摘要
Nd-Fe-B永磁材料具有高的剩余磁化强度、高的矫顽力和高的磁能积等优良的综合硬磁性能，广泛应用于机电、信息、通讯和医疗等领域。纳米晶Nd-Fe-B永磁体由于其温度稳定性、断裂韧性均好于传统微米晶烧结磁体，是目前稀土永磁合金的研究热点之一。但是，常规的烧结工艺由于升温速率慢、烧结温度高和保温时间长等缺点，不适用于高致密度纳米晶Nd-Fe-B永磁体的制备。
本文通过放电等离子烧结技术和热变形制备了各向同性与各向异性纳米晶Nd-Fe-B磁体。系统研究了材料的制备工艺、显微组织和硬磁性能。分析了各向同性与各向异性纳米晶Nd-Fe-B磁体中富钕相的扩散及其产生的微观结构特征。重点分析了热变形纳米晶磁体的回复曲线特点及回复曲线的张开对材料磁性能的影响。
通过放电等离子烧结（SPS）技术制备了各向同性纳米晶Nd-Fe-B磁体，研究了制备工艺对微观结构演化及性能的影响。结果发现，SPS烧结磁体中形成了两种具有不同晶粒尺寸的两区结构，即粗晶区和细晶区。放电等离子体在颗粒边界产生的暂态高温场是磁体形成粗晶区的原因。烧结温度和压力对粗细晶区的宽度、晶粒大小及其磁性能的变化有重要的影响。同时研究了不同成分及不同尺寸粉末制备的烧结Nd-Fe-B磁体的性能和微观结构。结果表明：对于富钕Nd-Fe-B磁粉，粉末颗粒尺寸对烧结磁体的矫顽力有较大的影响。而对于单相Nd-Fe-B磁粉，颗粒尺寸较大的粉末有利于磁体性能的提高。700oC/50MPa/5min烧结条件下，磁体的综合磁性能最佳，其剩磁、矫顽力和磁能积分别为J_r=0.82T、jH_c=1516kA/m和(BH)max=116kJ/m~3，剩磁和矫顽力温度系数分别为α=－0.107%K~(-1)和β=－0.483%K~(-1)，致密度高达99.5%。
以放电等离子烧结磁体为前驱体，采用热变形工艺成功制备了各向异性纳米晶Nd-Fe-B磁体。研究了不同应变速率和变形量对各向异性磁体微观结构、磁性能、温度稳定性和力学性能的影响。结果表明：热变形磁体继承了前驱烧结磁体中的两区结构特点，且粗细晶区显示了不同的热变形行为。0.0005S-1应变速率条件下，粗晶区由于蠕变也产生了一定程度的c轴取向。热变形所产生的扁平形状晶粒在宽度上的维度w的大小与热变形磁体的剩磁及矫顽力温度系数密切相关，较大的w意味着磁体较高的剩磁和较低的矫顽力温度系数。矫顽力机制分析表明：热变形磁体的矫顽力机制为钉扎机制。开展了对磁体不均匀变形及其微观结构对磁性能影响的研究。热变形磁体的最佳磁性能为：Jr=1.35T、jH_c=829kA/m和(BH)_(max)=336kJ/m~3，矫顽力温度系数β=－0.682%K~(-1)。
对各向同性与各向异性纳米晶Nd-Fe-B磁体中富钕相的扩散行为进行了细致的研究。结果发现，对于经机械混合富Nd成分和富Fe成分磁粉所制备的具有一定净稀土含量的烧结磁体，在两种成分边界区域存在着富钕相在富Fe成分区域扩散所形成的区域，即扩散区。扩散区晶粒尺寸从富Nd成分向富Fe成分区域呈由大至小分布，对应的Nd含量亦呈递减分布。富钕液相在富Fe成分区域的扩散是导致晶粒结构呈渐变式变化的原因。该扩散区域依然存在于后续热变形磁体中，并且形成了具有c轴取向特征的扁平形状晶粒。随着从富Nd成分到富Fe成分区域Nd含量的减小，扩散区域c轴取向呈由强到弱分布。
此外，本文还重点研究了放电等离子烧结磁体和热变形磁体的回复曲线特点，分析了导致热变形回复曲线张开的主要原因。结果发现，对于具有相同单相磁性相的放电等离子烧结磁体和热变形磁体，其回复曲线呈现不同的特点。所有采用单一磁粉制备的放电等离子烧结磁体的回复曲线都是闭合的，而后续热变形磁体的回复曲线却呈不同程度的张开。存在于前驱烧结磁体中的孔洞会造成后续热变形磁体中富钕相的聚集。聚集尺寸大小为几个微米到十几个微米的条形富钕相，不仅难以作为畴壁的钉扎点，而且会产生不可忽略且不均匀分布的退磁场，不均匀分布的退磁场会降低钉扎场，导致磁各向异性的不均匀性，最终导致了回复曲线的张开。较大的回复曲线张开程度导致了磁体矫顽力降低。最后，本实验通过改善SPS前驱烧结磁体的致密度，明显减轻了热变形磁体中富钕相的聚集，减小了回复曲线的张开程度，大幅度提高了材料的硬磁性能。密度为7.03g/cm~3、7.33g/cm~3和7.56g/cm~3的前驱烧结磁体经相同热变形条件后磁体所获得的矫顽力分别为226kA/m、288kA/m和995kA/m。
Nd-Fe-B magnets have been widely used in various fields such as electromechanicalequipments, information, communication, and medical devices, due to its excellent magneticproperties. Nanocrystalline Nd-Fe-B magnets have attracted much attention not only becauseof their good magnetic properties but also due to their exceptional thermal stability andfracture toughness. The conventional densification methods for preparing nanocrystallinemagnets is still a major concern, since magnetic properties will deteriorate dramatically due toslow heating rate, high sintering temperature, and long holding time.
In this work, isotropic and anisotropic nanocrystalline Nd-Fe-B magnets were fabricatedby spark plasma sintering (SPS) and SPS followed by hot deformation, respectively. Theprocessing-microstructure-magnetic properties were investigated in detail. The diffusion ofNd-rich phase and induced developing microstructure were discussed for the isotropic andanisotropic nanocrystalline Nd-Fe-B magnets. The characteristics of recoil loops and theirrelationships with the microstructure and properties for hot deformed nanocrystallineNd-Fe-B magnets were investigated.
Isotropic nanocrystalline Nd-Fe-B magnets were synthesized by spark plasma sintering(SPS) using the melt spun ribbons as the starting materials. Due to the local high-temperaturefield induced by spark plasma discharge at the vicinity of the particle boundaries, the distincttwo-zone structure was formed in the SPSed magnets. The SPS temperature and pressure haveimportant effects on the widths of coarse and fine grain zones, as well as the grain sizes intwo zones. The changes in grain structure led to the variations in the magnetic properties.Melt spun Nd-Fe-B ribbons with two compositions and various powder sizes were employedas the starting materials. For the magnets with RE-rich compositions, the influence of powdersizes on the coercivity of SPSed magnets is very significant. For single phase Nd-Fe-B alloyswith stoichiometric2/14/1composition, the starting powder with a larger particle size isbeneficial to achieve better magnetic properties. The best combination of magnetic propertiesare J_r=0.82T,jH_c=1516kA/m, and (BH)max=116kJ/m~3at the sintering condition of700oC/50MPa/5min. The values of α and β are－0.107%K~(-1)and－0.682%K-1, and therelative density is as high as99.5%.
Anisotropic nanocrystalline magnets were prepared by spark plasma sintering (SPS)followed by hot deformation (HD) using SPS precursors. The influences of strain rate andcompression ratio on the microstructure, magnetic properties, temperature stability have beeninvestigated. The platelet-shaped grain dimension perpendicular to the pressing direction wasrelated to the remanence and temperature coefficient of coercivity for hot deformed magnets.A strong domain-wall pinning model was valid to interpret the coercivity mechanism andcharacteristics of initial magnetization curves for hot deformed magnets. The influences ofnon-uniform plastic deformation on the microstructure and magnetic properties were alsoinvestigated. Good magnetic properties with Jr=1.35T,jHc=829kA/m, and (BH)max=336kJ/m~3have been obtained. The value of β is－0.682%K-1.
The role of Nd-rich phase in developing microstructure and properties of isotropic andanisotropic Nd-Fe-B magnets has been investigated. Melt spun Nd-richNd13.5Fe73.5Co6.7Ga0.5B5.6and Fe-rich Nd_(7.7)Pr_(2.6)Fe84.1B5.5alloy powders were mechanicallymixed with different ratios. The mixed powders were consolidated into isotropic magnets andanisotropic magnets by spark plasma sintering (SPS) and SPS followed by hot deformation,respectively. The composition and microstructure of diffusion area between Nd-rich andFe-rich compositions for isotropic and anisotropic magnets were investigated. The gradientdistribution of Nd content from Nd-rich to Fe-rich area due to the diffusion of liquid Nd-richphase during the SPS and hot deformation was observed, which leads to gradually changes ingrain structure. The c-axis crystallographic alignment decreased with the decrease of Ndcontent.
The characteristics of recoil loops and their relationships with the microstructure andproperties for hot deformed nanocrystalline Nd-Fe-B magnets were investigated. The recoilloops of all SPSed magnets are closed using single magnetic powders as the starting materials.However, the recoil loops of hot deformed anisotropic Nd-rich Nd-Fe-B magnets without softmagnetic phase were found to be open. The investigations showed that the aggregation ofNd-rich phase is related to the open loops. The Nd-rich phase aggregation with various sizes,lager than grains, which can produce non-uniform distribution of local demagnetization fields,reduced the local pinning field and lead to the inhomogeneity of magnetic anisotropy. Thisvariation of local magnetic anisotropy in the magnets should be responsible for the formation of open recoil loops. Less extent of Nd-rich phase aggregation obtained by increasing therelative density of SPS precursors, which can reduce the openness of recoil loops, and theenhanced magnetic properties can be obtained obviously. Using the SPSed magnets with thedensity of7.03g/cm~3,7.33g/cm~3, and7.50g/cm~3as the precursors, hot deformed magnetswith the coercivity of226kA/m,288kA/m, and995kA/m were obtained under the samedeformation conditions, respectively.
引文
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