Direct Atomic-Scale Observation of Ultrasmall Ag Nanowires that Exhibit fcc, bcc, and hcp Structures under Bending

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.

Figure 1

In situ Cs-TEM images along the fcc $[1\overline{1}0]$ zone axis showing the bending process of a $\sim 5\mathrm{nm}$-sized Ag NW. The inset image shows the corresponding FFT pattern of the blue-framed region.

Figure 2

(a)–(c) Enlarged Cs-TEM images showing the fcc-bcc-hcp-fcc phase transformation at the bottom of the NW during bending. (d)–(e) The corresponding enlarged Cs-TEM images of (a)–(c) show the bcc, hcp, and fcc phases in detail.

Figure 3

Enlarged Cs-TEM images and atomic model showing the fcc-bcc-hcp-fcc phase transformation path. (a)–(c) The crystal lattice between the (111) and $(11\overline{1})$ planes changes from the original fcc to a bcc and then to an hcp lattice. The inset in (b),(c) show simulated images of bcc- and hcp-structured Ag. (d) Calculated energy curves of the fcc-hcp phase transition through the bcc path. The inset in (d) shows simulated HRTEM images of fcc-, bcc-, and hcp-structured Ag and the corresponding 3D models. (e)–(h) 2D models of fcc-, bcc-, and hcp-structured Ag.

Figure 4

An examples showing the plastic deformation of the ultrasmall NW governed by a partial dislocation resulting in stacking faults and deformation twins during tensile loading.