Metals usually have three crystal structures: face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal-close packed (hcp) structures. Typically, metals exhibit only one of these structures at room temperature. Mechanical processing can cause phase transition in metals, however, metals that exhibit all the three crystal structures have rarely been approached, even when hydrostatic pressure or shock conditions are applied. Here, through in situ observation of the atomic-scale bending and tensile process of $\sim 5\mathrm{nm}$-sized Ag nanowires (NWs), we show that bending is an effective method to facilitate fcc-structured Ag to access all the above-mentioned structures. The process of transitioning the fcc structure into a bcc structure, then into an hcp structure, and finally into a re-oriented fcc structure under bending has been witnessed in its entirety. This re-oriented fcc structure is twin-related to the matrix, which leads to twin nucleation without the need for partial dislocation activities. The results of this study advance our understanding of the deformation mechanism of small-sized fcc metals.

中文翻译：

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弯曲下呈现 fcc、bcc 和 hcp 结构的超小银纳米线的直接原子尺度观察

金属通常具有三种晶体结构：面心立方 (fcc)、体心立方 (bcc) 和六方密堆积 (hcp) 结构。通常，金属在室温下仅表现出这些结构中的一种。机械加工会引起金属的相变，然而，很少接近具有所有三种晶体结构的金属，即使在施加静水压力或冲击条件时也是如此。在这里，通过原位观察原子尺度的弯曲和拉伸过程$～5\mathrm{纳米}$尺寸的Ag纳米线（NW），我们表明弯曲是促进fcc结构Ag访问所有上述结构的有效方法。从fcc结构转变为bcc结构，再转变为hcp结构，最后在弯曲下转变为重新定向的fcc结构的过程已经被完整地见证了。这种重新定向的 fcc 结构与基体是孪生相关的，这导致孪生成核而无需部分位错活动。这项研究的结果促进了我们对小尺寸 fcc 金属变形机制的理解。