Supplemental Material
This supplementary information discusses previous claims of TEM observations of
twinning in pure Al, and illustrates the formation of other defects such as stacking
faults and Frank loops, which could be misinterpreted as twins under TEM.
As mentioned in the main text, deformation twinning is rarely observed in pure Al.
There were only two experimetal papers, in difficult-to-find journals [see 1,2 in the
reference list at the end of this supplemental discussion], that report possible
indications of deformation twinning at a crack tip in Al. The images provided were of
low quality, and the brief studies were not systematic and convincing. No in situ or
high-resolution observations were made, either. Warner et al. [3] thus concluded that
deformation twinning was never confirmed in Al experimentally, and only happens at
high strain rates such as in computer simulations [3 and references therein], or in
nanocrystals that deform under very high stresses [4,5].
Recentely W.Z. Han et al. [6], claimed that they have observed deformation twinning
in Al. The Fig. 3 in Ref. 6 actually shows a stacking fault (SF), but not a microtwin.
By definition, a microtwin is composed of 2-5 atomic layers in thickness. In other
words, there should be at least two Shockley twinning partials sweeping across
adjacent lattice planes to form a microtwin. In Fig. 3(b) in Ref. 6, the “kink” plate
contains only one atomic layer and is not a microtwin but a SF. Although a white line
was placed in Fig. 3(d) in Ref. 6 to indicate a two-atomic-layer microtwin, the atomic
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positions were unfortunately misread. The kink is only one atomic layer again. We
thus conclude that all they have observed are SFs, not twins. Also, it is unlikely that
the SFs presented in Ref. 6 are due to the activity of Shockley partial dislocations.
The density and the morphology of the SFs in Ref. 6 suggest that the formation of
these faults has nothing to do with dislocation slip. In fact, these SFs observed in pure
Al are Frank loops viewed edge-on along the <110> zone axis. The Frank loops tend
to be on the {111} crystal planes in an FCC metal. There are two well known origins
for the Frank loops: one is induced by severe deformation [7-9], as shown for
example by X.L. Wu et al. [9] recently, and the other is induced by electron irradiation
[9-11].
In addition to severe deformation, which was actually used in Ref. 6 to process Al,
point defects produced by electron irradiation during TEM experiments (which was
the technique used in Ref. 6 for observations) can also form SFs. To illustrate this
latter scenario in more detail, we have performed the following experiments.
Cold rolled high-purity polycrystalline Al (99.999%) acquired from Goodfellow Inc
was used for the e-beam irradiation experiment, which was carried out inside a 300
kV Tecnai G2 F30 TEM. Two levels of beam intensity were used: low-flux (43000x
full screen) and high-flux (430000x full screen). The latter was often used for
acquiring HRTEM images.
2
Under the low-flux irradiation, the Al specimen was damaged slowly, as shown in Fig.
1. Compared with the as-prepared specimen (see Fig. 1a), some small interstitial
clusters were formed after 10 min irradiation (Fig. 1b). Frank loops formed after
longer durations, as shown in Fig. 1c for a specimen exposed to e-beam for 40 min.
Fig. 1d is the HRTEM image of the Frank loop, in which the SFs feature is similar to
that seen in Ref. 6. Under the high-flux irradiation (HRTEM imaging condition), the
Frank loops are introduced into the Al specimen very fast. Fig. 2 shows a high density
of Frank loops formed by irradiation after exposure for only 2.5 minutes.
To exclude the effects of the defects introduced by cold rolling (such as voids,
interstitial atoms, and dislocations in the as-received polycrystalline Al) on irradiation
damage, we annealed the as-received Al at 180℃ for 20 minutes. At both low and
high flux irradiation conditions, the process of the Frank loop formation in the
annealed samples (Fig. 3 and Fig. 4) is the same as that in the cold rolled sample.
These irradiation experiments clearly indicate that pure Al specimens are easily
decorated by Frank loops, even when the Al specimens have not undergone
deformation at all. Therefore, care must be taken to explain such kind of SFs in Al
specimens observed in the HRTEM images.
3
References
1.
R. C. Pond and L. M. F. Garcia-Garcia, Inst. Phys. Conf. Ser. 61, 495 (1981).
2.
Q. Chen, Y. Huang, L. Qiao, and W. Chu, Sci China (Ser E). 42, 1 (1999).
3.
D. H. Warner, W. A. Curtin, and S. Qu, Nature Mater. 6, 876 (2007).
4.
M. W. Chen, E. Ma, K. J. Hemker, H. W. Sheng, Y. M. Wang, and X. M. Cheng,
Science. 300, 1275 (2003).
5.
X. Z. Liao, F. Zhou, E. J. Lavernia, D. W. He, and Y. T. Zhu, Appl. Phys. Lett.
83, 5062 (2003).
6.
W. Z. Han, G. M. Cheng, S. X. Li, S. D. Wu, and Z. F. Zhang, Phys. Rev. Lett.
101, 115505 (2008).
7.
X. L. Wu, B. Li, and E. Ma, Appl. Phys. Lett. 91, 141908 (2007).
8.
Y. Matsukawa, K.Yasunaga, M. Komatsu, M. Kiritani, Mater. Sci. Eng. A 350,
17 (2003).
9.
I. Mukouda,Y. Shimomura, M. Kiritani, Mater. Sci. Eng. A 350, 37 (2003)
10.
M. Kiritani, K. Yasunaga, Y. Matsukawa, M. Komatsu, Rad. Eff. Defects Solids
157, 3 (2002)
11.
M. Kiritani, J. Nucl. Mater. 216, 220 (1994).
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FIG. 1.
Frank loop formation process under electron beam irradiation (low dose).
The electron beam is along <110> zone axis. (a) Irradiation time t=0 min. (b) t=10
min. (c) t=40 min. (d) A HRTEM image of the Frank loop, showing the SF feature
similar to that found in [6].
FIG. 2.
Frank loop formation process under high-flux electron irradiation, viewed
along <110>. (a) t=0 min. (b) t=2.5 min.
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FIG. 3.
Frank loops formed in the annealed Al sample under low-flux irradiation
along <110>. (a) t=0 min. (b) t=10 min. (c) t=40 min.
FIG. 4.
Frank loops formed in the annealed sample under high-flux irradiation,
viewed along <110>. (a) t=0 min. (b) t=5 min.
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