【作者】 曹扬；

【导师】 陈光； 陈国良；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2009， 博士

【摘要】 块体金属玻璃(BMG)具有优异的力学性能和物理性能,为新型结构材料和功能合金的发展开辟了新的领域。然而,BMG在室温下发生灾难性脆性断裂,已经成为制约该材料规模化工程应用的瓶颈。因此,BMG室温脆性已经成为亟需解决的关键问题。已有研究表明,铸态内生微米尺度的塑性固溶体／BMG复合材料是改善BMG室温脆性的有效手段,并认为内生微米尺度第二相本身为塑性相是BMG复合材料增塑的必要条件。本文基于BMG室温脆性的本质,认为只要第二相能够阻碍单一剪切带的扩展,使剪切带增殖就能提高室温塑性,无论第二相本身是否是塑性相。据此设计并成功制备出铸态内生金属间化合物／BMG复合材料,其不但具有高的断裂强度,也具有显著的室温塑性(塑性变形达到4.4%)。进一步的分析研究发现,第二相尺寸不同,对BMG塑性影响不同：纳米尺度的第二相诱发剪切带大量形核而增塑；尺寸大于剪切带间距的第二相可以有效阻碍剪切带的扩展、诱发多剪切带的形成而提高塑性；而尺寸小于剪切带间距又大于纳米尺度的第二相对BMG塑性没有显著影响。BMG的热稳定性与晶化行为是BMG研究领域普遍关注的基础问题。本文的研究发现,高密度电脉冲能够促进BMG的晶化,并改变晶体析出相的类型。以V1合金为例,常规等温退火时晶化发生于过冷液相区内(400-450℃),而以电流密度为1410A／mm2的电脉冲退火时,在玻璃转变温度以下(338℃)就可以发生显著的晶化。与常规等温晶化析出BeNi、Be2Zr、Zr2Cu等相不同,电脉冲作用下,在析出Be2Zr、Zr2Cu相的同时,还发现有NiTi2相和未知亚稳晶体相析出。电脉冲后等温晶化过程中,延长等温时间,非晶馒头峰的半高宽(FWHM)变窄,表明BMG内短程有序程度增加。连续加热过程中施加5Gpa的高压可以抑制V1合金晶化,发生晶化的温度为410-430℃,远高于相同加热速率下的起始晶化温度(393℃)。等温晶化过程中施加12T的强磁场,能够显著提高Fe基BMG中晶化相的体积分数,而磁场施加方向对析出相体积分数没有明显影响。迄今为止,块体NdFeB永磁材料由于缺乏有效的磁性取向控制方法,在工业生产中不得不采用破碎与制粉——磁场取向与成型——烧结或粘结——充磁与检测的工艺来制备,不仅工序繁杂、生产成本高,而且还无法获得完全致密的永磁体,造成不耐蚀等缺点。本文采用Fe基BMG进行磁场晶化,试图诱导磁性晶体相在形核长大过程中实现磁性取向,以期为摒弃制粉过程、直接由非晶固体或液体制备完全致密的各向异性永磁体探索可能途径。实验采用去除形状各向异性的(Fe71B21Nd8)96Nb4样品,在12T的强磁场下进行700℃晶化退火,发现施加平行磁场的内禀矫顽力和最大磁能积(BH)max均显著高于施加垂直磁场的样品,而不加磁场的介于两者之间,证明强磁场可以在晶化过程中诱发磁性各向异性,为短流程直接制备块体NdFeB永磁体探明了可能性,具有重大的科学价值和应用潜力。更多还原

【Abstract】 Bulk metallic glasses (BMGs) with excellent mechanical and physical properties open a new research field for structural and functional materials. However, the catastrophic brittle fracture at room temperature has restricted the extensive application of BMGs. The brittleness of BMGs at room temperature is one of the key problems to be resolved in the near future.Previous researches showed that the formation of plastic solid solution phase in micrometer scale in the BMG matrix during casting is very effective to improve the plasticity of BMGs at room temperature. It is normally deemed that the in situ formed second phase in micrometer scale should be plastic to improve the plasticity of the BMG-based composites. Based on the understanding of brittleness of BMGs at room temperture, a new route to improve plasticity is proposed in this dissertation, i.e. the second phase in the composite is effective to improve the plasticity of BMGs as long as it can block the propagation of single shear band and induce the formation of multiple shear bands, in despite of the plasticity of the second phase. Following this idea, in situ formed intermetallic/BMG composite was designed and fabricated. This composite showed both high fracture strength and remarkable plasticity at room temperature (plastic strain 4.4%). Further study showed that the second phase with different sizes can affect the plasticity of BMG-based composites with different mechanism: if the second phase is in nanometer range, it can induce the aboundant nucleation of shear bands and improve the plasticity of the composite; if the second phase is in micrometer range and larger than the shear band spacing, it can block the propagation of single shear band and induce the multiplication of shear bands effectively, resulting in an improved plasticity; if the second phase is larger than nanometer range, but smaller than shear band spacing, it does not affect the plasticity obviously.The thermal stability and crystallization behavior of BMGs is a well concerned basic problem. In this dissertation, the high density electric pulse was found to be effective to induce the crystallization and alter the type of crystalline phases in BMGs. ForⅥalloy, the crystallization started in the supercooled liquid region (400-450℃) during the isothermal annealing. However, obvious crystallization reaction was found at a temperature well below the glass transition temperature (338℃) after annealing under an electric pulse with a density of 1410A/mm2. After crystallization under high density electric pulse, the crystalline phases included NiTi2 and an unknown metastable crystalline phase, besides Be2Zr and Zr2Cu phases which precipitated after isothermal annealing. Prolonging the holding time of the electric pulse annealing, the full wave at half maximum (FWHM) of the halo peak for amorphous phase became narrow, indicating an enhanced short range order. During the continuous heating, the high pressure up to 5Gpa supressed the crystallization ofⅥalloy. The crystallization under 5Gpa started at about 410-430℃, which is much higher than the value of onset temperature for crystzllization (393℃) found in DSC at a same heating rate. During isothermal annealing, the application of a magnetic field of 12T increased the volume fraction of crystallized phases in Fe-based BMGs, while the direction of magnetic field did not affect the volume fraction distinctly.Up to now, because of the lack of effective method to control magnetic orientation, a complicated and costly work flow including ingot crushing and powder preparation-magnetic orientation and molding-mintering or bonding-magnetization and inspection is necessary to fabricate bulk permanent magnets. However, the magnets fabricated with sinitering or bonding methods are not fully compact, resulting in some disadvantages such as low corrosive resistance. In this dissertation, magnetic annealing of Fe-based BMGs was proposed to attempt to induce the orientation of magnetic phases during the nucleation and growth of these crystalline phases. It might be a new approach to fabricate fully density anisotropic permanent magnets from the BMG solids or melts directly, avoiding the powder preparation process. (Fe71B21Nd8)96Nb4 BMG samples were annealed at 700℃under a magnetic field with a density of 12T. After excluding the shape anisotropy, it was found that the inherent coercive and maxium energy product (BH)max for samples annealed with a parallel magnetic field were much higher than those with a perpendicular field, while the values for samples annealed without a field ranged between them. These results demonstrated that the application of high density magnetic filed was effective to induce anisotropic magnetic properties during the crystallization of BMGs. Magnetic annealing of Fe-based BMGs precursor is feasible to fabricate bulk NdFeB permanent magnets with a short workflow, indicating great scientific significance and application potential.更多还原