Microstructure Regulation Of Precursor Powders And Mechanism Of Magnetic Properties For Hot-deformed Magnets

In order to realize the high-efficiency utilization of rare earth resources and reduce the excessive dependence of heavy-rare-earth elements in high-coercivity Nd-Fe-B magnets,researchers are dedicated to develop non-heavy rare-earth Nd-Fe-B magnet with excellent properties.The hot-deformed Nd-Fe-B magnet is expected to achieve high coercivity due to its nanocrystalline structure（³00 nm）.Besides,the hot-deformation process has such advantages such as short flow,excellent temperature stability and strong corrosion resistance making it a new type of high-performance permanent magnet instead of non-heav rare-earth sintered Nd-Fe-B magnet partly.In fact,the coercivity of hot-deformed magnet is far from the theoretical prediction.Researchers find that grain boundary diffusion technology（GBDP）is an effective means to improve coercivity of hot-deformed Nd-Fe-B magnets.However,the coercivity is improved on the basis of the substantial sacrifice of remanence finally which restricts the application of permanent magnet.Melt-spun powders is used as the precursor magnetic powders of hot-deformed Nd-Fe-B magnet.Due to the inhomogeneous of microstructure for ribbons,there are non-oriented quasi-periodic coarse-grained region at the interface of the ribbons in hot-deformed magnet.Moreover,the addition of low-melting alloys further enlarges the coarse-grained region which is not conducive to the improvement of the comprehensive performance of the magnet.In this paper,the following three aspects are used to obtain hot-deformed magnet with high remanence and high coercivity.Firstly,the pre-diffusion technology makes the Pr-Cu eutectic alloys diffuse into the ribbons through the crack.The low-melting phase aggregation between boundaries is effectively reduced so that the coarse grain region produced by diffusion is obviously reduced.The effective utilization of Pr-Cu alloy increases thickness of non-magnetic phase in grain boundaries which weakens ferromagnetic coupling between the main phase lead to the enhancement of the reverse domain nucleation field and inhibits the grain size of Nd₂Fe₁₄B phase to increase the effective coercivity enhancement.Adding Pr-Cu by pre-diffusion process can eliminate its agglomeration to uniform the microstructure and renovate the defects on the surface of the grain.The narrow grain size distribution makes the local critical nucleation field similar so that the demagnetization curve is square.Secondly,in order to restrain grain growth in the coarse grain region caused by diffusing low-melting alloys,Nb is introduced to the precursor alloy to regulate the microstructure of the melt-spun powders.Due to the low solubility of Nb in Nd₂Fe₁₄B,excess Nb is presented as c-Nb and h-NbFeB at grain boundary in hot-deformed magnet.Due to the pinning effect of h-NbFeB phase at the grain boundary,the migration of grain boundary between hard magnetic phase become difficult.In other word,it increases the resistance of grain growth which effectively refines the grain size inner ribbons and the coarse grain region and reduces the stray magnetic field to improve the temperature stability and coercivity of the hot-deformed magnet.The Nb phase can drive the element of Nd into grain boundary and both c-Nb and h-NbFeB are non-magnetic phase which can enhance coercivity.However,the enhanced thickness of the grain boundary phase inside the ribbon releases the deformation pressure and makes the matrix phase misaligned which is one reason for remanent reduce.The NbFeB precipitates from the main phase resulting in structure defects which is not conducive to the remanence enhancement.Thirdly,lifting the remanence of matrix is another helpful way to obtain high coercivity and high remanence magnets.Combined with the effect of Nb on microstructure and magnetic properties of hot-deformed magnets,the FeSiCuNbB soft magnetic ribbon（SMR）is mixed with MQU-F powders to fabricate hot-deformed magnet.In the process of hot deformation,the extruded liquid phase exists at the interface of hard and soft magnetic ribbons and reacts with Fe and B in the soft magnetic ribbons to form Nd₂Fe₁₄B.The structure effectively reduces the accumulation of liquid phase at the interface and inhibits the grain growth of coarse crystal region resulting in texture enhancement.A part of Nb in soft ribbon diffuses into the transition zone during the process of the hot deformation and effectively refines the grain size of transition region.The composition of grain boundary phases in transition region are c-Nb and fcc-Nd₂O₃,respectively which reduce the mismatch degree between grain and grain boundary and the relative content of Fe in the grain boundary to improve the coercivity of transition zone.The high remanence magnets is prepared successfully H_c=15.2 kOe,B_r=14.05 kGs,（BH）_max=49 MGOe.In addition,although the precursor powders of hot-deformed magnet,melt-spun ribbons,have many advantages such as fine grain size,strong rheology,and mature technology.The production efficiency is low due to the limitation of the single-roll quenching technique so that the total cost is increased.Therefore,it is urgent to find a new magnetic powder to replace melt-spun powders.HDDR（hydrogenation-disproportionation-dehydrogenation-recombination）as a process for preparing nanocrystalline magnetic powders is widely considered to be an alternative way as precursor powders.Moreover,HDDR process is cost low and the anisotropy is high so that it has great potential to replace melt-spun ribbons.But the coercivity of HDDR powders is low and the rheological ability is poor.The internal mechanism is not clear.The following two aspects are used to clarify the mechanism in this paper.Firstly,refining and heat treatment is carried out on the precursor cast ingot to homogenize the distribution of RE-rich phase.The coercivity of the modified HDDR powders is increased to 16.4 kOe.With the addition of Pr-Cu and pre-diffusion treatment,the coercivity of the compact magnet is significantly increased to 15.5 kOe.The heat treatment of the mixed powder is beneficial to the diffusion of the RE-rich phase along the grain boundary to enhance magnetic isolation effect.The Nd₂O₃ and c-PrO₂ are present in the grain boundary to reduce the mismatch between the grain boundary and matrix grain and repair the defect at the interface which is beneficial to coercivity.However,most regions still lack grain boundary phases which are not conducive to the preparation of hot-deformed magnets.Therefore,low-melting alloys are added before nucleation.The coercivity and remanence of the prepared HDDR powders are reduced in various degrees.Microstructure analysis shows that the addition of low-melting eutectic alloy before desorption destroys the element proportion in local area and generates NdFe₂ phase which enhances the exchange coupling between grain surface and grain boundary.Secondly,the effect of grain size on microstructure of HDDR magnetic powder is studied.Found that the distribution of Nd-rich phase in HDDR powder prepared from nano-crystalline powders is uniform and grain size distribution is narrow.The lack of neodymium-rich phase region is eliminated.The influence of neodymium-rich phase distribution on the rheological properties of HDDR magnetic particles and magnetic properties of magnets is also studied.It is found that the deformation capacity of the hot-pressed magnets with uniform distribution of Nd-rich phase is remarkably improved and the coercivity of its finally hot-deformed magnet is also enhanced.The region which lack of Nd-rich phase is eliminated so that the friction force of grain rotation during the hot deformation process is reduced.It is easier to align along the external pressure for the matrix grains so that the remanence is greatly improved.The above analyses clarified that nano-submicron process provides an effective way to optimize the microstructure of HDDR powders and its deformation ability.