Electron backscattered diffraction - principles and applications

Milan Dopita

Institute of Materials Science, TU-BAF, Freiberg, Germany

dopita@gmail.com

The Kikuchi pattern formation in Transmission Electron Microscope (TEM) was first observed and explained in 1928 by S. Kikuchi [1]. It was immediately found that the Kikuchi pattern is a powerful tool for the crystal orientation determination, because “Kikuchi diffraction pattern is a projection of the geometry of the crystal lattice from a volume of specimen in which this geometry is constant, or nearly so (Kikuchi [1]).” In 1932 Meibon and Rupp observed high angle Kikuchi patterns from “reflected” electrons. Venables and Harland observed electron backscatter patterns (EBSPs) in the Scanning Electron Microscope equipped with 30 mm diameter fluorescent imaging screen and television camera. This method allowed examination of specimens and the measurement of crystal orientation at high spatial resolution, which was significantly improved by the use of field emission gun scanning electron microscopes. The first on-line working (automated) EBSD systems were developed in 1980 and in 1993 the Orientation Imaging Microscopy (OIM) or orientation mapping was established. The on-line orientation determination from the EBSD patterns is computationally time-consuming task, however within the last decade the EBSD technique underwent a great boom as a consequence of the computers hardware improvements and progresses in the scanning electron microscopes technique, as well.

The EBSD is surface sensitive method, measured information come from the depth of several tenths of nm, depending on the measured material atomic number (the penetration depth of electrons decreases with increasing atomic number). The spatial resolution of the EBSD depends on the used electron microscope type (used electrons source). In the case of scanning electron microscope equipped with field emission cathode it is in order of ~10 nm.

Fig 1 Inverse pole figure maps of the CuZr samples, after (a) 1 (b) 2 and (c) 8 ECAP passes. The HAGBs (misorientation > 15°) are plotted as black line. Misorientation profile through one deformed grain in n_p = 1 (d) and and n_p = 2 (e), respectively and misorientation profile through several grains after eight ECAP passes calculated around the dashed line indicated in the measured map.

Two types of information are essentially held by the electron backscatter patterns. First is the Kikuchi pattern quality measure and the second is the orientation of irradiated volume. The first one, Kikuchi pattern quality information, can be used for determination of the crystal “perfection”, estimation of the crystal defects types and its densities, because the presence of the lattice defects in irradiated volume has in general in consequence decrease of the Kikuchi pattern “sharpness” (blurring of the Kikuchi pattern). However, the Kikuchi pattern quality is strongly influenced by the sample surface preparation. The Kikuchi pattern from poorly polished specimen is not sharp as well and this effect correlates with influence of the lattice defects and imperfections. Therefore, the determination of the lattice defects and densities can be done only quasi-quantitatively.

Fig 2 Inverse pole figure coloured map of the WC-Co hardmetal (a). Distribution of the length frequencies determined from orientation map using the length intercept method - blue bins and corresponding fit with the lognormal function – red solid line (b). Evolution of the mean grain size determined from the EBSD measurements in the hexagonal WC hard phase for four hardmetals differing in the starting powder WC hard phase amount (c).

More interesting information about the investigated specimen are provided by the orientation of each infinitesimal measured sample volume which can be calculated from respective Kikuchi pattern. From the orientation map, we can simply obtain information on the specimen morphology (see Fig. 1), grains and sub-grains shapes and grain and sub-grain size distributions (see Fig. 2). Measured orientation information allows us to calculate the misorientation [2] between different measured points and to describe the properties and the character of grain boundaries (GBs), to quantify fractions of high/low angle grain boundaries, observe and investigate occurrence of special grain boundaries (for instance CSL grain boundaries) – see Fig. 3.

Fig 3 Special S2 and S4 coincidence site lattice grain boundaries present in hexagonal WC phase in the WC-Co hardmetal, its confirmation of occurrence and quantification in hardmetals differing in the starting powder WC hard phase amount.

Orientation information yield us the details on the preferred orientation of crystallites, where we are not restricted to the measurements of distribution of one (or several) lattice planes in different direction in sample, which is the case of texture measurements using the X-ray diffraction (and sometimes can lead to significant errors), but we simply determine the distribution of all possible crystal orientations in given direction in sample.

Fig 4 Inverse pole figures (IPF) (a) and pole figures (PF) for {100} {110} and {111} families of lattice planes (b) in the rolled Cu sheet metal.

Measured orientation data can be used for calculation of the orientation distribution function (ODF) or misorientation distribution function (MODF). In specimen containing more phases, quantitative (volume averaged) phase analysis can be done, and above described details can be constructed for each individual phase present in sample, of course. Moreover, we can investigate the orientation dependences between different phases in the specimen.

References:

[1] S. Kikuchi, Imp. Acad. Tokyo, Proc., June 1928, Volume 4, 271-278.

[2] V. Randle and O. Engler, Introduction to Texture Analysis, Gordon and Breach Science Publishers, 2000.

[3] M. Dopita, M. Janeček, D. Rafaja, J. Uhlíř, Z. Matěj, R. Kužel: Int. J. Mat. Res., 6 (2009).

[4] M. Dopita, D. Rafaja, H. J. Seifert, D. Janisch, and W. Lengauer: Proceedings of the 17th international Plansee seminar, Vol 3, 2009.