Fig. 1. (a) Schematic view showing the electron microscopy specimens of the straining sample. During heating, the different thermal expansion coefficients of the thin film and the substrate will lead the film to experience tensile or compressive strain. (b) Typical image of the Gatan straining holder. (c) Scanning electron microscopy image of the micro-electromechanical system. (d, e) Commercial Nanofactory TEM-STM/AFM holders and Hysitron picoindenter holder.

Fig. 2. Schematic view of the in situ controllable tensile testing device. (a) Uniform dog-bone shaped NC thin film. (b, c) Thin films attached on the surface of the bi-metallic extensor. (d) Upon etching away the substrate, free-standing thin films are released from the substrate. (e, f) During heating, the plastic deformation behavior of thin films can be observed in situ at the atomic scale.

Fig. 3. MD simulation shows the low-temperature plastic deformation of fully 3d NC copper. The results show the existence of GB sliding combined with grain rotation. (Reprinted with permission from Schiotz et al.[54] Copyright 1998 Nature Publishing Group.).

Fig. 4. (a-f) In situ observed grain rotation in NC Ni using dark-field TEM images (Figure from Shan et al.[110]). (g, h) High densities of dislocations were observed in Ni and Cu (Figure from Youssef et al. [118]).

Fig. 5. Low-magnified TEM image of the Pt film. (b) Distribution of grain sizes indicating the grain diameter range from 2 to 12 nm, with an average d of ～6 nm. Typical HRTEM images show that both the low-angle GBs (c) and high-angle GBs (d) are well crystalline.

Fig. 6. (a, b) In situ observation of full dislocation (marked with “T”) nucleation and motion in an ～11 nm sized grain. (c, d) In situ observation of partial dislocation (marked with red arrow) resulted SFs in an ～8 nm sized grain.

Fig. 7. Series of HRTEM images showing the GB dislocation mediated grain rotation process at the atomic scale. (a) GB angle is 8.3°. (b, c) During straining, the number of dislocations at the GB increases, and the average spacing of the GB dislocations decreases, leading the GB angel increase to increase from 8.3° to 13.5°.

Fig. 8. Statistical results show a clear trend with decreasing d. As noted by the squares, full dislocations were frequently observed in larger grains. For d between 6 and 10 nm, SFs resulting from partial dislocations become more frequent (as noted by dots). For d < ～ 6 nm, the GBs dominate the plasticity, as noted by the black triangles.