Research On Microstructure Andproperties Of High Stability FeCo-Based Permanent Materials

Rare earth is an important strategic resource in China,and rare earth permanent magnet materials are key materials to support national defense security,information,energy conservation and intelligent manufacturing.In recent years,with the rapid development of high-speed permanent magnet motors,higher requirements have been put forward for the thermal stability and mechanical properties of permanent magnet materials.Compared with Sm Co materials,sintered Nd Fe B has the advantages of high magnetic energy product and excellent mechanical properties,but there is a problem of poor thermal stability,resulting in its due limitation at high temperature.The use of Co instead of Fe can increase the Curie temperature of Nd Fe B,but it will reduce the intrinsic magnetism of the 2:14:1 main phase,and there are also problems such as the loss of thin-layer grain boundaries.In this paper,Nd-Dy-Fe-Co-B magnets（hereinafter referred to as:FeCo-based magnets）were prepared by using the combined addition of Dy and Co to replace Nd and Fe respectively.The composition,columnar crystal morphology and phase structure and their genetic mechanism in the sintering/annealing process were studied,and the magnets with high thermal stability were prepared by adjusting the sintering/annealing process;high Co+Dy and low Co+Dy two types of alloys,with different Co contents were prepared by the double alloy method,and the correlation between the magnetic properties,mechanical properties,corrosion properties and composition was systematically studied;the diffusion mechanism of the magnet with high Co content and its influence on the thermal stability were studied;according to the actual application requirements,the engineering preparation and research of the FeCo-based magnet were carried out.The main conclusions are as follows:（1）Taking a magnet with a composition of Nd_25.5Dy_6.5Co₁₃Fe_balM_1.05B_0.98as the research object,the distribution of columnar grains in the stript alloy is poor,and there are more dendrites and irregular rare earth-rich phases.There are hcp-Nd₂O₃phase and amorphous phase in the trifurcation grain boundary,Fe and Co elements are the main elements in the amorphous phase,and Nd and Dy rare earth elements are very few.Cobalt-poor and cobalt-rich regions exist in the grain boundaries of sintered magnets.There are many amorphous regions and crystallized regions with different shapes in the magnet.The Co-rich region in the crystallized region has an RM₂phase structure,and the Co content is as high as about 70 at.%,while the amorphous region is dominated by Nd and Co elements,and the composition fluctuates in a wide range,which is significantly different from the stript alloy.The proportion of amorphous phase in the tempered magnet is greatly reduced,and the rare earth-rich phase of hcp-Nd₂O₃and the Co-rich phase of RM₂（1:2）structure exist in the grain boundary at the same time.The element distribution of the amorphous region of the tempered magnet is almost the same as that of the sintered magnet.Compared with the SH brand(H_cj=20 k Oe)magnet with similar coercivity,the FeCo-based magnet obtained by optimizing the preparation process has a maximum service temperature T_wincreased by 35℃,and the remanence temperature coefficient is reduced by one times,which is-0.065%/℃（20-180℃）.（2）Magnets with different Co contents were prepared by the double alloy method.The EPMA scan shows that with the increase of the addition of low Co alloy,the aggregation of Co in the grain boundary phase is improved.Magnet powder XRD shows that the RM₂phase still exists in the high-Co magnets（Co=13wt.%）and double alloy magnets（Co=9.25～11.75wt.%）,while no the 1:2 phase is found in the low Co magnets（Co=0.5wt.%）.With the increase of the addition amount,the B_rincreased from 11.25k Gs（Co=13wt.%）to 11.71k Gs（Co=9.25wt.%）,and the（BH）_maxincreased from 30.65 MGOe to 33.62 MGOe.The H_cjis increased from 19.54 k Oe to20.77 k Oe.The magnet remanence temperature coefficient|α|and Curie temperature T_care strongly correlated with the magnet composition.The higher the Co content,the smaller|α|and the higher the T_c.The spin reorientation T_srtest shows that the T_srof the high-Co magnet is higher than that of the low-Co magnet,which may be due to the substitution of Co for Fe to increase the angle between the total magnetic moment of the transition elements and the[001]orientation.Fracture analysis shows that as the Co content decreases,the proportion of cleavage fractures in the main phase grains decreases and the flexural strength increases.The corrosion resistance of the magnet is closely related to the Co content.The corrosion resistance is closely related to the Co content in the magnet:under conditions of 120℃,100%RH,and 96 hours,the weight loss of high Co magnets is significantly reduced by about 2/3 compared to low Co magnets..（3）Grain boundary diffusion（GBD）can significantly increase the H_cjof FeCo-based magnets,and the thermal stability is also improved.The H_cjof 6wt.%Co and12wt.%Co magnets after diffusion are 25.04k Oe and 20.66k Oe,which are 35.79%and 31.59%higher than before diffusion.For 6wt.%Co magnets in the temperature range of 293-413K,the|β|decreases from 0.603%/K to 0.508%/K,and the|α|decreases from 0.099%/K to 0.091%/K.The test of intrinsic magnetic properties shows that the change of Co content（0～12wt.%）within a certain range has little effect on the saturation magnetization M_sand magnetocrystalline anisotropy field H_aof the magnet.The M_sof 6wt.%Co and 12wt.%Co are 1.434 T and 1.421 T,respectively,while that of the Co-free magnet is 1.429 T.The H_aof the two high-Co magnets is 9.938 T and 9.919 T,which is slightly lower than that of the Co-free magnet（10.115 T）.From the perspective of intrinsic magnetic properties,the H_cjof FeCo-based magnets still has a lot of room for improvement.The lack of thin-layer grain boundaries in the FeCo-based magnet matrix results in a shallow diffusion depth.Both the 1:2 phase and the amorphous phase exist in the FeCo-based magnet before and after GBD.Compared with the 2:14:1 main phase,the rare earths in the 1:2 phase and the amorphous phase are more difficult to be replaced by Tb.Tb preferentially enters the main phase,which can improve the utilization efficiency of heavy rare earths at the grain boundaries of FeCo-based magnets.After GBD,the bending resistance of the magnet is improved to a certain extent.The bending strength of the6wt.%Co magnet before and after the diffusion is 153 MPa and 206 MPa respectively.The increase in the grain boundary phase reduces the main phase grain stress concentration may be one of the reasons for the increase of the bending strength of the magnet.（4）In this paper,the engineering preparation of 100 kg FeCo magnets has been carried out.Through the optimized process,the B_r>11.60 k Gs,the（BH）_max>33MGOe,the H_cj>25 k Oe,and the typicalαvalue was-0.05%/℃（20～100℃）,and the typical value ofβis about-0.50%/℃.The comprehensive physical performance test of FeCo-based magnets shows that its bending strength is twice that of Sm Co,its temperature stability is much better than that of Nd Fe B,which can meet the application scenarios of high operating temperature and high mechanical strength.