【作者】 邓沛然；

【导师】 李建国；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2006， 博士

【摘要】 磁致伸缩材料的主要功能是实现电信号和机械信号之间的转换。具有实用价值的巨磁致伸缩材料Tb-Dy-Fe自上世纪70年代问世以来,最先用于军事方面的声纳装置。在上世纪90年代以来,在民用工业领域的用途不断取得拓展,到目前已形成5亿美元以上并在不断增长的市场。巨磁致伸缩材料的磁致伸缩性能具有各向异性,对于Tb0.3Dy0.7Fe1.9合金,晶体沿易磁化轴<111>取向可以获得最佳的磁致伸缩性能。但是采用传统的定向凝固技术只能获得<112>和<110>择优取向。运用籽晶技术在提拉法中可以获得<111>取向,但这种方法工艺较复杂,并且成分沿棒的轴向不一致。因而寻找一种有工业价值的方法制备沿轴向<111>取向的Tb0.3Dy0.7Fe1.9合金是有意义的。近20来年的研究表明,磁性材料在磁场中凝固可以直接制备沿易磁化轴或难磁化轴取向的晶体。在磁场中取向的机理与传统定向凝固中晶体取向生长的机理完全不同,在传统的定向凝固中,热流沿着单一方向导出,沿着热流方向形核晶粒经竞争生长后在液固界面以最小界面能的晶面不断向液相延伸,界面的过冷因素最终修正定向生长的晶体取向。而在磁场中非定向传热的凝固,形核单晶晶粒的磁能与环境的无序扰动能的对比决定晶体的取向,在液相占主体的固液相中环境的无序扰动因素主要包括热无序效应和熔体的湍流。静磁场的作用在于抑制熔体湍流的同时为形核单晶提供克服熔体粘性和热无序效应的平行于磁场方向的磁能,从而实现晶体在凝固过程的取向。对于有各向异性的磁性材料,抗磁性材料能够获得沿难磁化轴取向的晶体组织,铁磁性材料等则获得沿易磁化轴取向的晶体组织。由于设备方面的因素,对于高温和远高于居里点的情况下,磁性材料在凝固中取向的基本条件还缺乏较为定量的研究和认识,因而该技术到目前还未能得到工业化运用。我们自制了简易的磁场装置,该磁场装置可以安置在超高温度梯度定向凝固的真空容器中。当熔体以较慢的冷却速率凝固时,磁场装置可以提供140mT的磁场强度。试验采用高频加热。在140mT的静磁场中,在较慢冷却速率的样品中,平行于磁场方向的织构开始出现。对于φ16mm×18mm的Tb0.3Dy0.7Fe1.9合金,当冷却速率为0.8℃/min时,在试样的中部以平行于磁场的<111>取向的晶体组织为主,沿易轴取向的晶体组织约占70%。取向的晶体组织以平行于磁场方向的片状的Laves相为主。晶粒粗大,粗大的晶粒有利于磁致伸缩性能。当磁场为0时,在同样的冷却速率下,晶体组织是非取向的。在同为立方Laves相ReFe系的磁致伸缩材料中,室温状态与Tb0.3Dy0.7Fe1.9合金相比,TbFe2和DyFe2具有强的磁晶各向异性。在与Tb0.3Dy0.7Fe1.9合金接近的凝固参数和磁场条件下,TbFe1.9和DyFe1.9合金同样能获得沿易磁化轴取向的晶体,其中TbFe1.9以<111>为取向,DyFe1.9以<100>为取向。Tb0.3Dy0.7Fe1.9和TbFe2合金的磁晶各向异性常数K1为负,DyFe2合金的磁晶各向异性常数K1为正,因而他们的易磁化轴取向不同。但是在我们的试验中,TbFe2和DyFe2并未比室温下磁晶各向异性接近软磁的Tb0.3Dy0.7Fe1.9合金表现出更为优异的顺磁生长特性,可能这与它们在高温下的各向磁化率Δχ的差别减少有关。在试验中,对于影响晶体取向的其他因素也进行了初步研究。材料的成分被发现对晶体取向的影响较大,对于RFex合金,当铁含量趋于x≥2时,液固相中将有过量的RFe3相析出,其次,随着铁含量的增加,液固并存的两相区的温度区间大大减少,这样在凝固时随着温度的降低,固相迅速增加、熔体的粘度上升很快因而不利于晶体取向。因此控制好RFex合金的材料成分有利于获得良好的晶体取向。在磁场中定向的Tb0.3Dy0.7Fe1.9合金在18.7MPa的压应力和6900Oe的磁场中的磁致伸缩系数可以达到1700ppm,这一数值要高于磁场预取向粉末冶金法制备的产品,低于籽晶法制备的单晶<111>取向的性能。可以预计在实际运用中,随着取向度的增加和晶体质量的改善,用这种技术可以制备更高磁致伸缩系数的Tb0.3Dy0.7Fe1.9合金。试验表明,降低熔体的冷却速率,可以大大降低熔体的湍流程度,这意味着慢凝的熔体的湍流可以被较弱的静磁场所抑制。在这里需指出的是熔体的湍流被抑制与熔体内部的运动被抑制是有区别的,前者指湍流向层流转变,熔体内部的运动仍是存在的,熔体内部的运动被抑制需要很强的磁场。湍流被抑制就已经可满足晶体取向所需要的较均匀的温度场的要求。综合这些内容,文中给出了磁场中凝固取向控制的参考模型。许多铁磁性材料由于在高温熔点附近具有相对较高的残余磁晶各向异性,因而在磁场中凝固时应当有着优良的顺磁生长特性。更多还原

【Abstract】 It is the main function of magnetostrictive materials to achieve electromechanical convertibility. Giant magnetostrictive material Tb-Dy-Fe found in 1970s. One of its earliest applications was as hydroacoustic transducers for military purpose. With the increasing use in civil industry from 1990s, the increasing market is about 5 hundred millions dollar.Magnetostrictive properties of magnetostriction materials depend greatly on the grain orientation and microstructure for highly anisotropic magnetostriction. The best magnetostrictive properties can be obtained along the <111>direction of Tb0.3Dy0.7Fe1.9 alloy. However, <112> or <110> is the preferred growth direction in conventional directional solidification technique. The <111> oriented crystal of Tb0.3Dy0.7Fe1.9 alloy can only be grown by seeding technique, but this process is complicated and chemical composition along the axial direction of rod is heterogeneous. It is of significant that searching an industrial method to prepare Tb0.3Dy0.7Fe1.9 with <111> orientation.Studies in recent twenty years show that Solidification in a static magnetic field is a novel process, which can produce oriented magnetic materials with either their easy or their hard magnetization axes along the field. There are different orientation mechanism for conventional directional solidification and that in static magnetic field. Heat is transferred along a direction in conventional directional solidification; the crystal faces with minimum interface energy after competitive growth extend continually into liquid along the direction of heat, supercooling factors in interface will modify the direction of crystal growth eventually. In solidification with non-directional heat conduction under a static magnetic field, the rivalry between magnetic energy of single nuclei and disorder disturbance energy in environment will determine the crystal orientation. In a liquid-solid coexisting area with a majority of liquid, disorder disturbance factors include mainly thermal disorder effect and turbulence in melt. Static magnetic field will suppress the turbulence in melt, meanwhile, induce magnetic energy of single nuclei along the field to overcome melt viscosity and thermal disorder effect. Consequently, the orientation of crystals will be achieved during solidification course. For magnetic materials possessing magnetic anisotropy,crystal structure will be oriented along hard-magnetization axes in diamagnetic materials, and crystal orientation along the easy magnetization axes will be obtained in ferromagnetic materials etc..Due to absence of experimental equipment, no quantitative researches were done for essential conditions of orientation for magnetic materials solidified at high temperatures far above their Curie point. Therefore there is no industrial application of solidification in a magnetic field to achieve crystal orientation presently.We made a simple magnetic apparatus, which can be installed in a vacuum container of a directional solidification equipment with super high temperature gradient. The magnetic field of 140 mT can be maintained while solidification at slow cooling rate. Adaptation of high-frequency was to heat sample in experiment. Textures begin to appear along the field while sample solidified at slow cooling rate in a magnetic field of 140 mT. For Tb0.3Dy0.7Fe1.9 alloy with sizeφ16mm×18mm, the crystal structure with <111> orientation along the field is dominant in middle part of sample while at cooling rate of 0.8℃/min, crystal grains along the easy axes are about of 70% in specimen. Plate-like crystal grains of Laves phase in oriented specimen are along the field, size of crystal grains is coarser, which is beneficial to magentostrictive properties. The orientation of crystal grains are in random while at the same cooing rate in zero magnetic field.As a magnetostrictive material of cubic Laves phase in Re-Fe system, the TbFe2 and DyFe2 compounds possess the large magnetocrystalline anisotropy at room temperature. Due to the combination of the TbFe2 compound and DyFe2, the magnetocrystalline anisotropy of Tb0.3Dy0.7Fe1.9 alloy is reduced greatly. Crystal orientation along the easy axis for TbFe1.9 and DyFe1.9 also can be obtained by the similar condition of orientating Tb0.3Dy0.7Fe1.9 alloy. TbFe1.9 alloy is oriented along <111> direction while DyFe1.9 alloy along <100> direction. The TbFe2 compound and Tb0.3Dy0.7Fe1.9 alloy possess the minus magnetocrystalline anisotropy constant K1, and DyFe2 exhibits a large positive K1. Therefore, their directions of easy magnetic axis are along different crystal orientations. However, for those compounds, the great difference in magnetocrystalline anisotropy at room temperature seems to have no notable influence on the texture formation during their solidification course at high temperature respectively. It may be explained by that the values ofΔχ(the anisotropy of the paramagnetic susceptibility of crystal) for TbFe2、DyFe2 and Tb0.3Dy0.7Fe1.9 alloy tend to be close to each other at high temperatures.Other factors influencing the crystals orientation are also studied in experiment. The composition is found to affect the crystal orientation during solidification. For RFey alloys, excessive RFe3 phases will precipitate in liquid-solid phase when Fe content tends to y≥2. Furthermore, the temperature interval of liquid-solid phase is greatly reduced, then solid phase increase greatly as well as viscosity of liquid as temperature of melt decreases, which are unfavorable for crystal orientation. It is necessary to control the composition of RFex alloys for a better crystal orientation.The magnetostriction of the oriented Tb0.3Dy0.7Fe1.9 alloy along the field reaches 1700ppm under 18.7MPa and 6900 Oe. Such a value is higher than that in the grain-aligned sintered compact prepared by powder metallurgy technique, and lower than that in <111>-oriented single crystal. However, the higher orientation degree of Tb0.3Dy0.7Fe1.9 alloy should be achieved in practical application by slow cooling in a relatively strong magnetic field, the magnetostrictive properties would be enhanced further.Experiments show that using a low cooling rate can greatly weaken the disturbance degree inside melt, and a weak disturbance can be restrained in a relatively weak magnetic field. It necessary to make clear that there are differences for suppressed turbulence and restrained motion in melt. The former means that laminar flow replaces turbulence but the motion in melt still exists. Restraining the motion requires very strong magnetic field. The even temperature field can be fit for crystal orientation by suppressing turbulence in melt. Summarizing those investigation and application, a model of controlling crystal orientation by solidification in magnetic field is proposed. Many ferromagnetic materials may be readily oriented during solidification course in field due to their relatively large residual |Δχ| at high temperatures near melting point.更多还原