Online Lecture by dr. Stefan Zaefferer

Bringing the TEM to SEM: An introduction to the basics of controlled electron channelling contrast imaging (cECCI) and its application to the study of extended defects in metals and alloys

Max Planck Institute for Sustainable Materials,
Department Microstructure Physics and Alloy Design,
Germany

Title:
Bringing the TEM to SEM: An introduction to the basics of controlled electron channelling contrast imaging (cECCI) and its application to the study of extended defects in metals and alloys.

Abstract:
Electron channelling contrast imaging (ECCI) is an SEM based technique for observation of extended crystal lattice defects like dislocations and stacking faults. It exploits the dependence of the backscatter electron intensity on crystal orientation and atomic order.

For ECCI a crystalline sample is observed with the backscattered electron signal. The basic principle of contrast formation is that electrons channel into a crystal lattice when the incident beam enters the lattice along the Bragg angle of a set of crystal planes. In this case, very few electrons are backscattered and the observed crystal appears dark. Every defect that disturbs the order of the lattice planes, in contrast, leads to backscattering and is visible in the ECC image as bright features in a dark grain. Dislocations, for example, appear as bright lines, stacking faults as bright areas with similar contrast features as those known from transmission electron microscopy (TEM).

Important for good imaging is a small beam convergence for good contrast, a small spot size for good resolution and a high beam current for low-noise images. Additionally, a sample holder with eucentric tilt and rotation capabilities allows to tilt the sample into well-controlled channelling conditions.

The technique can be used very similar to TEM, however with the serious advantage that a bulk sample is observed and not a thin foil. This enables observation of much larger samples, simplifies sample preparation, and it facilitates in-situ experiments like deformation, heating, or gas reaction observations.

In the presentation the basic principles of the technique are explained and illustrated. We then show examples from studies on hydrogen-embrittlement of high-strength steels, creep of superalloys, precipitation at dislocations in aluminium alloys and observation of dislocations in ductile magnesium alloys.

This session was chaired by Erik Offerman, Delft University of Technology, the Netherlands.