Medium range order of bulk metallic glasses determined by variable ...

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828 J.W. Deng et al. / Micron 43 (2012) 827–831 (Treacy et al., 2005; Voyles and Abelson, 2003), and investigations on BMGs have also illustrated effects of minor alloying elements, structural relaxation, and deformation processes on MRO of BMGs (Li et al., 2003; Stratton et al., 2005; Wen et al., 2007). However, most of reported works are conducted with variable coherence FEM, while only a few variable resolution FEM experiments are preformed in STEM by varying the size of electron probe (Bogle et al., 2010; Daulton et al., 2010; Hwang and Voyles, 2011). In this study, variable resolution FEM was implemented in TEM by changing the size of objective lens aperture, and MRO lengths of amorphous materials with different bonding types were obtained. Additionally, the optimal resolution for FEM experiment is also discussed based on the situation of obtaining the maximum intensity variance. 2. Experimental Two Zr-based and one Fe-based BMGs were chosen for the FEM experiment. Zr 64.13 Cu 15.75 Ni 10.12 Al 10 (Zr-S2) and Zr 52.5 Ti 5 Cu 17.9 Ni 14.6 Al 10 (Zr-Vit105) samples are plates with 10 mm width and 1 mm thickness. Fe 48 Cr 15 Mo 14 C 15 B 6 Y 2 are rods with 3 mm in diameter. All of the BMG samples were prepared by arc melting and copper mold casting. The amorphous structure of BMG samples was confirmed by X-ray diffraction and highresolution transmission electron microscopy. TEM samples were prepared by electropolishing in a Struers Tenupol-5 twin-jet electropolisher with the electrolyte of 10% perchloric acid in ethanol at 238 K. In order to avoid subtle structure changes occurring in the samples during storage (Stratton et al., 2004), the prepared TEM specimens were examined within 48 h after thinning. FEM experiment was also conducted on amorphous silicon nitride (a-Si 3 N 4 ) membranes of commercial TEM grids. The thickness of a-Si 3 N 4 foil is 20 nm. FEM works were performed in hollow-cone illumination mode on a Tecnai G 2 F30 TEM operated at 200 kV. Details of the FEM experimental procedure are described in a previous paper (Wen et al., 2007). The variance V(k) curve is obtained from the average of experiment results taken from more than ten different areas in a sample. To assure the consistence of thickness in different sample areas, the electron transmittance for the selected areas was examined before acquisition of each image series. In this study, the desired transmittances for Zr-based and Fe-based BMG samples are 50 ± 2% and 40 ± 2%, respectively, which correspond to a thickness less than 30 nm. The specimen thicknesses were verified by using the log-ratio method based on EELS (Egerton, 1996). The thicknesses were below 0.3 multiplies of the inelastic mean free path in the 50% and 40% electron transmittance areas. The inelastic mean free paths for the Zr-based and Fe-based samples are about 82 and 91 nm, respectively, calculated by using the DigitalMicrograph script of Mean Free Path Estimator (Mitchell, 2006) based on Malis method (Malis et al., 1988). The magnification and the pixel size of the images are kept constant for all samples and all the resolutions during the experiment. To compensate the low image intensity of smaller aperture, longer acquisition time is applied. The mean intensity of each dark-field image at different k values (obtained by changing the tilt angle of the incident electron beam) is also kept constant by adjusting the collection time. The highest k for FEM curves was chosen appropriately (8 nm −1 for BMGs and 7.3 nm −1 for a-Si 3 N 4 ) to avoid too low image intensity in the high k region. In this work, the longest acquisition time for single image has not exceeded 30 s in the highest k and with the smallest aperture. Although a long exposure time may bring artifacts into hollow-cone dark-field images due to such as specimen drift, instrumental instability, and CCD shot noise, these effects have not shown significant influences on the results when the acquisition Fig. 1. Select area electron diffraction pattern of Zr-S2 BMG sample. The insets are images of different objective apertures in the diffraction plane, while the scale bars are same as that of the diffraction pattern. Table 1 Parameters of objective apertures used in the experiment. Q and R denote radii of the apertures in reciprocal space and nominal resolutions of FEM images, respectively. Aperture size (m) Q (nm −1 ) R (nm) 5 0.55 1.1 10 1.0 0.6 20 2.3 0.3 time is less than 30 s. The data acquisition and image processing were implemented on the DigitalMicrograph software using script programs (Wen et al., 2007). The variance V(k,Q) of dark-field image is calculated as, 〈〉 I 2 (k, Q ) V(k, Q ) = 〈〉 2 − 1 (1) I(k, Q ) where k is the scattering vector, and Q is the radius of the objective aperture in reciprocal space. 〈I 2 (k,Q)〉 and 〈I(k,Q)〉 are the mean square intensity and averaged intensity of the dark-field image, respectively. An objective aperture strip with a series of aperture sizes was used to carry out the variable resolution FEM. The objective aperture strip was ordered from Ladd Research Inc. (Williston, VT, USA). The Pt aperture strip contains circular apertures with different diameters and aperture diameters of 5, 10 and 20 m were used in this work. A select area electron diffraction pattern of the Zr- S2 sample is presented in Fig. 1. The halo ring demonstrates that the specimen is fully amorphous. The insets in Fig. 1 are images of 3 different objective apertures in TEM. By measuring reciprocal radii Q of the circular objective apertures, we have determined nominal resolutions of the dark-field imaging via R = 0.61/Q (Gibson et al., 2000) for different apertures. The measured Q and calculated R values for the apertures are listed in Table 1. 3. Results Fig. 2 shows the normalized intensity variance V(k) taken at different scattering vector k from the Zr-S2 sample with FEM. The scattering vector k was controlled by tilting the incident
J.W. Deng et al. / Micron 43 (2012) 827–831 829 Fig. 2. The normalized intensity variance curves of Zr-S2 BMG obtained by variable resolution FEM with different nominal resolutions R. electron beam against the optic axis with angle (k = sin/, is the electron wavelength). As the tiled incident beam is continually rotating around the optic axis, the obtained hollow-cone dark-filed image is a superposition of dark-field images over 360 ◦ azimuth angle at the given k. A series of these images were obtained with k varying from 2.8 to 8.0 nm −1 for each objective aperture. For the corresponding nominal resolution R (1.1, 0.6, and 0.3 nm, respectively), a variance V(k) curve was obtained subsequently. The maximum V(k) located at k = 3.8 nm −1 for all the V(k) curves, where k = 3.8 nm −1 corresponds to the maximal diffraction intensity in the electron diffraction patterns (see e.g. Fig. 1). Although these curves possess similar tendency and peak positions, their maximum variance values are considerably different. For the nominal resolution R of 0.6 nm, the maximum V(k) is 0.022; while for resolutions R = 1.1 and 0.3 nm, the maximum V(k) are 0.0094 and 0.017, respectively. Since the sample and imaging conditions are kept constant during the experiment, these differences in the maximum variance are assumed mainly due to the different nominal resolutions employed. This is also visible in the corresponding dark-field images. Fig. 3a–c are hollow-cone dark-field images acquired at the V(k) peak position (k = 3.8 nm −1 ) while the 1.1, 0.6, and 0.3 nm nominal resolutions were used, respectively. All of the images are collected and shown with the same intensity scale for comparison. The speckle contrast in Fig. 3b is clear when the 0.6 nm resolution was used, while they are blurred when the other two resolutions were employed. V(k) curve in the lower k region for the 0.3 nm resolution is absent in Fig. 2, this is because the transmitted beam was partly included in the lower k region when a large aperture is used. Nevertheless, a V(k) peak is clearly visible in the variance curve for the Fe-based BMG sample with the 0.3 nm resolution (as shown in Fig. 4a). For comparison, normalized intensity variance curves for a-Si 3 N 4 are also presented in Fig. 4b. The maximum V(k) at each nominal resolution R are summarized in Fig. 5 for all the amorphous samples (Zr-S2, Zr-Vit105, Fe-based BMG, and a- Si 3 N 4 ). Noticeably, the variation of the maximum V(k) values with the nominal resolution follows the same tendency for the different Fig. 3. (a)–(c) Hollow-cone dark-field images of Zr-S2 BMG sample acquired at the peak V(k) position (k = 3.8 nm −1 ) with different nominal resolutions R ((a) 1.1 nm, (b) 0.6 nm, and (c) 0.3 nm). Fig. 4. The normalized intensity variance curves of (a) Fe-based BMG and (b) a-Si 3N 4 samples obtained by variable resolution FEM with different nominal resolutions R.
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