Grain Contrast Imaging in UHV SLEEM

Transcription

1 Materials Transactions, Vol. 51, No. 2 (2010) pp. 292 to 296 Special Issue on Development and Fabrication of Advanced Materials Assisted by Nanotechnology and Microanalysis #2010 The Japan Institute of Metals Grain Contrast Imaging in UHV SLEEM Šárka Mikmeková 1, Miloš Hovorka 1, Ilona Müllerová 1, Ondřej Man 2, Libor Pantělejev 2 and Luděk Frank 1 1 Institute of Scientific Instruments of the ASCR, v.v.i., Královopolská 147, Brno, Czech Republic 2 Institute of Materials Science and Engineering, Faculty of Mechanical Engineering, Brno University of Technology, Technická 2, Brno, Czech Republic Study of the grain structure in the equal channel angular pressing processed copper by means of the cathode lens equipped ultrahigh vacuum scanning low energy electron microscope is reported. The grain contrast was found achieving its maximum at electron energies below about 30 ev where it alternated its sign and exhibited dependence on electron energy specific for the grain orientation. The energy dependence of the electron reflectance seemed to be capable of serving as a fingerprint enabling determination of the crystalline orientation. In the cathode lens mode at hundreds of ev fine details of the microstructure are also observable including twins and low angle grain boundaries. This is explained by acquisition of high-angle backscattered slow electrons, normally not acquired in standard scanning electron microscopes. The very low energy electron reflectance is promising as an alternative to the EBSD method owing to its high resolution and fast data acquisition. [doi: /matertrans.mc200919] (Received November 5, 2009; Accepted November 25, 2009; Published January 25, 2010) Keywords: scanning low energy electron microscopy, electron backscatter diffraction (EBSD), grain contrast, ultra-fine grained materials 1. Introduction Nowadays the ultrafine-grained materials (UFG) belong to the front line areas of research in material sciences. These materials are very attractive for many industrial applications (e.g. aerospace, automotive, chemical sensors, biomaterials, electronic, etc.) due to their interesting mechanical and physical properties surpassing those of common materials, such as excellent super-plasticity at low temperatures, a combination of high ductility with enhanced electrical resistivity, and other properties. UFG materials are defined as polycrystals with grains of the average grain size less than 1 mm. 1) Various techniques have been developed to prepare UFG materials, with majority of them based on severe plastic deformation (SPD). One of the SPD methods is the equal channel angular pressing (ECAP) procedure in which an extremely high strain is imposed on the bulk materials without any changes in the cross-sectional dimensions of the samples. The ECAP technique is very attractive because it can economically produce bulk UFG materials, being a relatively simple and effective method applicable to a wide range of metal and alloys. 2) For the bulk UFG materials the presence of a high fraction of high-angle grain boundaries is very important from the point of view of achievement of unique advanced properties. Grain contrast imaging can be performed with the help of several techniques. 3) The technique most widely used for observation of microstructure of UFG materials are the scanning electron microscopy (SEM), the electron backscattered diffraction method (EBSD), the scanning transmission electron microscopy (STEM), the transmission electron microscopy (TEM) and the focused ion beam (FIB) microscopy. A specific way to visualizing the grain contrast at high spatial resolution, to achieve a high contrast between grains and very fast data acquisition is to use the cathode lens (CL) mode in the SEM. The aim of this study is to analyze the microstructure of UFG copper by an ultra-high vacuum scanning low energy electron microscope (UHV SLEEM) equipped with CL, which retards the primary electrons and collects and accelerates the signal electrons, and with an ion-gun for cleaning the sample surface. 2. Experimental 2.1 Sample preparation The material under investigation was a commercial purity copper (99.98 mass%) prepared by the ECAP technique. Cylindrical billet of 120 mm in length and 20 mm in diameter was processed by 8 passes of ECAP using the B c route 2) with the workpiece rotated by 90 between passes with respect to its longitudinal axis. In this way the originally coarse grained copper acquired the UFG structure. Finally, the sample was annealed for 6 minutes at 180 C in the argon atmosphere. For UHV SLEEM and EBSD techniques, a representative part of the parent sample was separated by an electronic cutter. The sample surface was prepared by mechanical polishing and electro-polishing, which was to remove any remnant damaged layers and surface irregularities. In order to remove the native oxide films from the sample surface the sample was cleaned in-situ by argon ions for 40 minutes using the ion beam of 3 kev energy, 5 ma current and partial pressure of argon amounting to 10 3 Pa. Figure 1 shows the SLEEM image obtained from the ECAPed copper at 2 kev landing energy of electrons. The native oxide layer at the surface was impenetrable for primary electrons (PE) and prevented us from observing the microstructure of the sample (Fig. 1, site a). In order to reveal the real microstructure of the UFG copper, the oxide layer had to be removed (Fig. 1, site b). 2.2 Instrument The microstructure of the UFG copper was investigated

2 Grain Contrast Imaging in UHV SLEEM Contrast of Crystal Orientation As mentioned above, a contrast between polycrystal grains can be obtained with the help of several techniques. Although the TEM and STEM methods provide the highest spatial resolution, a very thin foil has to be prepared for the observation, which is usually difficult and time-consuming. The EBSD method in SEM provides information on the grains orientation and is most commonly used for evaluation of the microstructure of UFG materials. On the other hand, when high quality electron backscatter diffraction patterns are needed, any deformed layers and surface irregularities have to be removed by special preparation techniques. A further disadvantage is the very slow data acquisition and processing. As an alternative method for observation of grains, the SLEEM observation with electrons below, say, 25 ev of landing energy is very fast, with the preparation procedure comparable to the EBSD method and much simpler than that of TEM/STEM. Fig. 1 Microstructure of UFG Cu: (a) as-inserted oxidized area, (b) in-situ cleaned area. under ultrahigh vacuum conditions by means of the UHV SLEEM developed at ISI Brno for observation of clean and defined surfaces. The UHV SLEEM consisted of three vacuum chambers. The observation chamber was equipped with a two-lens field emission electrostatic electron optical column (FEI Company) with the CL system. The CL mode in the UHV SLEEM 4) enabled us to observe specimens at arbitrary landing energies of the primary electrons. The negatively biased specimen formed the cathode and the earthed scintillator of the detector the anode of the cathode lens. The primary electrons were retarded in the CL field to their final landing energy with which they impinged on the specimen surface. The emitted electrons were collimated to the optical axis and accelerated towards the detector so that very high collection efficiency was obtained. A very important aspect was represented by detection of the electrons backscattered under large angles from the optical axis and also the slow backscattered electrons, both of which are unavailable in conventional devices. Moreover, the combination of the nearly total fluxes of accelerated secondary and backscattered electrons that impacted the detector provided the optimum extraction of information. The detector was based on the bored single crystal YAG disc. Image resolution of the UHV SLEEM in the standard SEM mode is 12 nm at 25 kev while in the CL mode it amounts to 28 nm at 10 ev. The second chamber was intended for in-situ preparation of the samples and incorporated a scanning argon ion-beam gun for surface cleaning. The third chamber was the loading chamber of the air lock. The grain orientation maps were obtained with the help of the EBSD method in a Philips XL30 SEM. The EBSD system was equipped with the EDAX TLS s Digi View 1612 high sensitivity CCD camera. The OIM DC 3.54 software was used for data acquisition and OIM Analysis 3.08 for their processing. 3.1 Channeling contrast The origin of crystallographic contrast is anisotropy of electron scattering, i.e. influence of 3D distribution of the inner potential and diffraction on the crystal planes and channeling between them. Thus, emission of the backscattered (BSE) and the secondary (SE) electrons depend on the crystal orientation and on the angle of incidence of the primary electrons. The BSE yield is higher from planes of higher density of atoms. In the FCC crystal (Cu), the low index planes have a high atom density. 5) The grain orientation contrast is borne by a large fraction of BSE, and SE2 (secondary electrons released by BSE leaving the sample) also contribute. 3,6) Figure 2 shows the landing energy dependence of the image contrast, acquired using the cathode lens assembly, between grains of ECAPed copper with orientations (023) and (103). The contrast was calculated as C ¼ðS 1 S 2 Þ= ðs 1 þ S 2 Þ where S 1 and S 2 were image signals averaged over the grains in question. At energies above 30 ev the BSE signal from the 103 plane was higher owing to its higher Fig. 2 Energy dependence of the contrast between grains of ECAPed copper with orientations (023) and (103).

3 294 S. Mikmekova et al. Fig. 3 Contrast observed between (023) and (103) grains. The electron impact energies are (a) 4000 ev, (b) 3000 ev, (c) 2000 ev, (d) 1000 ev, (e) 216 ev, (f) 46 ev, (g) 16 ev, (h) 7 ev, (i) 3 ev, ( j) 2 ev. The boundary misorientation map obtained from the EBSD measurement is in (k), together with the color key (l). Fig. 4 UHV SLEEM images of the ECAPed copper, with the electron impact energies (a) 10 kev, (b) 9 kev, (c) 8 kev, (d) 7 kev, (e) 6 kev, (f) 5 kev, (g) 4 kev, (h) 3 kev, (i) 2 kev, ( j) 1 kev and (k) 10 ev.

4 Grain Contrast Imaging in UHV SLEEM 295 Fig. 6 Reflectance curves acquired by measuring the average image signal over the grains with orientation (023) and (103). Fig. 5 UHV SLEEM image at 11 ev (a), together with the EBSD map of the same area (b), the color key for the map (c) and the reflectance curves for three copper grains with orientations (100), (101) and (111) (d). density of atoms. However, below 30 ev the contrast behavior was less simple and straightforward. Figure 3 shows the corresponding series of micrographs. Fig Contrast of the local density of states In the landing energy range of units of ev, the elastic backscattering is the dominant process among the electron scattering mechanisms. The reflectivity of the very slow electrons (below, say, 25 or 30 ev) from the specimen surface is inversely proportional to the local density of states coupled to the incident wave.7) Differently oriented grains in the polycrystalline materials have specific density of states in the electron impact direction and its energy distribution, enabling to distinguish grains in very low energy images and to observe interesting contrast variations.4) As we see in Figs. 2 and 3, maximum contrast between the grains was met within the range of units of ev. SLEEM image of the ECAPed copper obtained at 2 kev (a) and the EBSD map of the same area (b).

5 296 Š. Mikmeková et al. Another energy series of micrographs, again for the ECAPed copper, in Fig. 4 also provides the grain contrast highest at the lowest energy and demonstrates the ability of the UHV SLEEM to image grains in polycrystalline materials. Figure 5 shows the reflectance curves acquired by measuring average image signals over three grains in copper with orientations (100), (101) and (111), together with the grain orientation map obtained from EBSD. In Fig. 6 there are the resulting reflectance curves for (023) and (103) (see Fig. 2), now within the very low energy range. 4. Visualization of Details in Microstructure Acquisition of the great majority of emitted species, in particular of the high angle BSE, enables to observe the micro- or even nano-structure at a high signal and contrast so that details can be observed that are not available with conventional methods. Figure 7 shows the UHV SLEEM image obtained in the CL mode at 2 kev, together with the EBSD map of the same area. Obviously in the SLEEM images more information is contained about the microstructure of ECAPed copper. You can see the twins not visible in the EBSD map at all, and also fine details of the internal microstructure of the larger grains. 5. Conclusion UHV SLEEM is a method especially useful for observing the grains in polycrystalline materials thanks to its high spatial resolution, fast acquisition of data and possibility of observing spacious bulk samples. The sample preparation corresponds to the small penetration depth of slow electrons, i.e. really clean surfaces are required. Ultrahigh vacuum conditions, dry vacuum pumps and in-situ ion beam cleaning seem to be unavoidable for acquisition of contrasts connected with the local density of electron states. However, the cathode lens mode secures detection of high angle BSE and of a combination of SE with (slow) BSE and provides micrographs with high grain contrasts even under conditions available in standard SEMs. The UHV SLEEM method can be used for identifying the grain orientation through its specific reflectance vs. energy curve in the very low energy range. Future development of the method should incorporate creation of a database of the reflectance curves to be used as reference fingerprints for recognition of grain orientations in the SLEEM micrographs. Imaging of polycrystalline samples in the cathode lens mode is very useful for observation of fine details in the microstructure such as the twins, low angle grain boundaries and so on. Acknowledgements The study is supported by the Eureka project no. OE REFERENCES 1) R. Z. Valiev and T. G. Langdon: Progress Mater. Sci. 51 (2006) ) R. Z. Valiev, R. K. Islamgaliev and I. V. Alexandrov: Progress Mater. Sci. 45 (2000) ) S. Canovic, T. Jonsson and M. Halvarsson: J. Phys. Conf. Series 126 (2008) :1 4. 4) I. Müllerová and L. Frank: Adv. Imag. Electron Phys. 128 (2003) ) A. Kiejna and K. F. Wojciechowski: Metal surface electron physics, (Pergamon Press, Oxford, 1996) pp ) L. Reimer: Scanning Electron Microscopy, (Springer-Verlag, Berlin, 1998) pp ) R. C. Jaklevic and L. C. Davis: Phys. Rev. B 26 (1982) 5391.