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mqjiang木虫 (著名写手)
Amorphous Solids
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[交流]
【原创】关于金属玻璃中剪切带失稳的物理起源
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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。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 (2009),doi:10.1016/j.jmps.2009.04.008 On the origin of shear banding instability in metallic glasses M.Q. Jiang, and L.H. Dai, 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 全文下载地址：http://www.sciencenet.cn/m/user_content.aspx?id=235805 [ Last edited by hslining on 2009-9-12 at 09:00 ] |
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2楼2009-06-03 22:13:03
mqjiang
木虫 (著名写手)
Amorphous Solids
- 应助: 12 (小学生)
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- 注册: 2008-12-25
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- 专业: 固体力学
3楼2009-06-04 08:56:40
