恭喜李毅老师在Science上发表文章。 Matching Glass-Forming Ability with the Density of the Amorphous Phase Y. Li,1,2* Q. Guo,1,2 J. A. Kalb,1,3 C. V. Thompson1,3* The density of the amorphous phase of metals is generally thought to be related to glass formation, but this correlation has not been demonstrated experimentally to date. In this work, systematic deflection measurements using microcantilevers and a combinatorial deposition method show a correlation between glass-forming ability and the density change upon crystallization over a broad compositional range in the copper-zirconium binary system. Distinct peaks in the density of the amorphous phase were found to correlate with specific maxima in the critical thickness for glass formation. Our findings provide quantitative data for the development of structural models of liquids that are readily quenched to the amorphous state. The experimental method developed in this work can facilitate the search for new glass-forming alloys. 详见附件： Science Solving The Mysteries Of Metallic Glass ScienceDaily (Dec. 24, 2008) Researchers at MIT have made significant progress in understanding a class of materials that has resisted analysis for decades. Their findings could lead to the rapid discovery of a variety of useful new kinds of glass made of metallic alloys with potentially significant mechanical, chemical and magnetic applications. The first examples of metallic alloys that could be made into glass were discovered back in the late 1950s and led to a flurry of research activity, but, despite intense study, so far nobody had solved the riddle of why some specific alloys could form glasses and others could not, or how to identify the promising candidates, said Carl. V. Thompson, the Stavros Salapatas Professor of Materials Science Engineering and director of the Materials Processing Center at MIT. A report on the new work, which describes a way to systematically find the promising mixes from among dozens of candidates, is being published this week in Science. Glasses are solids whose structure is essentially that of a liquid, with atoms arranged randomly instead of in the ordered patterns of a crystal. Generally, they are produced by quickly cooling a material from a molten state, a process called quenching. It is very difficult to make glasses from metals compared to any other class of materials, such as semiconductors, ceramics and polymers, Thompson said. Decades of effort by scientists around the world have focused on understanding and on exploiting the remarkable properties of these materials, and on understanding why some alloy compositions can be made into glasses and others cannot, he said. They still haven't solved that why, Thompson said. But this new work does provide a very specific and quantitative new insight into the characteristics of liquid alloys that can most readily be quenched into the glassy state, he said, and thus provides a much more rapid way of discovering new alloys that have the right properties. The research was the result of a collaboration between Thompson and MIT post-doc Johannes A. Kalb with Professor Yi Li and graduate student Qiang Guo at the National University of Singapore, working together across thousands of miles of separation through the Singapore-MIT Alliance. Essentially, the work consisted of producing an array of different alloys with slightly varying proportions of two metals, each deposited on a separate microscopic finger of metal. Then, they analyzed the changes in density of each different mixture when the glass crystallized, and found that there were a few specific proportions that had significantly higher density than the others -- and those particular alloys were the ones that could readily form glasses. Of three of these special proportions they found, two were already known glass-forming alloys, but the third was a new discovery. The new work could even lead to a solution to the longstanding puzzle of why only certain alloys make glasses, he said. I expect these new results, and the technique we developed to obtain them, will play a key, and hopefully decisive, role in solving the mystery of metallic glass formation. Such materials could have a variety of applications because of their unusual physical and magnetic properties, Thompson said. They are soft magnetically, meaning that it's very easy to change the magnetic orientation of the material. This is a highly desirable characteristic for the cores of transformers, for example, which must switch their magnetic orientation dozens of times per second. Transformers made from metallic glasses could potentially greatly reduce the amount of electricity wasted as excess heat in conventional transformers, reducing the need for new generating plants. In addition, these glasses are unusually hard mechanically and have a high degree of springiness (known technically as a high elastic modulus). This springiness could make them a useful material for some sports equipment such as golf clubs or tennis rackets, Thompson said. Although metallic glasses are relatively expensive, he said, for some people interested in the best-performing sports equipment, or in virtually unbreakable housings for cellphones, for example, no expense is too high. The new research is a major accomplishment for the Singapore-MIT Alliance, Thompson said, and would not have been possible without the high-quality communications and collaboration tools it provides. Despite their physical separation, Prof. Li and I have been working together now for almost ten years, he said. We routinely meet via video conferencing and have both been deeply involved in the co-supervision of the remarkable PhD student, Qiang Guo, who carried out this research. Thompson said he sees such collaborations as a significant example of a growing trend. I think this and other accomplishments within the SMA program demonstrate that the future of research lies in technology-mediated collaborations among people with common interests and complementary capabilities, regardless of where the different parts of the team are located, he said. The research was supported by the SMA program, and Kalb was partially supported through a fellowship from the German Alexander Von Humboldt Foundation.
非晶态金属的原子排列是无规密堆的，没有长程序，只是局域地保持一定的短程序。由于这种特殊的微观结构，块体非晶合金的机械物理性能都有极大的变化。 Ti 基非晶合金延续了块体非晶合金的以上优点，除了具有较高的抗拉强度、和高的抗腐蚀性能，此外，由于主合金元素 Ti 的低密度（ 4500 kg /m 3 ），使其具有极高的比强度，具有诱人的应用前景。截至目前，在已开发的 Ti 基非晶合金中，具有大于 5 mm 临界尺寸的成分大多是借助于有害元素或者贵金属提高玻璃形成能力，或者制备出的非晶合金易于发生脆性断裂而缺乏实用价值，因此，开发一种新型无害轻质高强 Ti 基非晶合金显得尤为必要，可以极大地拓宽非晶合金的应用范围。 作者研究了合金元素 Sn 的添加对 TiZrHfCuNiSi 玻璃形成能力的影响，寻找具有最佳玻璃形成能力的合金成分并对其进行相应的研究。研究结果表明，含 5 %Sn 合金的临界尺寸达到 6 mm 。据我们了解，到目前为止， 6mm 为不含有毒元素（例如 Be ）以及贵金属（例如 Pd ）的 Ti 基块体非晶合金系中临界尺寸的最大值。适量的 Sn 元素的加入导致更加复杂的原子间作用力和更大的原子密排度，结晶所需要的长程扩散难以进行，从而显著提高了合金的玻璃形成能力，同时提高了合金的热稳定性。 研究结果发表在 Journal of Alloys and Compounds （ 427 (2007) 171 ）上，相关摘要内容如下 Title ： A new TiZrHfCuNiSiSn bulk amorphous alloy with high glass-forming ability Authors ： Y.J. Huang, J. Shen , J.F. Sun, X.B. Yu Abstract ： The effect of Sn substitution for Cu on the glass-forming ability was investigated in Ti41.5Zr2.5Hf5Cu42.5 x Ni7.5Si1Sn x ( x = 0, 1, 3, 5, 7) alloys by using differential scanning calorimetry (DSC) and X-ray diffractometry. The alloy containing 5% Sn shows the highest glass-forming ability (GFA) among the TiZrHfCuNiSiSn system. Fully amorphous rod sample with diameters up to 6mm could be successfully fabricated by the copper mold casting Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 alloy. The activation energies for glass transition and crystallization for Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 amorphous alloy are both larger than those values for the Sn-free alloy. The enhancement in GFA and thermal stability after the partial replacement of Cu by Sn may be contributed to the strong atomic bonding nature between Ti and Sn and the increasing of atomic packing density. The amorphous Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 alloy also possesses superior mechanical properties. 全文见附件 A new TiZrHfCuNiSiSn bulk amorphous alloy with high glass-forming ability
非晶合金由于其独特的长程无序、短程有序结构，因而具有比常规晶态金属材料更为优异的物理和力学性能，如高强度、高硬度、高耐蚀性及高耐磨性，成为近年来材料界的研究热点。然而，块体非晶合金也有其固有的缺点，在远低于玻璃转变温度以下，变形通常沿单一主剪切带发生，表现出很小的整体塑性变形能力，从而限制了其在工程材料领域的应用。块体非晶合金塑性的提高，对了解材料的力学行为及塑性变形机理有重要的意义。 近来，一些所谓的延性块体非晶合金也被相继报道，它们在断裂前体现较大的塑性应变。断裂后，这些合金的表面上均出现多重剪切带、剪切带的交割和分枝。截止到目前，虽然学者对这种塑性增强现象给了一些解释，但是其本质机理仍然不清楚。 作者对相同试样不同部位样品进行系统的力学分析，合金试样的微观结构对于控制室温变形行为起主要作用。对于冷却速度大的试样， 1-2 nm 尺度的短程序团簇被认为是促进试样发生塑性变形的原因。这些团簇可能使快速凝固过程中从液体中遗留下来的预存晶核。这些弥散分布的团簇极大地改变了非晶合金自由体积的分布，它们作为钉扎点阻止了剪切带的进一步扩展。这种钉扎效应使得材料的应力模式由单轴转变为复杂的多重方向。多重剪切带的出现阻止了会导致灾难性断裂的裂纹的产生。因此，材料的整体塑性得到大幅度的提高。 对于冷却速度较小而言，在铜模铸造过程中形成了 8-10 nm 尺度的纳米晶弥散分布于非晶基体上这种复相结构。由于这些细小的纳米晶原位形成于合金熔体冷却过程中，因此，它们与基体结合较好。纳米相与非晶相之间弹性性能的不同诱发局部应力集中，导致剪切带在纳米颗粒周围形成。这些细小的纳米晶有效地阻碍了剪切带的扩展，并诱发新的剪切带的形成，使得剪切带发生分支，进而导致材料大的宏观塑性。对于冷却速度最低的试样，其纳米颗粒的尺寸及体积分数都太大，基体中自由体积大量减少，脆性的纳米颗粒不能阻止剪切带沿整个试样扩展，导致材料的脆性增加。 该研究结果发表在 Journal of Materials Research （ 2007, 22:3067 ）上，相关的摘要信息如下。 Title ： Plasticity of a TiCu-based bulk metallic glass: Effect of cooling rate Authors ： J. Shen, Y.J. Huang, and J.F. Sun Abstract ： Compressive deformation was experimentally investigated for Ti41.5Cu42.5Zr2.5Hf5Ni7.5Si1 bulk metallic glass (BMG) fabricated at different cooling rates. It was found that the ductility of the BMG alloy increased with increasing of the cooling rate in solidification. The alloy with a monolithic amorphous structure exhibited a large ductility, up to 12%. The effect of cooling rate on the ductility of the BMG alloy is interpreted in terms of the variation in amorphous nature and free volume of the as-cast materials. 全文见附件。 Plasticity of a TiCu-based bulk metallic glass: Effect of cooling rate
一直以来，如何评价非晶合金的塑性是一个重要问题。对于不同尺寸的非晶合金，塑性变形能力究竟如何呢？对此，作者进行了实验以及理论分析。 随着直径的减小，非晶合金试样产生破坏前所体现的塑性应变呈指数增加，强度则不断减小，这表明所研究的非晶态金属具有愈小亦愈软的趋势。对于具有相同化学成分的玻璃态合金而言，试样尺寸愈小，试样在凝固过程中所经历的冷却速度则越大，单位原子体积内自由体积的数量愈大，合金体内包含更多的具有高自由体积数量的区域，进而，促使剪切带的形核点愈多，导致合金在塑性变形过程中产生数量更多的剪切带，合金所体现的塑性更为优异。这说明试样尺寸在非晶态金属合金的塑性变形中占有至关重要的作用。 该结果发表在 Applied Physics Letters （ 2007, 90: 081919 ） 上,摘要信息如下： Title：Bulk metallic glasses: Smaller is softer Authors：Y.J. Huang, J. Shen, J.F. Sun Abstract: The authors report on the dramatic effect of sample size on the plastic deformation capability of a Ti-based bulk metallic glass. Compared with the larger one, the smaller size glassy alloy with the same chemical composition exhibits significantly enhanced plasticity, suggesting a smaller is softer trend of metallic glasses. The phenomenon is attributed to the fact that the smaller size alloy, which experienced a faster cooling rate during solidification, contains a larger amount of heats of relaxation and crystallization, favoring the preferential nucleation of shear bands and thus allowing enhanced plasticity upon compressive loading. 全文见附件. Bulk metallic glass: Smaller is softer
标签: 非晶合金
