The increasing interest in nanostructured materials has raised the need for high spatial resolution orientation mapping and large-scale quantitative characterisation of such microstructures. Because Electron Back Scatter Diffraction (EBSD) does not achieve such high spatial resolution on bulk samples, these kind of studies are often done using a Transmission Electron Microscope (TEM). However, TEM-based orientation mapping techniques suffer from small field of view. As a result, Transmission Kikuchi Diffraction (TKD) in Scanning Electron Microscope (SEM) was developed as a technique capable of delivering the same type of results as EBSD but with a spatial resolution improved by up to one order of magnitude [1,2]. TKD analysis is conducted on an electron transparent sample using the same hardware and software as for EBSD system.  But when using conventional EBSD geometry, the transmitted patterns (TKP) are captured by a vertical phosphor screen with a considerable loss of signal and with strong distortions induced by gnomonic projection. Also, with standard TKD detector configuration, most of the transmitted signal does not reach the phosphor screen and results in lower quality patterns which can have negative effect in the measurement quality.

The limitations of such non-optimal sample-detector geometry can be overcome by an on-axis detection system. With a horizontal phosphor screen placed underneath the sample, the transmitted signal is captured where it is the strongest and TKPs will have minimal distortions. Using low probe currents, the spatial resolution can be increased and the beam-induced specimen drift reduced as compared to standard TKD detector configuration [3]. The improved stability and high spatial resolution allow the user to conduct large-area TKD orientation mapping.

Using a partially recrystallized ultrafine stainless steel sample, we will demonstrate that statistical data can be obtained for the quantitative characterisation of nanostructured materials in the SEM (figure 1).

References:

[1] R.R. Keller and R.H. Geiss, Journal of Microscopy, Vol. 245, Pt. 3, pp. 245-251, 2012.

[2] P. W. Trimby, Ultramicroscopy, 120, 16-24, 2012.

[3] M. Abbasi et al., ACS Nano, vol.9, no.11, 10991- 1002, 2015.

Figures:

Figure1: Unprocessed orientation map (IPFz) of a partially recrystallized steel acquired at a speed of 100 points per second using the on-axis detector configuration from a 20x15μm2 area at a 10nm resolution. The dataset contains more than 3600 grains which can be considered as a sufficiently large number to correctly assess the local average grain size and crystallographic texture. Authors would like to thank Prof. Zeng Yi from Shanghai Institute of Ceramics, China for generously providing the steel sample.

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