Effect Of Grain Boundary Diffusion On Structure And Properties Of Ce-containing Hot-deformed Magnets

The rapidly growing application demands of Nd-Fe-B permanent magnets have caused the excessive consumption of major rare earth raw materials such as Nd,Pr,Dy,and Tb,while the high abundance rare earths La and Ce have been overstocked,which has led to serious imbalance in the use of rare earth resources.Because of the abundant reserves and low price of rare earth element Ce,the preparation of Ce-containing magnets by introducing Ce into Nd-Fe-B has attracted wide attention.However,the magnetic properties of magnets,especially the coercivity,deteriorate seriously with the increase of Ce content.Therefore,how to enhance the coercivity of magnets with high Ce content is currently the main problem.Hot-deformed magnets with nanocrystalline structure have the potential to obtain high coerciveity and the grain boundary diffusion technology is an effective means to improve the coercivity.In this paper,high performance hot-deformed magnets with high Ce content were fabricated by hot-pressing and hot-deformation process and the coercivity was further improved by Pr-Cu and DyF₃ grain boundary diffusion,respectively.The effect of grain boundary diffusion on the structure and properties of Ce-containing hot-deformed magnets was discussed,and the mechanism of coerciveity enhancement was explained.The main results are as follows:High performance hot-deformed magnets with Ce content up to 30 wt.%were prepared,in which the coercivity exceeded 14.0 kOe,which is comparable to commercial sintered Nd-Fe-B magnets of grade 38M.After 3 wt.%Pr-Cu diffusion,the magnetic properties are comparable to grade 33H magnets,indicating the advantages of low cost and high magnetic properties.The microstructure shows that the continuous and thick non-ferromagnetic or weak ferromagnetic grain boundary phase formed after Pr-Cu diffusion isolates adjacent grains and weakens the exchange coupling between grains.It also suppresses the size and aspect ratio of platelet shaped grains,which is beneficial to improve the coercivity.The decrease of the remanence and maximum magnetic energy product is caused by the poor texture and the increased volume ratio of non-magnetic phase.According to the stability analysis,after the diffusion of Pr-Cu,the temperature stability of the coerciveity is improved and the resistance to humidity and heat is not changed much.The coercivity of Ce-containing hot-deformed magnets was greatly increased after DyF₃diffusion,and further improved after heat treatment.It shows that the increase range in coercivity of magnets after DyF₃ diffusion is higher than that of Pr-Cu.After DyF₃ diffusion,it can also form the continuous and thick grain boundary phase and suppress the size and aspect ratio of platelet shaped grains,which is conducive to the coercivity enhancement.The heat treatment promotes the grain boundary diffusion and forms a thicker grain boundary phase,which is beneficial to further strengthen the coercivity.But the growth of some grains limits the increase of coercivity to some extent.The Dy-containing shell structure formed by the partial diffusion of Dy into the main phase increases the magnetic anisotropy field,which is the main reason to improve the coercivity.After DyF₃ diffusion,the poor texture,the increased volume ratio of non-magnetic phase and the decreased saturation magnetization from Dy into the main phase are the reasons for the decrease of the remanence and maximum magnetic energy product.The magnetic domain structure shows that the original"dendritic-like"reverse domain is transformed into a"dot scattered-like"reverse domain.The main reason is that the formation of the Dy-containing shell structure enhances the nucleation field of the main phase,and the thickened grain boundary phase improves the pinning effect.The temperature stability of the coerciveity and remanence is improved after DyF₃ diffusion,and the temperature stability of the coerciveity is further optimized after heat treatment.The high performance Ce-containing hot-deformed magnets studied in this paper not only develop a resource-saving magnet preparation method for low and middle-end permanent magnet applications,but also have certain guidance and reference significance for the research of high abundance permanent magnet materials.