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大块纳米结构金属 简介  

2011-05-06 13:04:36|  分类: 默认分类 |  标签: |举报 |字号 订阅

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       元素周期表中,超过80%的元素是金属元素,每一种金属元素显示多种物理特性和特征。在我们的生活中,利用金属这些多方面的个性,使用了大量的金属和金属合金;现代科技高度发展的社会不可能没有金属材料。金属材料最大的特征是在一点上受到高压强从不破碎(具有塑性和韧性),由于其很好的机械性能,很多被用作结构材料。结构金属材料的需求,随着科技发展已经变得越来越迫切。特别是当今那些需要克服环境,资源,能源问题的地方,发展利用合金化材料改善性能的方法,就像在过去增加一个新元素那么容易,但不可能满足社会多层面的需要。其他方式,不是传统冶金和材料科学的延伸,而是需要间断的,金属材料快速发展。大块纳米结构金属就是在这样的建议下有望实现金属材料发展的革命性新材料,而其化学成分与传统金属没有明显差别。

More than 80% of elements in the periodic table are metallic elements and each metal element show various physical properties and characteristics. In our life, a large amount of metals and alloys have been used by utilizing those various individuality and modern highly developed science & technology society is impossible without metallic materials. The biggest characteristics of metallic materials are in the point that they never break fragilely while having high strength (have ductility and toughness), and many are used as structural materials for its mechanical characteristic. The demand for metallic material for the structure has become increasingly stringent with the development of science and technology. Especially where nowadays it is required to overcome environmental, resources and energy problems, developing methods of improving the performance by alloying materials as easy as adding a new element such on the past, may not meet the demands of a multifaceted society. In other way, not an extension of conventional metallurgy and material science, but a discontinuous, rapid development of metallic materials is required. Bulk Nanostructured Metals in this proposal is expected to be revolutionary new material that enables the rapid development of metallic materials while its chemical composition not significantly different with conventional metals.
       我们使用的大多数金属材料是多晶材料,具有不同晶体取向组合的晶粒。一般经验的认为,材料的多样特征通过改良组成多晶体的晶粒尺寸来获得改善,在材料结构控制方面,晶粒改良总是一个重要的问题。然而,甚至到现在,大块金属材料的最小平均尺寸大约为10微米?。顺便说说,晶界具有不同的方向,原子三维的周期排列是界限。然而,面向晶界的原子和相邻晶测的原子保持结合,使得多晶体保持为一个没有破坏的大块形状。但,几何分布相位和他们的结合与在晶粒内部有序排列原子是不同的。此外,必然形成几何形貌,自由区小于一个原子尺寸也在晶界区域出现。

Most of the metallic materials that we use are polycrystalline material (polycrystal) where number of grains having different crystal orientations assembled. It is empirically known that various characteristics of materials improved by refining the grain size that composed polycrystals, and the crystal grain refinement was always an important problem on structure control of materials. However, even to the present the minimum average size of the bulk metallic material is about 10mm. By the way, the boundaries of neighboring grains have different orientations (grain boundary), atoms’ three-dimensional periodic arrangement is cut-off (Fig.1). However, the atom that faces the grain boundary keeps the bonding with the atom on the adjacent grain side, as the polysrystal maintained over the bulk shape without destruction. But, aspects of geometrical arrangement and its bonding are different from the orderly periodic arrangement of atoms in the grain interior. In addition, form the aspect of geometric inevitability, free area less than an atom size is also present in grain boundary area.

Fig.1 Polycrystalline Metal and Grain Boundary
Fig.1 Polycrystalline Metal and Grain Boundary

晶界的体积分数可以作为平均晶粒尺寸的函数来计算,假设原子毁坏区发生在晶界附近1nm的厚度范围内。图二显示,晶界的体积分数,没有大于10微米的晶粒。按照传统来讲,这样的纳米结构多晶金属几乎是没有晶界的。相反,当平均晶粒尺寸小于1微米,晶界的体积分数迅速增长,毕竟,大块多晶体,是由晶粒或者相的尺寸小于1微米组成的矩阵,被称作大块纳米结构金属。
The volume fraction of the grain boundary can be calculated as a function of the mean grain size by assuming the thickness of the area where such an atomic disruption occurs in the vicinity of grain boundaries to 1nm. This is shown in Fig.2. From this figure, the volume fraction of grain boundaries in polycrytal having size more than 10mm proves to be nearly 0%. That is, the metals that had been used so far were polycrystals with “ almost no grain boundaries”. In contrast, the volume fraction of grain boundaries increases rapidly when the mean grain size become 1mm or less. Afterwards, the bulk polycrystals, having matrix composed by crystal grain or phase size less than 1mm, will be called Bulk Nanostractured Metals.

Fig.2 Bulk Nanostructured Metals are full of grain boundaries
Fig.2 Bulk Nanostructured Metals are full of grain boundaries

因为在晶界区的原子结构和晶粒内部的原子结构有很大差别,几乎全是晶界的大块纳米结构金属,显示和传统金属不同机械性能和特征。有关金属材料的畸变是有与晶格的滑移位错带来的。在大块纳米结构金属中,每个晶体被高密度的晶界牢牢的束缚,位错使得能量发生变化,机械性能表现超越传统金属的一般常识。例如:大块纳米结构金属显示其强度达到传统晶粒尺寸的四倍。结果是,这样铝的强度可以达到传统钢铁的强度。不但强度更高,而且能够实现具有柔韧性的高强度,这被看做是替代传统方法的一种方向。此外,随着晶界密度的快速增加,通过原子扩散剂量控制的各种各样的高温现象, 也能出现在低至室温。而且,由于晶界和其他高密度晶格缺陷,自由能大幅增加,每个合金系统热动力相稳定化改变,转化,析出,再结晶现象,这些在传统金属中不可能发生,显示新的结构。一个要点是,大块纳米金属源自只是晶界结构的不寻常特征,这个现象也能在纯金属和低合金系统中显现。大块纳米结构金属作为具有再循环能力,颠覆传统合金设计概念,简单的化学组成,没有消耗稀土资源,显示优越特性,成为一种优良的结构材料。换句话说,大块纳米结构金属使得金属材料跳跃式的快速发展成为可能,作为一种支持新的环境和能源技术的新材料,能够有助于社会发展的可持续性。
       As the atomic structure in the grain boundary area is greatly different with that grain interior, the Bulk Nanostructured Metals, which consists “only grain boundary”, shows different mechanical properties and a characteristic different from conventional metals. Deformations on metallic materials are carried by the sliding motion of the lattice defects called dislocation. In Bulk Nanostructured Metals, each crystal is strongly bounded by the high density of grain boundaries and changes on dislocation energy occurs, and that behavior (that is, mechanical properties) is beyond conventional common knowledge of conventional metals. For instance, the Bulk Nanostructured Metals shows strength that reaches four times of that with conventional grain size. As a result, aluminum that has strength as conventional steel material can be achieved. Not only become stronger, but also both strength with toughness and ductility, which were considered trade-off in conventional way, can be achieved. Moreover, by a drastic increase in grain boundary density, various high-temperature phenomena, which are rate-controlled by atomic diffusion, can also appear in such low temperature of room temperature. In addition, free energy increases greatly due to the grain boundary and other high density lattice defects, a thermodynamic phase stabilization in each alloy system changes, the transformation, precipitation and the recrystallization phenomenon that are impossible in the conventional materials occur and new structures appear. An important point is, as anomalous characteristic of the Bulk Nanostructured Metals originates on the structure of the “only grain boundary”, this phenomenon can also appear in both pure metals and low alloy systems.The Bulk Nanostructured Metals achieves a excellent structural material as for recyclability overturned an conventional alloy design concept, showing an excellent characteristic in a simple chemical composition, without consuming rare resources. In other word, the Bulk Nanostructured Metals enables a discontinuous, rapid development of a metallic material and can contribute to the sustainable social development as a new material that supports a new environment and energy technology.

资料来源

http://www.bnm.mtl.kyoto-u.ac.jp/outline/background_e.html 

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