How to characterize defects on bulk specimen?

The Electron Channeling Contrast Imaging (ECCI)

Full characterization of dislocations is not limited to Transmission Electron Microscope (TEM) experiments on thin foil samples. Scanning Electron Microscope (SEM) can also access diffraction contrast on bulk materials through the phenomenon of electron channeling. This occurs when electrons channel down crystal planes, creating paths where electrons can penetrate to a deeper depth before scattering. This can result in different orientations of the crystal backscattering more electrons than others, creating orientation contrast.

a. Closed channel (red); b. open channel (green) and c. conversion of an open channel to a closed channel.

Accurate Electron Channeling Contrast Imaging (A-ECCI) is a non-destructive technique that allows for TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about 100 nm) on centimeter-sized bulk specimens with unparalleled resolution, inside a SEM. This is achieved by utilizing the fact that the yield of BackScattered Electrons (BSE) varies greatly depending on the orientation of the crystal relative to the incident electron beam, also known as the optic axis of the SEM.

Detailed analyses of a dislocation dipole on a bulk IF steel (Kriaa et al. Sci. Rep., 2017)

The ability to characterize dislocation configurations on a bulk sample in as much detail as TEM is due to our extensive research on A-ECCI theory (Kriaa et al., Sci. Rep., 2017; Kriaa et al. Materials, 2019), and the fact that modern FEG-SEMs offer high imaging performance thanks to adapted characteristics such as high beam current mode, small beam convergence (quasi-parallel beam), and very small spot size, which result in high lateral resolutions (a few nanometers) with a good signal-to-noise ratio.

Through these improvements, I am strongly convinced, that A-ECCI is now mature for exploring new horizons. Indeed, we recently showed the full potentiality of this technique for following the evolution (before and after deformation) of microstructures near interfaces (Guitton et al. Materials, 2018).

A-ECCI characterization of defects before (left) and after (right) µN-nanoindentation near a twin boundary in a TiAl alloy (Guitton et al. Materials, 2018)

Furthermore, in order to overcome the limitations of TEM for in-situ mechanical testing, we recently reported also the feasibility of macromechanical testing coupled with in-situ A-ECCI and its complementarity with simulations (Ben Haj Slama et al. Materials, 2019).

Evolution of deformation microstructures during loading in a Ti21S bulk specimen. Dislocation network and slip traces are evolving (Ben Hal Slama et al. Materials, 2019)

Therefore, we have access to different scales with one unique tool while loading a bulk sample: from sample scale to dislocation scale.

Some of my publications on ECCI

In situ macroscopic tensile testing in SEM and Electron Channeling Contrast Imaging: pencil glide evidenced in a bulk beta-Ti21S polycrystal.
M. Ben Haj Slama, N. Maloufi, J. Guyon, S. Bahi, L. Weiss, A. Guitton
MATERIALS, 2019, 12 (15), 2479
DOI: 10.3390/ma12152479

Modeling dislocation contrasts obtained by accurate-Electron Channeling Contrast Imaging for characterizing deformation mechanisms in bulk materials.
H. Kriaa, A. Guitton, N. Maloufi
MATERIALS, 2019, 12 (10), 1587
DOI: 10.3390/ma12101587

A dislocation-scale characterization of the evolution of deformation microstructures around nanoindentation imprints in a TiAl alloy.
A. Guitton, H. Kriaa, E. Bouzy, J. Guyon, N. Maloufi
MATERIALS, 2018, 11 (2), 305
DOI: 10.3390/ma11020305

Fundamental and experimental aspects of diffraction for characterizing dislocations by electron channeling contrast imaging.
H. Kriaa, A. Guitton, N. Maloufi
DOI: 10.1038/s41598-017-09756-3

Electron channeling contrast imaging of dislocations. Theory and experiments.
H. Kriaa
PhD Thesis, Université de Lorraine, 2018
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