【作者】 吴尉；

【导师】 张湘义；

【作者基本信息】 燕山大学 ， 材料学， 2009， 博士

【摘要】 纳米晶双相复合永磁材料是一种引人关注的新型永磁材料,这类材料通过在纳米尺度下的软磁相和硬磁相晶粒之间的磁交换耦合作用获得高的综合磁性能。理论预计,取向排列的纳米晶双相复合磁体的理论磁能积可高达1000 kJ/m3,被称为“兆焦耳永磁体”,高于任何一种单相永磁材料。这类材料还具有稀土含量低,价格便宜,化学稳定性好等特点,具有潜在的开发应用前景,有望发展为新一代高性能永磁材料。然而在块体纳米晶复合永磁材料中,如何获得晶粒均匀细小且取向排列的纳米晶则是一项具有挑战性而又有意义的工作。非晶晶化的方法是制备纳米晶材料的一个重要方法,高压退火非晶合金的晶化规律研究也是当前材料科学研究的前沿之一;开展高压下磁性非晶合金晶化过程中纳米晶形成规律的研究,不但有助于开发高性能的磁性材料,而且有助于了解纳米晶在高压下的形成规律。本文采用六面顶压机(Six-Side Anvil Cell)、X射线衍射分析(XRD)、透射电子显微分析(TEM)和高分辨电子显微分析(HRTEM)等研究手段针对磁性非晶合金Nd9Fe85B6开展了如下几个方面的研究工作:研究了在高压下非晶Nd9Fe85B6合金基体中纳米晶体的形核和生长过程。研究发现:压力可以诱导Nd2Fe14B相纳米晶体的取向生长。分析结果表明:这是由于压力对不同半径原子的扩散抑制程度不同造成的,不同的晶面其含有不同种类原子的比例是不同的,当晶面含有较多压力抑制作用较弱的原子时,其晶面将易于形核和生长。在高压下,多组元的晶体会具有较高的生长各向异性。因此可以通过高压退火非晶粉末或条带的方法,制备出具有一定晶体取向的块体纳米晶合金。在6GPa压力条件下,获得了晶粒尺寸<10 nm,具有强晶体学取向的块体纳米晶合金。研究了非晶合金Nd9Fe85B6高压退火过程中,压力对形成的纳米晶的微观组织结构的影响。研究发现:在高压下可以获得比常压条件下晶粒尺寸分布更均匀的纳米晶。分析结果表明:这是由于非晶原子做无规则密堆积排列,其密度低于晶态合金,压力可以通过压缩非晶的体积,促使非晶合金中原子的短程重排,短程原子的重排促使在非晶合金基体中形成一些细小的均匀分布的原子团簇,这些原子团簇成为晶化的优先形核的位置,因而促进了晶粒尺寸的均匀分布。而压力对非晶合金Nd9Fe85B6晶化过程的动力学影响主要表现为两个方面:一个方面是压力降低晶化相的临界晶核形成自由能,促进晶化相的形核和生长;另一个方面是压力会使非晶基体的体积压缩,限制原子的运动能力,降低原子的扩散系数,抑制晶化相的长大。对于非晶合金Nd9Fe85B6而言,在较低的压力范围内(0—1GPa),压力降低晶化相的临界晶核形成自由能的因素是主要的影响因素,α-Fe相的晶粒尺寸随压力的增加而增加;在高压的范围内(1-6GPa),压力抑制扩散的因素成为主导因素,晶化相Nd2Fe14B相和α-Fe相的晶粒尺寸随压力的增加而降低。同时,压力对不同晶化相的晶化过程的相对影响程度也是不同的,压力改变了各晶化相之间的自由能关系,因此显著影响晶化相的析出次序,在低压下先析出相为α-Fe相,而在高压下,先析出相为Nd9Fe85B6相。另外,在室温高压(6 GPa)条件下,压力可以使非晶合金Nd9Fe85B6发生纳米晶结构转变。研究了高压退火过程中晶化相原子扩散的激活体积。研究结果表明:在晶化的过程中,α-Fe相和Nd2Fe14B相的激活体积分别为10.3立方埃和7.73立方埃;α-Fe相的Fe原子的扩散机制以空位扩散为主;Nd2Fe14B相中的Nd、Fe和B原子以集体的扩散方式(Collective diffusion mechanism)为主,原子的集体扩散方式导致了Nd2Fe14B相形成了一个较小的激活体积。更多还原

【Abstract】 Nanocomposite permanent magnetic materials consist of both nano-scale hard and soft phase, which get excellent magnetic performance through exchange coupling between their neighboring atomic magnetic moments. If the nanocrystals of the two phase nanocomposite magnets are orientated, their potential upper limit of maximum energy product can exceed 1000 kJ/m3, which is higher than that any single phase permanent magnetic materials. In addition to the high maximum energy product that may be achieved, nanocomposite magnets are of commercial interest because they require less of an expensive rare earth element. However, in the bulk nanomaterials, it is a difficult and significant work to prepare the orientated, ultra-fine and homogeneous nanocrystals.Amorphous crystallization method is an important technique to prepare the nanomaterials; moreover, the research of amorphous crystallization under high pressure is the front edge in the materials sciences. Therefore, to investigate the nanostructure transformation of magnetic amorphous alloy Nd9Fe85B6 under high pressure is not only help to develop the nanocomposite permanent materials, but also help to learn the rule of amorphous crystallization under high pressure.By means of X-ray diffraction (XRD), Transmission electron microscopy (TEM), Electron diffraction (ED) and Six-Side Anvil Cell, the nanostructure transformation of amorphous alloy Nd9Fe85B6 under high pressure has been investigated in this paper. The main research contents and results are followed:The oriented growth of nanocrystals in the amorphous matrix under high pressure has been investigated. The results show that the pressure can induce the preferential growth of nanocrystals of Nd2Fe14B phase in amorphous matrix. Nd2Fe14B nanocrystals with a strong crystallographic texture along with [410] orientation have been produced under a pressure of 6 GPa at 923 K. This is attributed to that the diffusion of different size atom is discrepant in amorphous matrix under the high pressure. The plane which is mainly composed of the smaller atom is easier to form under the high pressure.The effect of high pressure on the microstructure in crystallizing amorphous Nd9Fe85B6 alloy has been studied. It is found that application of high pressure makes the microstructure of crystallized alloy much more homogeneous. This is attributed to the homogeneous distribution of the cluster which is formed under high pressure. There two effects of pressure on the microstructure of crystallized alloy: One is to constrain atomic diffusion, which makes atomic mobility more difficult. The low atomic mobility constrains the growth of crystals during the crystallization process. The other is to decrease the critical free energy required to form a nucleus, which promotes the growth of crystals. The average grain size of Nd2Fe14B phase decreases with the increase of pressure, while the size ofα-Fe phase first increase when a pressure of 1 GPa was applied and then decreases with further increase of pressure. The pressure also change the sequence of crystalliztion. Under the low pressure (1 GPa), theα-Fe phase is the first crystallized phase, while under the high pressure (6 GPa), the Nd2Fe14B phase becomes the first crystallized phase. Furthermore, the amorphous Nd9Fe85B6 alloy can transform into the nanocrystals under high pressure at room temperature.The activation volume for nanocrystals growth in amorphous Nd9Fe85B6 alloy has been investigated. The activation volume ofα-Fe and Nd2Fe14B phase areΔV ?= (0.76±0.04) ? and (0.57±0.05) ?, respectively. This indicates the growth ofα-Fe and Nd2Fe14B nanocrystals is dependent on atomic diffusion mediated by vacancy-type thermal defects. The growth of Nd2Fe14B nanocrystals in the amorphous Nd9Fe85B6 may take a collective diffusion mechanism involving Nd, Fe and B atoms, which leads to a relatively small activation.更多还原