Abstract: Glasses are normally brittle materials with no tensile ductility at room temperature. Using in situ, quantitative nanomechanical tests inside a transmission electron microscope, we demonstrate that certain nanoscale metallic glass samples are exceptions to this general rule. Such metallic glasses can be intrinsically ductile, capable of elongation and necking under uniaxial tension, in lieu of catastrophic fracture caused by severe shear-banding. Beam-off tests confirm that the ductile behaviors are not artifacts due to electron-beam effects during the in situ tests. Additional experiments indicate that ductile necking gives way to fast shear banding failure at increased samples sizes and elevated strain rates. The observed spread-out shear transformations delaying localization and severe shear banding are explained in terms of the propensity for participation in deformation, while the tendency towards necking is attributed to the lack of strain hardening mechanism and inadequate strain rate hardening.

 

       我中心关于金属玻璃的研究取得了又一项进展。近日一篇题为“纳米尺度金属玻璃在室温下单轴拉伸中的塑性颈缩行为”的论文在国际材料科学领域一流学术刊物《Acta Materialia》上发表。该论文是西安交大金属材料强度国家重点实验室“微纳尺度材料行为研究中心(CAMP-Nano)”( http://www.campnano.org/web)博士生田琳，在单智伟教授指导下完成的。

       在室温下，亚微米尺寸以上的金属玻璃通常表现出以剪切带为主要特征的脆性断裂行为。继非晶的弹性极限研究（Tian et al, Nat. Commun., 2012）和强度的尺寸效应研究(Wang et al, Acta Mater., 2012)之后，微纳尺度材料行为研究中心的研究人员通过巧妙的实验设计，首次在透射电镜下实现了对直径小于100纳米的非晶样品的定量单轴拉伸测试，并以令人信服的证据证实当把样品的直径减少到80纳米左右时，金属玻璃呈现明显的本征颈缩塑性。实验进一步证实，尽管当样品受到超常强度电子束辐照时，非晶样品的塑性变形能力明显提高；但是正常的透射电镜成像条件对所研究的金属玻璃的力学性能却没有明显影响。同时，微纳尺度材料行为研究中心的研究人员还首次研究了应变速率对颈缩现象的影响。结果发现低应变速率和小尺寸试样有利于以颈缩为特征的塑性变形而高应变速率和大尺寸的试样将促进以剪切带为特征的脆性断裂。这些发现为了解小尺寸非晶试样的形变特征，揭示其物理本质提供了亟需的坚实实验证据。