic phase and strong exchanged coupling effect.The highest coercivity of 3019 Oe was obtained at x=18 and the highest maximum magnetic energy product of 4.97 MGOe and remanence ratio of 0.67 were obtained at x=17.The substitution of Fe for Co decreases the magnetic properties in some extent.In addition,Co-Zr based alloys show excellent temperature stability and corrosion resistance,which can be further improved by Fe addition.The highest Curie temperature is up to 785 K at y=8.Secondly,in order to further improve the coercivity of Co-Zr based alloys,the melt-spun Co_78-xZr₁₆Cr_xSi₃B₃（x=0-4）alloys were prepared.The effects of Cr element and underlying mechanism were studied.The results show that the coercivity increases from 4.6 k Oe at x=0 to7.7 k Oe at x=4.For the sample with x=3,the coercivity was even enhanced to 8.0 k Oe after annealing treatment,increased by 74%.The roles of Cr element in Co-Zr-Si-B alloys include（1）suppressing the formation of fcc-Co phase,（2）the precipitated Cr elemental cannot only refine the grains but also enhance the coercivity by acting as pinning center,and（3）improving the exchange coupling effect resulting in a closed recoil loop.In addition,bulk Co_78-xZr₁₆Cr₃Si₃B₃ magnets have been sucessfully prepared by spark plsma sintering,though the process is in need of optimization.Finally,in order to reduce the the amount of rare-earth and improve the comprehensive magnetic properties,the Nd₂Fe₁₄B and Co₈₂Zr₁₈ alloys were combined with each other as an initial attempt.The results show that the melt-spun 70 wt.%Nd₂Fe₁₄B-30 wt.%Co₈₂Zr₁₈ alloy obtained at 42 m/s after annealing at 700℃for 60 min has the best magnetic properties with H_c=4497Oe,J_r=0.95T,J_s=1.37T,（BH）_max=10.37MGOe,and T_c=775 K.Although the further confirmation of two hard phases and the optimization of magnetic properties need to be conducted,the innovative idea of combining the advantages of two hard phases can be meaningful for the exploration of novel permanent magnetic materials in the future.