Technical Comments

Comment on "Grain Boundary-Mediated Plasticity in Nanocrystalline Nickel"

Science  15 Apr 2005:
Vol. 308, Issue 5720, pp. 356
DOI: 10.1126/science.1107143

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 ±1 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.

Fig. 1.

Contrast-inverted images of figure 3 in (4). The small grains with less contrast change during loading were linked to form a trapezoidal frame.

Fig. 2.

Measurements of line lengths, angles, and grain areas using NIH Image. (A) The distance changes between the smaller grains around the marked grain G as a function of loading time (see Fig. 1B). The slope of each line approaches zero, which suggests that no systematic deformation occurs accompanying the continuous contrast changes of grain G. (B) The relation between the angles (see Fig. 1B) and loading time. (C) Changes in the area of grain G as a function of time. The linear relation between S and t is consistent with the classical grain growth equation.

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 - S0 = kt, where S0 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) reveals continuous rings with an increasing number of bright spots that correspond to coarsened grains; this, in turn, suggests that the change of diffraction patterns results from the grain growth, rather than the thickness decrease claimed by Shan et al. (4). Additionally, the contrast in the DFTEM [figure 2E in (4)] cannot be solely attributed to the grain boundaries. Crystal defects—for example, dislocations, as revealed by their high-resolution electron microscope image [figure 4C in (4)]—can provide the similar contrast caused by their elastic strain fields (10). After contrast inversion of figure 2E in (4), the dark contrast that Shan et al. suggested represented grain boundaries shows discontinuous features (fig. S2) that are characteristic of dislocations in bright-field TEM. It is not surprising to see dislocations in the nanograin because the grain size, at around 50 nm in diameter, is large enough to contain a number of perfect dislocations. Actually, the appearance of the edge dislocations as observed by Shan et al. is also consistent with the rotation growth theory, as suggested by recent MD simulations (11, 12) and the classical rotation growth model (13).

In summary, the TEM results reported in (4) can be interpreted as nanograin growth caused by electron-beam irradiation and applied stresses. Although nanograin growth may not be the whole story, and although a small amount of deformation through grain boundary mediation may occur accompanying the observed grain rotation, the grain contrast change reported by Shan et al. appears to result mainly from nanograin coalescence and growth rather than visible plastic deformation.

Supporting Online Material

Figs. S1 and S2

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