今晨在科学网上看到一则新闻： 25 位华人科学家新当选美国物理学会会士（ http://news.sciencenet.cn/htmlnews/2010/12/241038.shtm ） 。 出于好奇，想看一下都有哪些牛人？有没有咱认识或者听说过的牛人。结果很是惊喜。居然有两位非常熟悉的朋友金榜题名。 9. 马恩 （男，美国约翰斯霍普金斯大学教授） Ma, En Johns Hopkins University Citation: For pathbreaking research and outstanding publications on metastable, amorphous, and nanocrystalline metals and alloys, and international outreach in the metallic materials field. Nominated by: Materials Physics (DMP) 。 20. 王循礼 （男，美国橡树岭国家实验室高级科学家） Wang, Xun-Li Oak Ridge National Laboratory Citation: For sustained contribution in neutron diffraction studies of structure, phase transformations, and mechanical behavior in materials and engineering systems and leadership in the design and construction of a versatile engineering diffractometer at the Spallation Neutron Source. Nominated by: Industrial and Applied Physics (FIAP) 。 首先，向两位获此殊荣的朋友表示最衷心的祝贺！ 附加一点感想 ： 西方国家的学术荣誉都是在获得者本人不知情的情况下由业内资深的科学家们提名获得的。获殊荣者本人并不知道哪些人是提名人。对于他们来说，标志着以往的学术成就和贡献得到了业内的高度认可。对他们提名的资深科学家们也远非出自于个人（或者集团）的利益。 换句话说，学术上的殊荣决非是经过个人申请（填表、提供一大堆的佐证材料），再经评审、答辩之类的途径获得的！ 两种方式的区别从根本决定了荣誉的含金量、价值与意义！ 2010 年 7 月 7 日 ，马恩博士在瑞士联邦理工学院（ ETH ）举行的 第 17 届亚稳态、非晶态、纳米结构材料国际会议 上做大会报告。 2008 年 9 月 9 日 ，王循理博士访问博主的研究组，作了题为 Competing Order Parameters and Crystallization Pathways in Metallic Glasses 的报告。 相关链接 ISMANAM 2010 花絮 参加 BMG-VII 国际会议感受 祝贺 ISMANAM 2009 举办成功！
众所周知，晶态合金的塑性流动机制通常是位错运动。但是，非晶态合金中是不存在位错的，其塑性流动是通过原子团簇的非弹性剪切运动（通常称为剪切转变区）介导的，见下图。由于原子的长程无序性，这种原子团簇的剪切运动中往往包含体胀变形，即所谓的（原子团簇尺度）剪胀效应。宏观上，导致塑性流动具有压力或者正应力敏感性。但是这种剪胀效应和压力敏感性之间的定量关系，目前还不清楚。 最近，我们通过在剪切带转变区运动中考虑剪胀效应，并基于Johnson和Samwer提出的协同剪切模型(CSM) ，理论推导了塑性流动压力敏感系数与剪胀因子的之间的定量关系，并且得到了一个包含压力敏感效应的塑性屈服准则。相关结果发表在Scripta Mater 63(2010) 945-948 全文下载： Scripta Mater-2010-L Sun
块体金属玻璃由于其独特的微观结构（原子排列长程无序、短程有序，无晶界、位错等传统意义的结构缺陷），赋予这类材料一系列优异的物理、力学、化学等性能。但是，在推进金属玻璃的实际应用过程中，最大的一个瓶颈是它在室温下极差的拉伸延性，其原因是金属玻璃在室温下的塑性变形高度局部化，极易形成纳米尺度的剪切带（见下图【Li et al., Philos Mag 2002】）。因此，对于金属玻璃中剪切带相关的研究就显得尤为重要。 关于金属玻璃中剪切带的形成机理或者物理起源，虽然经过几十年大量的探索，目前其清晰的物理图像仍然不清楚。一般认为，金属玻璃中某些不均匀处的粘度降低并进而导致塑性变形能力急剧下降是形成局部化剪切带的主要机制。但是，对于这种粘度降低的原因一直以来存在着两种截然不同的观点：自由体积软化和绝热软化。虽然，两种观点各自都有实验证据，但是有些现象它们单独仍然无法合理解释。 我们课题组在前面通过一系列的实验【Liu et al., MCP 2005; JNS 2005; JMR 2006】和理论分析【Dai et al., APL 87, 141916 ( 2005); Dai and Bai, IJIE 35, 704 (2008)】，初步揭示金属玻璃中剪切带形成是一种热力耦合的失稳过程，并不是某一因素（自由体积软化或者热软化）单独作用的结果。尤其是在高应力和高应变率条件下，应力驱动的自由体积产生和功热转变导致的温升都有可能发生，并且互相影响。这两个过程天然耦合在一起。最近发展的一些热力耦合模型能够很好地描述金属玻璃的（非）均匀塑性变形行为，但是对于在这种耦合过程中，自由体积和热的相互作用以及谁是剪切带的起源等关键问题没有给出明确的答案，仍然需要进一步的研究。 我们这个工作就是针对上述这个问题进行系统而深入的理论分析，重点关注在剪切带形成过程中，热和自由体积的相互作用图像；自由体积聚集、温升、体胀对于剪切带失稳的影响。相关工作已经发表在固体力学领域最有影响力的杂志 J. Mech. Phys. Solids 57 (2009) 1267-1292 On the origin of shear banding instability in metallic glasses M.Q. Jiang a , and L.H. Dai , a , a State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, PR China Received 20 October 2008; revised 13 April 2009; accepted 21 April 2009. Available online 4 May 2009. Abstract To uncover the physical origin of shear-banding instability in metallic glass (MG), a theoretical description of thermo-mechanical deformation of MG undergoing one-dimensional simple shearing is presented. The coupled thermo-mechanical model takes into account the momentum balance, the energy balance and the dynamics of free volume. The interplay between free-volume production and temperature increase being two potential causes for shear-banding instability is examined on the basis of the homogeneous solution. It is found that the free-volume production facilitates the sudden increase in the temperature before instability and vice versa. A rigorous linear perturbation analysis is used to examine the inhomogeneous deformation, during which the onset criteria and the internal length and time scales for three types of instabilities, namely free-volume softening, thermal softening and coupling softening, are clearly revealed. The shear-banding instability originating from sole free-volume softening takes place easier and faster than that due to sole thermal softening, and dominates in the coupling softening. Furthermore, the coupled thermo-mechanical shear-band analysis does show that an initial slight distribution of local free volume can incur significant strain localization, producing a shear band. During such a localization process, the local free-volume creation occurs indeed prior to the increase in local temperature, indicating that the former is the cause of shear localization, whereas the latter is its consequence. Finally, extension of the above model to include the shear-induced dilatation shows that such dilatation facilitates the shear instability in metallic glasses. Keywords: Metallic glasses; Free volume; Temperature increase; Shear-banding instability; Dilatation 全文下载： JMPS-2009
最近，我们结合分子动力学模拟和连续介质力学建模分析，揭示出金属玻璃中的锯齿状流动行为主要取决于内部不可逆原子重排的程度，而这种重排必然是在时间上非均匀的，而与空间上的非均匀行为（或者剪切带）没有直接关联。 本文工作进一步验证了W.H. Jiang在2008年国际塑性上关于非均匀变形的时空特性的猜想图像。相关结果发表在计算材料科学杂志。 M.Q. Jiang-CMS-2009 Smaller Deborah number inducing more serrated plastic flow of metallic glass M.Q. Jiang a , S.Y. Jiang a , Z. Ling a , L.H. Dai a,b,* a State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, PR China b State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, PR China a b s t r a c t Spherical nano-indentations of Cu46Zr54 bulk metallic glass (BMG) model systems were performed using molecular dynamics (MD) computer simulations, focusing specifically on the physical origin of serrated plastic flow. The results demonstrate that there is a direct correlation between macroscopic flow serration and underlying irreversible rearrangement of atoms, which is strongly dependent on the loading (strain) rate and the temperature. The serrated plastic flow is, therefore, determined by the magnitude of such irreversible rearrangement that is inhomogeneous temporally. A dimensionless Deborah number is introduced to characterize the effects of strain rate and temperature on serrations. Our simulations are shown to compare favorably with the available experimental observations. 锯齿状流动与时间非均匀原子不可逆重排的内在关联
金属玻璃 作为一种具有广阔应用前景的新型材料，在推进其工程应用的过程中，高效率、高精度的加工（包括切削、车削和钻削等）是必不可少的一个环节。然而，对于金属玻璃在切削过程中的变形行为及其物理机制的研究要远远落后于晶态合金材料，其中对于切屑形成机理的理解至关重要。金属玻璃在切削过程中常常会产生一种独特的连续的片状切屑，即使是在切削速度非常低的条件下。由于金属玻璃较低的热导率，这种片状切屑一般认为也是由于绝热剪切带的周期形成导致的。但是，与传统晶态材料相比，金属玻璃具有截然不同的塑性流动机制。因此，基于STZ的自由体积的非线性演化及其动态失稳有可能在片状切屑的形成过程中起重要作用，而这一点在以前的研究中还未见报道。此外，在金属玻璃切削过程中极限环分叉现象是否仍然存在？如果存在，那么主控机制是什么？对于这些问题的回答不仅对于金属玻璃的应用至关重要，而且将有助于我们更好地来理解金属玻璃的流动机理，甚至剪切带的形成机制。基于上述考虑，我们进行了Vit 1块体金属玻璃的低速切削实验，发现产生的独特的片状切屑主要归因于刀具前面主剪切区内剪切带的周期形成(见图1）。基于实验观察，考虑在主剪切区内力、自由体积和温度的平衡，发展了一种热力耦合的正交切削模型来定量表征片状切屑的形成。采用线性扰动分析，揭示了片状切屑形成的临界条件。这种切屑的形成可以理解为一种自我维持的极限环失稳现象（见图2），即在主剪切区内应力、自由体积和温度的自动反馈。其背后的物理机制是自由体积流和源的对称破缺，而不是热失稳。 相关工作已经发表在Acta Mater 57 (2009) 5730-5738. M.Q. Jiang-Acta Mater 2009 投稿及审稿过程见以前博文： http://www.sciencenet.cn/m/user_content.aspx?id=216213 Formation mechanism of lamellar chips during machining of bulk metallic glass M.Q. Jiang, L.H. Dai * State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China Received 27 November 2008; received in revised form 19 February 2009; accepted 19 February 2009 Available online 28 March 2009 Abstract The unique lamellar chips formed in turningmachining of a Vit 1 bulk metallic glass (BMG) are found to be due to repeated shearband formation in the primary shear zone (PSZ). A coupled thermomechanical orthogonal cutting model, taking into account force, free volume and energy balance in the PSZ, is developed to quantitatively characterize lamellar chip formation. Its onset criterion is revealed through a linear perturbation analysis. Lamellar chip formation is understood as a self-sustained limit-cycle phenomenon: there is autonomous feedback in stress, free volume and temperature in the PSZ. The underlying mechanism is the symmetry breaking of free volumef low and source, rather than thermal instability. These results are fundamentally useful for machining BMGs and even for understanding the physical nature of inhomogeneous flow in BMGs. Keywords: Bulk metallic glass; Lamellar chip; Orthogonal cutting model; Free volume; Limit cycle 图1 Vit1 金属玻璃切削过程中形成的切屑形貌 图2 极限环分叉行为
Prediction of shear-band thickness in metallic glasses M.Q. Jiang, a W.H. Wang b and L.H. Dai a ,* aState Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, Peoples Republic of China bInstitute of Physics, Chinese Academy of Sciences, Beijing 100190, Peoples Republic of China Received 20 November 2008; revised 31 January 2009; accepted 20 February 2009 Available online 27 February 2009 We derive an explicit expression for predicting the thicknesses of shear bands in metallic glasses. The model demonstrates that the shear-band thickness is mainly dominated by the activation size of the shear transformation zone (STZ) and its activation free volume concentration. The predicted thicknesses agree well with the results of measurements and simulations. The underlying physics is attributed to the local topological instability of the activated STZ. The result is of significance in understanding the origin of inhomogeneous flow in metallic glasses. 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Metallic glasses; Shear band; Shear transformation zone; Free volume Figure 3. Prediction of shear-band thickness W. (a) Dependence of Poissons ratio m and activation free volume concentration n* on W for fixed STZ size d = 1.5 nm. (b) Dependence of n* and d on W for fixed m = 0.36. 相关工作发表在 Scripta Materialia 60 (2009) 10041007 M.Q. Jiang-SMM-2009
Formation of magnetic Fe-based bulk metallic glass under low vacuum 近来，我们开展了在低真空环境下制备磁性铁基块体金属玻璃的研究。发现适量的稀土元素Y能够有效的提高抗氧化能力，在低真空环境下（1.5Pa），成功获得直径达2mm以上的铁基块体金属玻璃，磁性能研究发现，该合金具有良好的软磁性能，饱和磁化强度达95 emu/g。研究结果在线发表在 Journal of Alloys and Compounds上。 1. Formation of magnetic Fe-based bulk metallic glass under low vacuum Journal of Alloys and Compounds, In Press, Accepted Manuscript , Available online 6 December 2008 S.F . Guo , L. Liu, X. Lin 附上文章： Accepted Manuscript
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
标签: 金属玻璃
