Texture Growth And Magnetic Properties Of Bulk Nd₂Fe₁₄B/α-Fe Nanocomposite Magnet

Nd₂Fe₁₄B-based nanocomposite magnets,which are made up of Nd₂Fe₁₄B hard-magnetic phase and soft-magnetic phase（e.g.,α-Fe,Fe₃B）,provide a high theoretical energy product and save rare-earth（RE）metals due to the addition of high-fraction soft phase.However,the experimental results are still far less than the theoretical predicted value（¹00 MGOe）with a high soft-phase fraction over 20%in bulk Nd₂Fe₁₄B/α-Fe nanocomposite magnets.The main difficulty is that the well controlled microstructure unable to realize due to the trade-off between the formation of the crystallographic texture of Nd₂Fe₁₄B nanocrystals and the control over the structural properties of soft phase,such as its content,grain size and distribution.In this paper,we successfully prepared the bulk anisotropic Nd₂Fe₁₄B/α-Fe nanocomposite magnets with strong（00l）texture of Nd₂Fe₁₄B hard-phase by some new designs in preparation processes and of alloying elements.In addition,the microstructure and magnetic properties of synthesized magnets were studied as following:Anisotropic bulk Nd₂Fe₁₄B/α-Fe nanocomposite magnets have been fabricated by synchronous High-pressure Thermal Compression deformation（HPTC）from Nd-lean amorphous Nd₁₀Fe₈₄B₆ alloys.The deformed magnet with optimum properties yielded at the deformation temperature from 530 ^oC to 700 ^oC in 2s with the deformation amount up to 77%combined with annealing at 600 ^oC for 5 min under high stress.This nanocomposite exhibits much larger magnetic properties than that in annealing amorphous Nd₁₀Fe₈₄B₆ alloy,yielding an increase by 38%in remanence B_r,31%in coercivity H_ci,and140%in maximum energy product（BH）_max.Therefore,the bulk nanocomposite magnet shows a maximum energy product of（BH）_max=19.2 MGOe prepared by synchronous HPTC.The synthesized nanocomposite magnets exhibit an obvious（00l）texture for Nd₂Fe₁₄B nanograins,and the average grain sizes ofα-Fe and Nd₂Fe₁₄B phases are determined to be 19.2 and 26 nm,respectively.The bulk Nd₂Fe₁₄B/α-Fe nanocomposite magnets with enhanced crystallization texture have been prepared by designing alloy composition combined with synchronous HPTC deformation.The fraction of soft magnetic phase and the texture strength of hard magnetic phase can increase with adding appropriate amount of Cu element at once,the results lead to the enhanced remanence without reduction of coercivity.Another,the fraction of soft magnetic phase,the texture strength of hard magnetic phase and the remanence of deformed magnets can also increase with decreasing the Nd and B content.However,the decrease of Nd content results in the deteriorate of coercivity,and the coercivity remains basically unchanged with the reduction of B atomic stoichiometry from6 at.%to 4 at.%.Then,we study the effect of deformation amount on the microstructure and magnetic properties of deformed magnets with the optimum component alloy Nd₉Fe_85.5Cu_1.5B₄.The results show that the texture of hard magnetic phase and the magnetic properties of nanocomposites increase with increase of deformation amount.At last,the synthesized magnet exhibits an enhanced maximum energy product of（BH）_max=21.6 MGOe and（00l）texture of Nd₂Fe₁₄B nanocrystals with a high soft-phase fraction up to 33 wt.%.Using two-step high-pressure thermal compression,we successfully fabricated a bulk anisotropic Nd₂Fe₁₄B/α-Fe nanocomposite with a strong（00l）texture for Nd₂Fe₁₄B nanocrystals combined with a high fraction（³1 wt%）and small size（¹8 nm）forα-Fe nanograins.The deformed magnet exhibits a superior energy product of（BH）_max=24.3MGOe compared with the typical values of the bulk Nd₂Fe₁₄B-type nanocomposites with soft-phase fractions above 20 wt%.To achieve the optimum deformation process,we study the effect of various different deformation techniques and the deformation parameters in the two-step isothermal deformation on the microstructure and magnetic properties of bulk Nd₂Fe₁₄B/α-Fe nanocomposite magnets.Experimental results demonstrate that the two-step isothermal deformation mode is the most beneficial to the formation of（00l）texture of Nd₂Fe₁₄B hard-phase in these deformation modes.The bulk magnets,which made by two-step deformation at 530 and 650 ^o C for first-and second-step with an optimum deformation time of 5 s respectively,show an enhanced（00l）texture of Nd₂Fe₁₄B nanocrystals and superior magnetic properties.The nanocomposite magnet made by two-step isothermal deformation yields and enhancement by 49.4%in（BH）_max,13.6%in B_r,and 17.6%in coercivity H_cii compared with the magnets produced by conventional isothermal deformation.