Stuady On The Microstructure And Properties Of Nd-Fe-B Magents Based On Grain Boundary Reconstruction

The sintered Nd-Fe-B,with excellent magnetic properties,has been widely used in medical,military and aerospace devices,and daily appliances.With the scientific progress and technological innovation,the market demand for sintered NdFeB has increased dramatically,leading to the overconsumption of traditional heavy rare earths Dy and Tb,which causes the uneven use of heavy rare earth resources.In addition,the technology of rare earth alloying has accordingly caused the overuse of heavy rare earths and the inability to maximize the use of heavy rare earth resources with low reserves.Therefore,we should develop the other heavy rare earths to reduce the consumption of high quality heavy rare earths Dy and Tb,combining with the grain boundary reconstruction techniques.Therefore,the sintered NdFeB is modified by using low melting point rare earth erbium alloy based on the grain boundary reconstruction technique in this paper,and the finally conclusions are as follows:（1）Based on the grain boundary reconstruction technique,what does the low melting point alloy Er₃₀Cu₇₀has caused on the properties and organizational impacts of the sintered NdFeB were investigated.The experimental results show that the Er element form a low H_A（Er,Nd,Pr）₂Fe₁₄B shell layer structure at the grain edge of the main phase by diffusion in the grain boundary region,which makes the magnetic properties show a slight downward trend.However,only the coercivity decreased by 0.39 k Oe（2.5%）at the 0.2 wt%Er₃₀Cu₇₀addition,the target magnet’s microstructure defects were improved.In addition,the high electrode potential Cu enriched in the grain boundary region reduces the potential difference between the grain boundary phase and the main phase,which results in a reduction of the corrosion current density(I_corr)from 128.7μA/cm² to 38.41μA/cm² with a70.1%reduction rate at the addition amount of 0.6 wt%Er₃₀Cu₇₀,and the corrosion resistance of the magnet is greatly improved.（2）Based on the study of Er₃₀Cu₇₀,the low melting point alloy Er₆₉Fe₃₁ was designed to investigate the effect of Er elements on the sintered magnet’s properties and organization in order to exclude Cu elements.The experimental results show that a small part of Er elements exist in grain boundary regions,while most of them form a（Er,Nd,Pr）₂Fe₁₄B shell structure with low magneto-crystalline anisotropic field coercivity.However,when the Er₆₉Fe₃₁ alloy addition was 0.2 wt%,the maximum magnetic energy product（BH）_maxincreases from 35.88 MGOe to 36.30 MGOe（increases by1.17%）;the remanence B_rincrease from 12.17 k Gs to 12.23 k Gs;In addition,the corrosion potential(E_corr)increased from-1.109 V to-1.003 V,and the corrosion current density(I_corr)decreased from 128.7μA/cm² to 53.62μA/cm².（3）The effects of heat treatment and second cycle heat treatment on the properties and organization of magnets with 0.6 wt%Er₃₀Cu₇₀ and 0.4 wt%Er₆₉Fe₃₁ additions were investigated,respectively.By varying the low-temperature tempering temperature in the first heat treatment and the high-temperature tempering temperature in the second cycle heat treatment.It was found that the main impact of heat treatment is by optimizing magnet’s microstructure.Finally the optimal heat treatment process for the second cycle heat treatment was determined:0.6 wt%Er₃₀Cu₇₀ addition:880°C×2.5h+480°C×5h;0.4 wt%Er₆₉Fe₃₁ addition:940°C×2.5h+480°C×5h,respectively.