【作者】 曹玲飞；

【导师】 徐根应； 汪明朴；

【作者基本信息】 中南大学 ， 材料物理与化学， 2006， 博士

【摘要】 本文结合国家十五攻关项目“非晶合金粉末固化成型——磁力控制退火晶化法制备大块状FeMB合金纳米晶的研究”（编号2001BA310A03-1），研究了Fe-M-B纳米晶软磁块体材料的制备方法及其所涉及的基础问题。首先，根据合金热力学知识，计算了Fe和B与相关元素各自形成二元有序合金或固溶体时混合焓的大小，发现最大的负混合焓均分别出现于Fe-Nb系和B-Nb系，其次为Fe-V系和B-V系，并且形成有序合金时能量最低。在此基础上，进一步考虑了应变能、弹性能和结构能的影响，并结合Slater-Pauling关系，预测了利用机械合金化法或利用快速凝固—非晶晶化法制备Fe₈₄（NbV）₇B₉高磁通密度的纳米晶软磁材料的可行性。在此基础上，采用机械合金化-高压成形法制备了Fe₈₄（NbV）₇B₉纳米晶块体材料。研究表明，在机械合金化过程中，随着球磨时间的增加，Fe、Nb（V）、B混合粉末合金化程度逐渐升高，晶粒尺寸逐渐减小，最终形成了具有bcc结构和微纳层状组织的、晶粒为10～15nm的Fe₈₄（NbV）₇B₉非平衡纳米晶固溶体。V元素的添加可加速合金化过程。完全合金化的粉末，其M_s可达150～170Am²／kg。粉末边缘生成的非晶层有利于交换耦合作用的进行，改善了合金的矫顽力。在退火过程中，随着退火温度的升高，Fe₈₄（NbV）₇B₉纳米晶粉末晶粒尺寸逐渐长大，内应力逐渐松弛。但在750℃以下退火，纳米晶长大速度很慢，且无新相生成。对于Fe₈₄Nb₃V₄B₉合金，其晶粒尺寸可保持在10～20nm范围内。当退火温度高于750℃以后，纳米晶尺寸急剧长大，并且产生了NbFeB等杂质相。在机械合金化过程中，位错泵机制和层状结构为元素间原子扩散提供了快速通道。机械合金化使粉末内产生了大量的微纳层状结构，原子可以通过层间界面扩散形成非平衡固溶体。机械合金化过程中，大量位错积累的结果导致位错胞的形成，并最终发展为纳米晶。当晶格畸变和位错密度增至系统自由能足够高时可获得非晶结构。在上述工作基础上，采用5.5GPa的成形压力和1530w加热功率的高压成形条件，成功制备了相对密度大于97％的Fe₈₄（NbV）₇B₉纳米晶块体，晶粒尺寸约为10～15nm，饱和磁化强度约为150Am²／kg，矫顽力约为0.85KA／m。退火时，纳米晶块体晶粒长大，应力松弛以及新相形成的规律与纳米晶粉体基本相同。研究发现，提高超高压压形压力，不但能有效抑制Fe-Nb、Fe-B等杂质相生成，而且能有效防止块体纳米晶尺寸的长大。这一结论对今后纳米晶软磁块体材料超高压成形技术的发展具有重要的指导意义。本文还采用水冷铜模快速凝固法首次成功制备了具有非晶与纳米晶双相结构的Fe₈₄（NbV）₇B₉块体材料，其晶粒尺寸约在10～20nm之间，且均匀分布在非晶基体中。退火时，块体中的非晶部分逐渐产生晶化现象，同时，块体材料的软磁性能逐步提高。550℃退火可获得最佳综合软磁性能：B_s=1.52～1.54T，H_c＜5.0～8.0 A／m，μ_e（1KHz，0.4A／m）=18000～20000。本文研究的水冷铜模快速凝固+非晶晶化法是一种非常有前途的制备高性能纳米晶软磁块体材料的短流程方法，该方法制备的Fe₈₄（NbV）₇B₉纳米晶块体材料软磁性能远优于目前报导的各种方法制备的纳米晶软磁块体材料的软磁性能，甚至不逊于快速凝固非晶薄带+非晶晶化法制备的二维带状纳米晶软磁材料，其前景是非常诱人的。根据Hill微系统热力学理论，本文还建立了一个同时包含尺寸和形状效应的磁性纳米颗粒的居里温度模型，该模型对自由态和嵌入态Fe、Co、Ni磁性纳米微粒居里温度的计算结果与纳米晶体居里温度下降的实验现象吻合良好。最后，本文根据纳米晶体结合能的计算公式，建立了纳米晶体熔化与过热的等效模型。在此基础上计算了Fe、Co、Ni等磁性纳米晶体的最小临界尺寸和最低熔化温度，分析了其在使用过程中可能存在的热稳定性问题。此外，还建立了纳米晶体表面能和空位形成能模型，该模型对Fe、Co、Ni等金属的预测结果与大部分实验数据和其他理论计算相吻合。在此基础上，利用纳米晶体结合能的包覆界面模型对Fe-M-B纳米晶双相合金进行了描述，并根据熔化与过热的等效模型对其熔化温度、熔化熵、熔化焓等参量进行预测。更多还原

【Abstract】 Preparation methods for Fe₈₄（NbV）₇B₉ nanocrystalline soft magneticalloys and related fundamental theory were investigated in this paper,supported by the National Key Technologies R & D Program of Chinaduring the 10th Five-year Plan Period （No.2001BA310A03-1）.First, the mixing heat of formation for Fe and B that combine withother elements to form binary ordered alloys or solid solution wascalculated on the basis of thermodynamic theory. It’s found that thenegative maximum value appears in the Fe-Nb and B-Nb systemsrespectively, next are Fe-V and B-V systems, and the energy requiredreaches its minimum for the formation of ordered alloys. Furthermore,considering the influence of strain energy, elastic energy and structureenergy, and combining the relationship of Slater-Pauling, the possibilityof high magnetic Fe₈₄（NbV）₇B₉ prepared by mechanical alloying or rapidsolidification- crystallizing amorphous solid is predicted.Accordingly, Fe₈₄（NbV）₇B₉ nanocrystalline bulk was obtained bymechanical alloying-high pressure forming. It shows that with theincreasing of milling time, Fe、Nb（V）、B mixing powders tend to higheralloying lever and the grain size decreases gradually. In the end,nonequilibrium Fe₈₄（NbV）₇B₉ nanocrystalline solid solution with bccstructure, microthin-layer morphology and grain size of 10～15nm isformed. The addition of V element can accelerate the alloying process.Completely alloyed powders exhibit high M_s of 150～170Am²/kg.Amorphous layer generated on the boundary of powders can benefit theexchange of coupling, which improves the coercive force of alloys. Withthe increment of annealing temperature, grain size of Fe₈₄（NbV）₇B₉nanocrystalline powder increases gradually and its inner stress relaxes.However, when annealed below 750℃, the nanocrystalline size growsslowly without new phases formed. For Fe₈₄Nb₃V₄B₉ alloy, its grain sizekeeps within 10～20nm. When the annealing temperature beyond 750℃, the grain size increases rapidly and new phases, such as NbFeB appear.During mechanical alloying, the dislocation pump mechanism andlayer structure supply fast channel for atom diffusion between elements.Mechanical alloying forces plentiful micro-layer structure inside powders,and atoms can diffuse via interlayer interface to form nonequilibriumsolid solution. During the alloying, lots of dislocations are accumulated,which results in the formation of dislocation cell and later develops intonanocrystalline structure. When the system free energy caused by latticedistortion and dislocation density rises high enough, amorphous structurecan be obtained.Based on the above work, Fe₈₄（NbV）₇B₉ nanocrystalline bulk withrelative density over 97％was successfully produced under 5.SGpapressure and 1530w heating power. Its grain size is about 10～15nm,saturation magnetization is about 150Am²/kg and coercive force is about0.85KA/m. During annealing, its regulation of the growth ofnanocrystalline grain, stress relaxation and formation of new phase fornanocrystalline bulk is well constant with that of nanocrystalline powders.It’s also found that the increase of forming pressure during high pressureprocess can effectively restrain the formation of impurity phases, such asFe-Nb and Fe-B, and prevent the growth of bulk nanocrystalline grains aswell. Such conclusion is meaningful to the technique development ofsuperhigh pressure forming.The Fe₈₄（NbV）₇B₉ bulk with amorphous and nanocrystalline binarystructure was also successfully prepared by rapid solidification method inthe cooling copper mould. Its grain size is about 10～20nm, distributedhomogenously in the amorphous matrix. During annealing, amorphousparts in the bulk is crystallized gradually, and the soft magnetic propertyof the bulk is improved. Optimal soft magnetic data can be obtained whenannealing at 550℃, namely, B_s=1.52～1.54T, H_c＜5.0～8.0 A/m,μ_e （1KHz, 0.4A/m）=18000～20000. The shortened process of rapidsolidification by cooling copper mould plus amorphous crystallization israther promising for the preparation of high performance nanocrystallinesoft magnetic bulks. The soft magnetic properties of Fe₈₄（NbV）₇B₉nanocrystalline bulk prepared by this method are much better than nanocrystalline bulks obtained by other methods. Especially, our bulkseven perform as well as those nanocrystalline soft magnetic ribbons thatprepared by rapid solidification ribbon plus amorphous crystallization.Therefore, it exhibits remarkable application potential.According to the Hill’s thermodynamic theory for small systems, aCurie temperature model for magnetic particles was developed to showboth size effect and shape effect. Using this, the prediction of Curie pointfor freestanding and embedding magnetic particles Fe、Co、Ni is in goodagreement with experimental results. Meanwhile, the model for meltingand superheating of nanocrystals was developed on the basis ofnanocrystal cohesive energy. Accordingly, the minimum critical size andmelting temperature of Fe、Co、Ni magnetic nanocrystals were calculated,and their thermal stability during application was discussed. At last,models for higher surface energy and vacancy formation energy ofnanocrystals were developed. The predicted results for Fe、Co、Ni metalsare agreeable with mostly corresponding experimental values and othertheoretical calculations found in the literature. Based on this, Fe-M-Bbinary structure alloy was described by the nanocrystal cohesive energymodel with embedded interface, and its melting temperature, meltingentropy and melting enthalpy can be further predicted by our melting andsuperheating equivalent model.更多还原