Control Of Microstructure And Magnetic Properties Of Pr₂Fe₁₄B/?-Fe Nanocomposite Magnets Via Composition Optimization

Nanocomposite magnets are expected to possess the potential of being next-generation superstrong permanent magnets due to their theoretical maximumemergy product up to 100MGOe.However,the practically obtained maximum energy products do not exceed 25MGOe,which are still significantly lower than the theoretical predicted value.The discrepancy between the actual and predicted energy products is attributed to the difficulty in achieving the optimum microstructure employed for theoretical models,and the most difficult one is the alignment of the easy axis of hard-magnetic phases on the premise of high fraction of soft-magnetic phases.For the abovementioned scientific issues,the Pr₂Fe₁₄B/?-Fe nanocomposite magnets were used as the model alloys on the basis of the extensive experimental research of the pioneers.In this dissertation,the subjects about the fabrication of bulk Pr₂Fe₁₄B/?-Fe nanocomposite magnet with high fraction of soft-magnetic phase,strong magnetic anisotropy and excellent magnetic properties through the optimization design of the alloy composition have been investigated by employing melt-spinning conbined with thermal compaction and rapid two-step high-pressure thermal compression deformation techniques.Firstly,the microstructure and magnetic properties of the deformed Pr-lean ternary Pr-Fe-B magnets were studied through controlling the B contents by employing melt-spinning conbined with thermal compaction ang rapid high-pressure thermal compression deformation.With increasing B contents,the magnetic anisotropy and the remanence?4?M_r?of the deformed magnets decrease gradually,while the coercivity increases gradually.And the Pr₂Fe₁₄B/?-Fe nanocomposite magnets with maximum energy product?BH?_max=18.1MGOe,the fraction of soft-magnetic phase above 25 wt.%and certain?00l?crystallographic alignment of hard-magnetic phase were prepared at the B content of 5 at.%.Bulk Pr₂Fe₁₄B/?-Fe nanocomposite magnets with strong magnetic anisotropy were fabricated by adding Cu in the Pr-lean ternary Pr-Fe-B alloys and adjusting the ratio of Pr,Cu,and B appropriately using melt-spinning conbined with thermal compaction ang rapid high-pressure thermal compression deformation.The effects of each component on the microstructure and magnetic properties of the deformed magnets were studied.The results show that the addition of Cu can effectively promote the formation of?00l?crystallography texture of Pr₂Fe₁₄B hard-magnetic phase in the deformed nanocomposite magnets.With increasing Cu addition,the magnetic anisotropy,the fraction of soft-magnetic phase and remanence of the deformed magnets increase synchronously.In addition,the remanence,coercivity and magnetic anisotropy of the deformed magnets are increased simultaneously with increase Pr contents.However,the coercivity of the deformed magnets increases while the remanence and magnetic anisotropy are significant decreased with increasing B contents.Finally,we prepared the Pr₂Fe₁₄B/?-Fe nanocomposite magnet with maximum energy product of 21.3 MGOe,the fraction of soft-magnetic phase of³1 wt.%,and strong crystallographic orientation of I_?004?/I_?220?=784%at the alloy composition of Pr₉Fe₈₅Cu₁B₅.The bulk Pr₂Fe₁₄B/?-Fe nanocomposite magnet with high soft magnetic phase fraction,strong magnetic anisotropy and good magnetic properties was prepared by adding Ga/Nb element into Pr-lean Pr-Fe-Cu-B alloy using high pressure thermal compression deformation.The effects of combined addition of Cu and Ga/Nb on the microstructure and magnetic properties of the deformed magnets were studied.The results show that the combined addition of elements make up for the deterioration of coercivity caused by single Cu addition,and the magnetic anisotropy of the deformed magnets was enhanced together with the coercivity.The deformed Pr₂Fe₁₄B/?-Fe nanocomposite magnet with coercivity of H_ci=4.63 kOe,remanence of 4?M_r=13.1 kG and maximun energy product of 27 MGOe was prepared at the composition of Pr₉Fe₈₅Cu₁Nb₁B₅.The coercivity mechanism of bulk anisotropic Pr₂Fe₁₄B/a-Fe nanocomposite magnets fabricated by rapid two-step high-pressure thermal compression deformation was investigated.It was found that the coercivity was mainly controlled by domain wall pinning mechanism,and the pinning effect of domain wall was more obvious with the addition of alloy elements in the process of demagnetization due to the grain refinement.In addition,the formation of?00l?crystallographic alignment of hard-magnetic phase in the deformed magnets is attributed not only to the preferential nucleation and growth of hard-magnetic Pr₂Fe₁₄B phase along the direction of the lowest strain energy density under high stress and large strain,but also to the small amount of low melting point Cu₂Pr phase produced by the introduction of Cu element.