Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China.
Proc Natl Acad Sci U S A. 2013 Dec 3;110(49):19725-30. doi: 10.1073/pnas.1320235110. Epub 2013 Nov 19.
Metallic glasses (MGs) exhibit greater elastic limit and stronger resistance to plastic deformation than their crystalline metal counterparts. Their capacity to withstand plastic straining is further enhanced at submicrometer length scales. For a range of microelectromechanical applications, the resistance of MGs to damage and cracking from thermal and mechanical stress or strain cycling under partial or complete constraint is of considerable scientific and technological interest. However, to our knowledge, no real-time, high-resolution transmission electron microscopy observations are available of crystallization, damage, and failure from the controlled imposition of cyclic strains or displacements in any metallic glass. Here we present the results of a unique in situ study, inside a high-resolution transmission electron microscope, of glass-to-crystal formation and fatigue of an Al-based MG. We demonstrate that cyclic straining progressively leads to nanoscale surface roughening in the highly deformed region of the starter notch, causing crack nucleation and formation of nanocrystals. The growth of these nanograins during cyclic straining impedes subsequent crack growth by bridging the crack. In distinct contrast to this fatigue behavior, only distributed nucleation of smaller nanocrystals is observed with no surface roughening under monotonic deformation. We further show through molecular dynamics simulation that these findings can be rationalized by the accumulation of strain-induced nonaffine atomic rearrangements that effectively enhances diffusion through random walk during repeated strain cycling. The present results thus provide unique insights into fundamental mechanisms of fatigue of MGs that would help shape strategies for material design and engineering applications.
金属玻璃（MGs）比其晶态金属对应物具有更大的弹性极限和更强的抗塑性变形能力。它们在亚微米长度尺度下抵抗塑性应变的能力进一步增强。对于一系列微机电应用，MG 对热和机械应力或应变循环下的局部或完全约束下的损伤和裂纹扩展的阻力具有相当大的科学和技术兴趣。然而，据我们所知，在任何金属玻璃中，都没有实时、高分辨率透射电子显微镜观察到的受控施加循环应变或位移时的结晶、损伤和失效。在这里，我们展示了一项独特的原位研究的结果，该研究在高分辨率透射电子显微镜内部对基于 Al 的 MG 的玻璃到晶体形成和疲劳进行了研究。我们证明，循环应变逐渐导致起始缺口的高变形区域的纳米级表面粗糙度增加，从而导致裂纹成核和纳米晶体的形成。这些纳米晶粒在循环应变过程中的生长通过桥接裂纹来阻碍随后的裂纹扩展。与这种疲劳行为形成鲜明对比的是，在单调变形下，仅观察到较小纳米晶的分布式成核，而没有表面粗糙度。我们通过分子动力学模拟进一步表明，这些发现可以通过应变诱导的非仿射原子重排的积累来合理化，这有效地增强了在重复应变循环过程中的随机行走中的扩散。因此，目前的结果为 MGs 的疲劳基本机制提供了独特的见解，这将有助于制定材料设计和工程应用的策略。
