Structural Evolutions And Their Effects On Magnetic Properties In Sm₂(Fe,M)₁₇N_x Permanent Magnetic Material Processed By HDDR

Since Coey et al discovered the interstitial compound Sm₂Fe₁₇N_x, it hasattracted many researchers' attention. As a kind of permanent magnetic material,Sm₂Fe₁₇N_x exhibits fairly high saturation magnetization(1.54T), high anisotropyfield(14T) under room temperature, high Curie temperature(476℃), good thermalstability, resistance to oxidation and corrosion resistance. These intrinsic magneticproperties are comparable with or better than those of Nd₂Fe₁₄B compound. Itshows to be a candidate for intending permanent magnets, and comes to be a hot anddifficult area on research. In this thesis the isotropic Sm₂Fe₁₇N_x permanent magneticpowders have been prepared by HDDR (Hydrogenation-Disproportionation-Desorption-Recombination). The phase changes, microstructure evolution and effecton magnetic properties during the solidification, homogenization, HDDR andnitration processes were investigated by means of XRD, SEM, EDX, TEM, HRTEM,Rietveled, DTA/TG, VSM etc. Innovation results achieved are as follows:(1)The mechanism and regularity of microstructure evolution and phasecomposition during the solidification and heat treatment process of Sm-Fe alloy havebeen systemic researched. The key technology of obtaining a single Sm₂Fe₁₇ phasealloy by adding Sm lagging and thin ingot aiming at improving the cooling speed ofSm-Fe alloy was created firstly. It was successful in solving the problem, which is hard to obtain homogeneous single Sm₂Fe₁₇ phase alloy during the productionprocess for the volatilization of Sm. It has laid a good foundation for industrialproduction.(2) The evolution of the microstructure and phase composition in the process ofHDDR of Sm-Fe alloy have researched systematically. And microstructure evolutionmechanism and grain refinement mechanism during the HDDR process wererevealed. It was found that Sm₂Fe₁₇ alloy adsorbs a large number of hydrogen at 200℃to form Sm₂Fe₁₇H_x accompanied with the appearance of many micro-cracks.Hydrogenation is gradually slowed and stopped at about 350℃. When thetemperature is above 500℃, hydrogenation of the alloy takes place again, and thedisproportionation of the alloy near the cracks took precedence of other parts underthe same condition. Disproportionation at 600℃, the microstructure of Sm-Fe alloyis layer structure of alternated SmHx phase in nano-crystalline andα-F phase innano-crystalline."Rayleigh Collapses" phenomena takes place in the layer structurewith alternated phases and turned it into isotropic spherical structure at 750℃. Withthe rise of temperature, grains of Sm-Fe alloy grows up. The Disproportionation-recombination process of Sm-Fe alloys is realizes by rising the temperature andreducing the hydrogen pressure at the same time. This process is a step of short rangediffusion, phase change and nucleation at the grain boundaries. The recombinationtakes place at 700℃in which the inter-diffusion of Sm atoms from the SmH_x withhigh activity and the Fe atoms fromα-Fe occurred. Nucleation were found at thegrain boundaries between SmHx andα-Fe phases. Meanwhile, the Sm-rich phaseappears due to inhomogeneous during the diffusion process, and with increasingdegree of diffusion, the Sm-rich phase is also changed from SmFe₂ to SmFe₃. Thenew phase formed in the recombination is not the Sm₂Fe₁₇ with the Th₂Zn₁₇ structuretype but the Sm₁₀Fe₉₀ metastable phase with the TbCu₇ structure. Heat treatment at750℃for 15 minutes started the change of the metastable phase Sm₁₀Fe₉₀ to stablephase Sm₂Fe₁₇. After 30 min, recombination was completed with almost the Sm₂Fe₁₇and Sm₁₀Fe₉₀ except some oxide phase. After a holding time of 1h, most of theSm₁₀Fe₉₀ metastable phase changed into Sm₂Fe₁₇ stable phase with a grain size of about 150nm. The completeness of this change was improved by rising thetemperature to 800℃. However, the contents of phases other than Sm₁₀Fe₉₀ andSm₂Fe₁₇ were increased due to the accelerated volatilization and oxidation of Sm atthis temperature. Moreover, the grain of the alloys would become larger. In HDDRprocess H plays an important role in grain refinement of the alloy. On one hand, thedisproportionation reaction between hydrogen and Sm-Fe alloys greatly reduces thegrain size of Sm-Fe alloy, and changes the alloy microstructure (interface amongphases). This provides more nucleation position for the formation of Sm₂Fe₁₇ phasethrough inter-diffusion between Sm atoms and Fe atoms. Thus, the nucleation rate Iof the Sm₂Fe₁₇ phase is improved and the grain sizes are reduced. On the other hand,growth of grains are constrained by decreasing of the hydrogen pressure due to theimprovement of nucleation rate in recombination and the changeless growth rate ofthe grains. As a result, the grain size of the Sm-Fe alloy changes from severalhundred micrometers in the casting state to about one hundred nano-meter after theHDDR process.(3)It is fn'st to apply the Neural Network Technology into the preparation ofSm₂Fe₁₇N_x permanent magnets creatively. Magnetic properties estimate model andprocess parameters forecasting model about the effect of temperature and time ofHydrogenation-Disproportionation, Desorption- Recombination on the magneticproperties of Sm₂Fe₁₇N_x permanent magnets were established by the Neural NetworkTechnology, which have been tested through experiments.(4) The nitride process, nitride mechanism and decomposition mechanism ofSmFe alloy after HDDR process were investigated systematically. In addition, theoccupying of nitrogen atoms in the lattice of atoms Sm₂Fe₁₇ alloys, such as theoccupying ratio were studied. The results showed that SmFe alloys processed byHDDR nitrogen begin at 287℃. Rapid nitrogen takes place at 342℃, after that,intensive heat release and dramatic increase in weight are observed. Diffusion of Natoms in the alloy take place at 450℃to 530℃and these will not change the alloystructure. With the rise of temperature, the lattice constants, unit cell volume andnitrogen content in the alloy are increased and reaches their peaks at 490℃tested by the X-ray diffraction (XRD). Although the decomposition reaction occurred duringthe nitride process, the nitrogen dominates the whole reaction at the temperaturerange of less than 490℃due to the slow atomic migration in decomposition reaction.At the temperature range of above 510℃, the intense decomposition lead to lowernitrogen content which result in the increase of non-magnetic and soft magneticphases. Magnetic properties of the alloy nitrides first increase and then decrease withincreased temperature. The magnetic properties achieves the maximum at 490℃.Grain size have great influence on the nitride process of SmFe alloys. The SmFealloy without milling are in a state of coarse particles that have the lowest magneticproperties in the same condition. After 10 minutes of ball milling, most of the coarseparticles were reduced to a gain size range within 10 um. At this processing time, thenitrogen content in the alloy got the highest value and magnetic property valuesreached their peaks. Extension of milling time results in finer particle sizes of thealloy, and higher oxygen content. This causes serious decomposition of the nitride inthe same processing condition. Therefore, the contents of non-magnetic phasesincrease and the magnetic properties get lower. With the increase in the pressure ofnitrogen gas, activity of nitrogen atoms raised which is helpful to the nitride of SmFealloys. Increasing the partial pressure of nitrogen can effectively inhibit thedecomposition reaction, but by the equipment limits a 0.3Mpa nitrogen pressure isappropriate. After HDDR process the surfaces of the alloy are purified and activated,accompanied with a large number of micro-cracks that increase the surface area ofthe alloy. Thus the nitride activation energy of SmFe can be significantly reduced.Nitrogen take place firstly at surfaces and defects regions of the alloy in nitrideprocess. After that, the N atoms diffused from surfaces of the alloy particles to theircores and took a precedence to occupy the 9e lattice positions of the Sm₂Fe₁₇ alloy.After 4h occupying ratio of the 9e lattice positions in the alloy reached 99ï¼…, whichindicated very high nitrogen content in the alloy. At the same time, content of themain magnetic phase (Sm₂Fe₁₇N_x) in the alloy come to 97.9ï¼…wt. Consequently,preparation of Sm₂Fe₁₇N_x rare earth permanent magnetic powder with high nitrogencontent, uniform distribution of N atoms and high content of magnetic phases are successfully realized. Mixed with 2.5ï¼…wt of binder, the bonded Sm₂Fe₁₇N_x rare earthpermanent magnets with B_r=0.67T, intrinsic coercivity H_cj=1415kA/m, maximumenergy product (BH)_max=73.7kJ/m³ were prepared. Moreover, productionexperiments of Sm₂Fe₁₇N_x powder at a level of kilograms were successfully carriedout on the above basis.(5)It is first to research the effect of adding W and other elements onmicrostructure, phase composition and magnetic properties of Sm-Fe alloy during thecast, HDDR, and nitride process. And the effect mechanism of additional elementson Sm-Fe alloy were revealed. On these basis, the homogeneous Sm₂Fe₁₇ alloywhich need short time of homogenization treatment or even did not need anyhomogenization treatment by the optimization of composition and process wasprepared. And the manufacture performance was significantly improved also theproductive costs was reduced in terms of magnetic little lower.