Influence of surface oxidation on transmission electron microscopy characterization of Fe–Ga alloys

MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 9 8– 6 0 2

available at www.sciencedirect.com

www.elsevier.com/locate/matchar

Influence of surface oxidation on transmission electron microscopy characterization of Fe–Ga alloys Q. Xing⁎, M.J. Kramer, D. Wu, T.A. Lograsso Division of Materials Sciences and Engineering, Ames Laboratory, Ames, IA 50011, USA

AR TIC LE D ATA

ABSTR ACT

Article history:

Fe–Ga alloys are rapidly oxidized when exposed in air, forming both amorphous and

Received 11 February 2010

crystalline surface oxides. These oxides hinder the observation of the ordered phases of B2

Received in revised form

and D03 in Fe–Ga alloys by dark-field imaging and high-resolution imaging of transmission

4 March 2010

electron microscopy (TEM) techniques. Proper imaging techniques and reduction of surface

Accepted 11 March 2010

oxides are necessary to obtain representative microstructural features to the bulk alloys by TEM.

Keywords:

© 2010 Elsevier Inc. All rights reserved.

Iron Gallium Oxide Dark-field imaging Fourier transform High-resolution electron microscopy (HREM)

1.

Introduction

Magnetostrictive Fe–Ga alloys have been of great research interest, due to their excellent combination of magnetostriction with mechanical properties and low production costs compared with other magnetostrictive materials [1]. Transmission electron microscopy (TEM) has proven to be an effective research tool in connecting the observed macro magnetostrictive behavior to the microstructure in these alloys [2–4]. The alloys' surfaces are prone to oxidization when exposed in air [2,3] and other oxidizing environments. The surface oxides make it difficult to properly operate darkfield (DF) imaging, interpret diffraction patterns, and analyze lattice fringes acquired by high-resolution electron microscopy (HREM), which will be presented in this work.

2.

Experimental

Fe–Ga single crystals were grown at the Ames Laboratory's Materials Preparation Center [5] by using Bridgman technique [6], subsequently annealed at 1000 °C for 168 h, and cooled at a controlled rate of 10 °C/min. Pure iron single crystals were obtained by solid state annealing. TEM samples were made by electro-jetting with a perchloric acid and methanol mixture, and examined in an FEI Tecnai G2 F20-XT microscope fitted with a Gatan image filter (GIF). Some samples were also ion-milled with Ar+ under conditions of 2 kV beam voltage, 3 mA beam current, and 8° beam glancing angle for 20–30 min to minimize surface oxides, formed during sample storage in air after electro-jetting. HREM images were simulated with NCEMSS [7], a software package to simulate HREM images, based on a

⁎ Corresponding author. Tel.: +1 515 294 4693; fax: +1 515 294 8727. E-mail address: [email protected] (Q. Xing). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.03.004

M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 9 8 –6 0 2

multislice approximation. The program was quite flexible and supported input of structures to view unit cells, calculate images and diffraction patterns, provide various layout options—contrast transfer functions, Pendellossung plots, etc. Structure models for HREM simulation were built with CaRIne™. Fourier processing was performed with Digital Micrograph™. For DF-imaging, the TEM objective lens astigmatism caused by the magnetic samples was corrected by a method using caustic curves [8]. To study oxidation behavior, thermogravimetric analysis (TGA) was completed for some samples. Single crystal samples of pure Fe, Fe–11 at.% Ga, and Fe- 18.5 at.% Ga were made into disks, 1.5 mm thickness and 4 mm diameter, by electric discharge machining. The disks were then polished with a series of sand papers, finishing with 800 grit. These samples were placed on the thermobalance's platinum pan of a TGA and heated to 700 °C at a heating rate of 20 °C/min. Isothermal oxidation was completed in an air environment with an air flow of 100 ml/min for 5 h. Plots of mass gain with time during isothermal runs were continuously monitored.

3.

Results and discussions

3.1.

Oxidation of Fe and Fe–Ga

rapidly to be avoided during the transfer from the ion-miller to the TEM, even though the sample transfer time was only several minutes. There are both crystalline and amorphous surface oxides for pure Fe and Fe–Ga alloys, e.g., see Fig. 5 in Ref. [2] and Figs. 2 and 3 in Ref. [3] for crystalline oxides, and Fig. 3 in this work for amorphous oxides. As stated before [2], on [001] zone axis (ZA), the diffraction spots from crystalline oxides coincide with superlattice diffraction spots of Fe–Ga alloys, making it difficult to determine whether there is any ordered B2 or D03 phase, if only using [001] ZA. The diffraction pattern from [011] ZA allows us to differentiate the oxides from the ordered phase [2]. However, the presence of crystalline oxides increases the difficulty in DF-imaging of ordered phase by selecting a (200) superlattice spot, as described below. Energyfiltered diffraction patterns on [011] ZA revealed more oxide features/spots around the direct beam. The oxides could be in a transition state. Investigation into the nature of the oxide formation is out of the scope of this work. The distribution of the crystalline surface oxides shows no link to the distribution of D03 domains, indicating the oxidation rate does not depend on ordering in the same material.

3.2.

TGA data (Fig. 1) clearly show that pure Fe and Fe–Ga alloys are prone to oxidization in air, while the solid solution of Ga into Fe slows the macro oxidation process. The effect of Ga addition on the macro oxidation process is not necessarily the same as that for the micro oxidation process. For example, a higher Ga content may result in a more rapid formation of dense surface oxides, which hinders further oxidation. We found TEM foils prepared by electro-polishing with perchloric acid and methanol were more resistant to oxidation than foils prepared by ion-milling with Ar+, because the passive films on the surfaces of the electro-polished foils were protective. The sample oxidation process after ion-milling took place in air too

Influence of crystalline oxides on DF-imaging

The technique and difficulties in DF-imaging B2 and D03 phase have been reported previously [3]. Particular care on a precise two-beam condition setup of the interested region is required to ensure (i) the weak superlattice spot is most excited and (ii) an objective aperture size is small enough to significantly reduce the background noise. If the orientation setup and the objective lens aperture used are inappropriate, it is possible to only obtain the nano crystalline surface oxides in a (200) DFimage, even though the sample contains a B2 or D03 ordered phase. These oxide domains could be mistaken as the D03 phase. This is illustrated in Fig. 2 with an Fe–20.1 at.% Ga alloy, which contains a chemically-ordered cubic phase, D03, and disordered body-centered cubic phase, A2. The foil was initially electro-polished, subsequently ion-milled, and then stored in air for a few days before TEM examination. The insertion in Fig. 2a shows a crystalline-oxide-only DF-image obtained under a wrong DF operation (the orientation was not precisely aligned). Fig. 2a shows D03 domains with the crystalline surface oxides (speckled feature). The foil was then taken from the TEM, ion-milled, and immediately placed back into the TEM. It is clear the speckled surface oxides are greatly reduced after ion-milling (compare Fig. 2a and b).

3.3.

Fig. 1 – TGA data of pure Fe and Fe–Ga alloys. The mass gain is caused by oxidation.

599

Influence of oxide on HREM

In a heavily oxidized region, the lattice fringes are discontinuous and not representative of bulk material. In a slightly oxidized region, the surface oxide may produce artifacts during windowed Fourier-filtration of HREM images to examine crystallographic defects on a nano scale, as shown in Fig. 3. Fig. 3a shows the original micrograph of lattice fringes on the [001] zone axis. To filter the background noise and enhance image quality, only (200) superlattice reflections were selected for windowed filtration by fast Fourier transform (FFT). Dislocations and lattice distortions were discovered in the

600

MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 9 8– 6 0 2

Fig. 2 – Dark-field images of Fe–20.1 at.% Ga formed by a (200) superlattice reflection showing (a) bright D03 domains, dark A2 matrix, and surface crystalline oxide, and (b) the foil was ion-milled and then immediately placed into a TEM. Insertion in (a) shows only surface crystalline oxides when the orientation deviated from a precise two-beam condition.

reconstructed image (Fig. 3c). The Burgers vector of the dislocations is identified as 1/2 <200>. The magnitude of this particular Burgers vector is sufficient to result in lattice

distortions visible in the original image (Fig. 3a), yet the distortions and dislocations are absent in the original image. Similar results have also been reported recently [9]. No misfit dislocation would be expected at the D03/A2 interface, due to the small difference in lattice parameter between D03 and A2 [10]. The sizes of the domain-like regions in Fig. 3c change with window size and are much smaller than the D03 domains, about 20–60 nm, shown in Fig. 2. These discrepancies suggest artifacts are generated during the FFT processing. It is well known the presence of an amorphous phase together with a crystalline phase may generate artifacts during the windowed Fourier processing of HREM images [11– 15]. To demonstrate this effect in the case of Fe–Ga, an amorphous carbon layer of 4 nm thickness on the top of [001] ZA pure iron (5.8 nm and 2.9 nm thickness) was simulated by the multi-sliced modeling of the layered structure. The presence of the amorphous phase is invisible in the HREM image of the 5.8 nm iron layer (compare Fig. 4a and b), but is evident for the 2.9 nm iron layer (compare Fig. 4d and e). However, the presence of the amorphous layer is obvious in frequency space in both cases—not shown here. The reconstructed images of Fig. 4b and e, using the same window size and locations in frequency space, are given in Fig. 4c and f, respectively. The lattice perfection of the 5.8 nm iron layer was unchanged, while that of the 2.9 nm iron layer was strongly distorted. In frequency space, the signal-to-noise (S/N), defined as the intensity ratio, is 1.2 and 1.03 for windowed regions for Fig. 4c and f, respectively. This clearly demonstrates the reconstructed image, using windows of low S/N ratios, is strongly affected by the background noise. In Fig. 3b, the superlattice reflections are almost invisible, which means a poor S/N. As a result, it is reasonable to see artifacts in Fig. 3c. A larger area of HREM image results in more visible superlattice reflections in its FFT pattern, but does not reduce the artifacts because noise from regions other than the region of interest also contributes to the background in frequency space. As the diffraction spots from crystalline oxides coincide with (200) superlattice diffraction spots of Fe–Ga alloys [2], it is impossible to distinguish the superlattice spot signal of the alloy from the noise of the crystalline oxide. Namely, the

Fig. 3 – Artifact creation during Fourier processing of an experimental HREM image of Fe-20.1 at.% Ga. (a): original HREM image, (b) Fourier transform pattern, and (c) reconstructed image using the masked windows of (200) superlattice reflections as shown in (b). One edge dislocation is arrowed in (c). Streaking of the spots in (b) is a result of aliasing during FFT, caused by the image boundaries. The background in (b) shows the presence of amorphous surface oxide.

M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 9 8 –6 0 2

601

Fig. 4 – Fourier processing of HREM images of pure iron with a top amorphous carbon layer of 4 nm thickness. (a)–(c) are for an iron layer of 5.8 nm thickness, and (d)–(f) for 2.9 nm.

crystalline oxide also imposes noise on the (200) superlattice reflecttions. This increases the probability to obtain artifacts by windowing (200) superlattice spots only for noise filtration. If the S/N is poor, the artifact's appearance also relates with window size, as the window size affects the noise variation within the windows. A window size too small filters out some signal and artificially creates perfect lattice fringes, while a window size too large includes too much noise and artificially introduces more lattice distortions and/or dislocations. Better filters have been developed for enhancement of noisy HREM images to avoid the problems caused by the window filter [14,16]. A script to compute the Wiener and average background subtracted filter images of HREM has been developed by Mitchell and can be downloaded for free to plug into Digital Micrograph™ [16]. Any feature which produces a noise background in frequency space can produce artifacts when subjected to improper Fourier processing. This is particularly important, since an amorphous surface layer exists in most TEM foils to one degree or another. Even if the bulk crystalline material is initially free from an amorphous phase, TEM foils can be subjected to beam damage/contamination in a TEM and/or oxidation during foil preparation and storage. The specific nature of the foil surface layer is dependent on the reactive nature of the material, coupled with how the foil is processed. Even the amorphous film used to support small crystalline

particles and fibers can introduce artifacts, if care is not taken to avoid these features.

4.

Summary

Fe–Ga alloys are prone to oxidation in an oxidizing environment, forming both crystalline and amorphous surface oxides. In a TEM study of Fe–Ga alloys, it is important to account for oxides as phases, in addition to the bulk material. Crystalline surface oxides increase the difficulty to obtain the ordered domains by dark-field imaging. Surface oxides may result in artifacts during windowed Fourier filtration of HREM images, as confirmed by the FFT processing of simulated HREM images. The artifact creation also relates to window size and signal-to-noise ratio. False interpretation of artifacts can be avoided by a visual inspection in the original image and a consistency check of the results with complimentary techniques.

Acknowledgments This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering. The research was performed at the

602

MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 9 8– 6 0 2

Ames Laboratory which is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC0207CH11358.

REFERENCES [1] Summers EM, Lograsso TA, Wun-Fogle M. Magnetostriction of binary and ternary Fe–Ga alloys. J Mater Sci 2007;42:9582–94. [2] Xing Q, Du Y, McQueeney RM, Lograsso TA. Structural investigations of Fe–Ga alloys: phase relations and magnetostrictive behavior. Acta Mater 2008;56:4536–46. [3] Xing Q, Lograsso TA. Phase identification of quenched Fe–25 at.% Ga. Scr Mater 2009;60:373–6. [4] Xing Q, Lograsso TA. Magnetic domains in magnetostrictive Fe–Ga alloys. Appl Phys Lett 2008;93:182501. [5] Materials Preparation Center, Ames Laboratory, US DOE Basic Energy Sciences, Ames, IA, USA, available from: www.mpc. ameslab.gov. [6] Lograsso TA, Ross AR, Schlagel DL, Clark AE, Wun-Fogle M. Structural transformations in quenched Fe–Ga alloys. J Alloy Comp 2003;350:95–101. [7] M.A. O'Keefe and R. Kilaas, Users Guide to NCEMSS (National Center for Electron Microscopy, Materials Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA), 1994. Also see http://ncem.lbl.gov/ frames/source/software.htm.

[8] Xing Q, Lograsso TA. A rapid method to correct objective lens astigmatism in a TEM. Ultramicroscopy 2009;109:287–90. [9] Bhattacharyya S, Jinschek JR, Kachaturyan A, Cao H, Li JF, Viehland D. Nanodispersed DO3-phase nanostructures observed in magnetostrictive Fe–19%Ga Galfenol alloys. Phys Rev B 2008;77:104107. [10] Lograsso TA, Summers EM. Detection and quantification of D03 chemical order in Fe–Ga alloys using high resolution X-ray diffraction. Mater Sci Eng 2006;A416:240–5. [11] Fan G. Enhance the contrast of defects in lattice images. Proc EMSA 1987;45:106–7. [12] Pradère P, Revol JF, Nguyen L, John Manley RST. Lattice imaging of poly-4-methyl-pentene-1 single crystals: use and misuse of Fourier averaging techinques. Ultramicroscopy 1988;25:69–80. [13] Pradère P, Thomas EL. Image processing of partially periodic lattice images of polymers: the study of crystal defects. Ultramicroscopy 1990;32:149–67. [14] Marks LD. Wiener-filter enhancement of noisy HREM images. Ultramicroscopy 1996;62:43–52. [15] Nihoul G, Sack-Hongehl H, Urban J. Electron microscopy structural characterisation of nano-materials: image simulation and image processing. Cryst Res Technol 1998;33: 1025–37. [16] Mitchell D. HRTEM filter, digital micrograph script in digital micrograph™ script database; 2007. .