A NEW APPROACH TO STUDYING CAST CB2 STEEL USING SLOW AND VERY SLOW ELECTRONS

A NEW APPROACH TO STUDYING CAST CB2 STEEL USING SLOW AND VERY SLOW ELECTRONS Šárka Mikmeková 1 Josef Kasl 2 Dagmar Jandová 2 Ilona Müllerová 1 Luděk Frank 1 1 Institute of Scientific Instruments of the ASCR, v.v.i, Královopolská 147, 612 64 Brno Czech Republic Tel. +420 541 514 263 sarka@isibrno.cz 2 Research and Testing Institute Plzeň Tylova 1581/46, 301 00 Plzeň Czech Republic Abstract The Scanning Low Energy Electron Microscopy (SLEEM) is beneficial for investigation many different types of materials. This technique can be used for improving of image parameters, such as atomic number and crystallographic contrast. The aim of this study is to analyze the microstructure of cast CB2 steel by ultra-high and standard high vacuum scanning electron microscopes equipped with cathode lens mode, which enables us to observe samples at arbitrary landing energy of primary electrons. In the SLEEM images more information is contained about the microstructure of CB2 steel in comparison with standard scanning electron microscopy (SEM). Study of materials by slow and very slow electrons open the way to better understanding of their microstructures. Keywords scanning low energy electron microscopy, crystallographic contrast, atomic number contrast, cast CB2 steel, scanning electron microscopy 1 Scanning Low Energy Electron Microscopy The microstructures of specimens were studied under standard vacuum in scanning electron microscopes Tescan VEGA MM 5130 and JEOL 6170F and in the ultra-high vacuum SEM (UHV SLEEM) of our own design (Fig. 1). UHV SLEEM consists of three separated vacuum chambers. One of them is observation chamber equipped with a two-lens field emission electrostatic electron optical column (FEI Company) and CL system. The second chamber was intended for in-situ cleaning of the samples and incorporated an argon ion beam gun for surface cleaning. The third chamber is the loading chamber of the air lock. All of these microscopes are equipped with cathode lens (CL) mode which enables us to observe specimens at arbitrary landing energy of primary electrons. The principle of CL mode is shown on Fig. 2 and as you Fig.1 An ultra high vacuum scanning low energy electron microscope equipped with the cathode lens operating at landing energies from 25 kev to zero ev. 161

can see the cathode of the CL is formed by negatively biased specimen and anode of the CL is created by detector based on the ground potential. Primary beam is retarded in the field of the CL on final landing energy and emitted electrons from the specimen are collimated and accelerated to detector. [1] The information about crystallographic orientation was obtained using electron backscattered diffraction (EBSD) method in a Philips XL30 SEM. investigated using scanning electron microscopes equipped with cathode lens mode not only under standard high vacuum but also under ultra-high vacuum conditions. Observation of specimens by very slow electrons requires in-situ cleaning of surface from native oxide layers. The native oxide is impenetrable for low energy primary beam and its presence prevent us from observing of real microstructure. This situation is demonstrated by Fig.1. Fig. 1 shows the comparison between the insitu cleaned and rough areas together with X-Ray Photoelectron Spectroscopy (XPS). From XPS analysis follows that the specimen surface is covered by Fe 3 O 4, Fe 2 O 3 and Cr 2 O 3 oxides. 3 Results and Discussion Fig. 2 Schematic sketch of the cathode lens. 2 Experimental material The cast CB2 steel was chosen as the specimens under investigations. Microstructure of the virgin material and crept specimens (tested at 650ºC and stresses at 60 and 85 MPa) were The atomic number and crystallographic contrast is borne mainly by backscattered (BSE) electrons (i.e. electrons that are leaving the sample with energy higher than 50eV and by elastic scattering). The BSE leave the specimen surface at wide range of angles relative to the optical axis. The electrons backscatter under low angles from optical axis carry mainly information about chemical composition and electrons backscattered under high angle are sensitive on density of atoms within crystal lattice. [2] 3.1 Tuning of material contrast As mentioned above, the BSE coefficient depends on atomic number of specimen. The BSE yield is higher from materials with higher atomic number and these areas appear brighter in image. Fig. 3 Surface of CB2 steel: (a) as-inserted oxidized area, (b) in-situ cleaned area, together with the XPS spectra of the oxidized area. 162

But the BSE yield becomes nonlinear with decreasing electron energy. SLEEM is very sensitive to distribution of components on specimen surface and make it possible to observe the contrast between areas with very similar chemical composition which is not possible in standard SEM. This benefit of the SLEEM is demonstrated with study of dispersion of the precipitates in CB2 steel. Fig. 4 shows the contrast evolution between some precipitates and the basic material. The images were taken using a Tescan VEGA MM 5130 SEM equipped with the CL mode. The comparison shows that atomic number contrast from the images obtained in the CL mode is more intensive. Fig. 5 shows images of in-situ cleaned CB2 steel specimen obtained under ultra high vacuum conditions (Fig. 5 a) and b)) in UHV SLEEM microscope, together with the image of the same area in a JEOL 6170F SEM (standard high vacuum, without in-situ cleaning). Fig. 5 demonstrates that the small precipitates are not visible in standard SEM and can be successfully imaged only in the CL mode. Thus, using the CL mode is possible to visualize areas with very small difference in chemical composition and this technique provides a powerful tool for examination of multi-component materials. Fig. 4 Micrographs of CB2 steel, acquired with a backscattered electron detector at 10 kev landing energy of electrons (a, d, g) and with using of the cathode lens mode. The primary electrons were retarded in the field of the cathode lens to their final energy 5 kev (b, e, h) and 2 kev (c, f, i). Images a), b) and c) show the cast CB2 steel and the next images display specimens after creep testing at 650ºC and stresses at 60 MPa (d, e, f) and 85 MPa (g, h, i). Experiments made under standard high vacuum conditions without in-situ cleaning. Fig. 5 The effect of beam energy and cleanness of the specimen surface on material contrast. UHV SLEEM images of CB2 steel obtained at 6017 ev (a) and at 10 ev (b) (the primary beam energy was 6017 ev). Fig. c) shows standard SEM image obtained at 5 kev landing energy of electrons. 163

3.2. Sensitivity to crystallographic orientation Backscattering of electrons is the source of one of the most important image signals in the SEM. In the CL mode of the SEM a combination of secondary electrons with (slow) backscattered electrons (including electrons backscattered under high angles from the optical axis) is acquired, providing micrographs with high crystallographic contrast even under conditions available in a standard SEM. Fig. 6 shows the UHV SLEEM images obtained from CB2 steel at 4 kev (a) and 1 kev (b) landing energy of electrons, together with corresponding crystallographic orientation map obtained from the EBSD measurement (c) and with simulations of the BSE trajectories (d, e). Acquisition of the high angle electrons enables to observe the contrast between martensite needles with extremely high contrast. As mentioned above, the CL mode in the SEM enables us to detect slow but not only slow, high angle scattered electrons that carry mainly information about crystallography of the specimen. The sensitivity of the SLEEM to the crystallographic orientation is demonstrated with a series of UHV SLEEM images of CB2 steel taken with various landing energy of primary electrons (Fig. 7). [3,4,6,7] 4. Conclusion The main advantages of the scanning low energy electron microscopy consist in many Fig. 6 UHV SLEEM images of CB2 steel obtained at 4 kev (a) and 1 kev (b), together with EBSD map of the same area (c) and corresponding simulations of BSE trajectories in the Electron Optical Design (EOD) software (d, e) [5]. Fig. 7 UHV SLEEM images of CB2 steel. The incident electron energies at with the images were taken were 6012 ev (a), 5000 ev (b), 4000 ev (c), 3000 ev (d), 2000 ev (e), 1000 ev (f), 500 ev (g), 400 ev (h), 300 ev (i), 200 ev (j), 100 ev (k) and 50 ev (l), together with the EBSD map of the same area (on the right). 164

opportunities the method provides not only in fundamental research but also for practice. This method offers the possibility to study of materials with extremely high sensitivity to the crystallography and chemical composition. Knowledge of the crystallography and chemical composition of materials is important for better understanding their microstructures as well as providing the key for development of materials with unique mechanical and physical properties. Acknowledgements The financial support of the project no. TE01020118 (Competence centre: Electron microscopy) from the Technology Agency of the Czech Republic is greatly acknowledged. References [1] I. Müllerová, L. Frank: Advanced Imaging Electron Physics, Vol. 128, 2003, p. 309-443 [2] L. Reaimer: Scanning electron microscopy, Springer Verlag, Berlin, 1998 [3] Š. Mikmeková et al.: Materials Transactions, Vol. 51, 2010, p.292-296 [4] Š. Mikmeková et al.: Key Engineering Materials, Vol. 465, 2011, p. 338-341. [5] B. Lencová, J. Zlámal: Microscopy and Microanalysis, Vol. 13, 2007, p. 2-3 [6] E. Bauer: Surface Rewiev and Letters, Vol. 5, 1998, p. 1275-1286 [7] J. Heiner: Microscopy and Microanalysis, Vol. 14, 2008, p. 1226-1227 165