Comment on "Grain boundary-mediated plasticity in nanocrystalline nickel".

Nanograin rotation via grain boundary sliding has been predicted as an important deformation mode in nanocrystalline materials as grain sizes approach less than 10 nm (1–3). However, definite experimental evidence beyond molecular dynamics (MD) simulations has been long sought. Recently, Shan et al. (4) reported in situ straining dark-field transmission electron microscope (DFTEM) observations of grain rotation in nanocrystalline Ni and claimed that the plastic deformation of nano-Ni is mediated by this grain boundary behavior. Although the experimental results reported by Shan et al. are interesting, their assessment and analysis of the TEM images are problematic. Using the images presented in (4), we have quantitatively measured the relative displacements and grain sizes. Both results suggest that the grain rotation and associated contrast change reported by Shan et al. more likely come from low-temperature nanograin growth, caused by electron-beam irradiation and applied stresses, than from plastic deformation. In Fig. 1, we show contrast-inverted images from figure 3 in (4). Small grains with less contrast change are linked with lines to form a trapezoidal frame surrounding grain G, which exhibited significant contrast change during loading. Overlaying the trapezoidal frame in Fig. 1, B to F, shows that all the joint points of the frame match well with the original small grains. Precise measurements of the line lengths were performed using NIH Image (5), and the dependence of the line lengths on loading time was plotted (Fig. 2). The mean error of these measurements is about T1 nm and the corresponding strain smaller than 0.5%. The measurements do not suggest any systematic length changes and, thus, any relative displacements and strains. To rule out the possible bending and torsion deformation, which might not significantly alter the line lengths, we measured the angles marked in Fig. 1B. We were also unable to observe any systematic angle changes with time. These measurements unambiguously show that no detectable deformation occurred during loading. If the significant contrast change of grain G were caused by plastic deformation, relative displacements, either in plane or out of plane, should have been observed among the surrounding grains, because plastic deformation cannot be accomplished solely by a single grain rotation. As several attempts have well demonstrated (6–8), it is extremely difficult to get uniform plastic deformation in nanocrystalline samples, and localized deformation and cracking cannot be avoided during in situ straining TEM observations. Although the data in Fig. 1 were recorded during in situ tensile tests, it is quite possible that the region observed by Shan et al. did not experience visible plastic deformation and that the observed contrast change came mainly from nanograin growth caused by electron-beam irradiation and applied stresses. The time-related size change of grain G, measured using the NIH Image (5), revealed a linear relation between grain size in the area (S) and time (t) (Fig. 2C) that is exactly consistent with the classical grain growth equation (9), S – S 0 0 kt, where S 0 is the initial grain size and k is a constant. The diffraction patterns shown in figure 2, B and D, in (4) also indicate nanograin growth during DFTEM observations. Slightly adjusting the brightness of figure 2D in (4) to be close to that of figure 2B in (4) (fig. S1) TECHNICAL COMMENT