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Quantifying deformation under indentations by TEM diffraction

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This talk will describe a quantitative diffraction technique in the transmission electron microscope (TEM) to measure crystal lattice rotations in materials deformed by low-load indentation. These rotations are equivalent to the accumulation of an excess of dislocations of a particular sign [1], known as geometrically necessary dislocations.

A variety of soft metals and hard ceramics have been deformed with low-load indentations using a Berchovich tip (nanoindentation) or a Knoop tip (microindentation). The crystals are then cut perpendicular to the indentation surface and thinned using a focused ion beam workstation so that the deformed region under the indentation can be studied in the TEM . The electron transparent foils are about 200 nm thick.

Convergent beam electron diffraction patterns are generated from spots with a 1 nm diameter in EF-STEM. The orientation of the local crystal lattice is measured from the Kikuchi lines in the diffraction patterns. These Kikuchi lines become blurred if the plastic or elastic strain gradients are high within the diffracting volume, but in-plane rotations can still be collected with selected area diffraction patterns.

By applying this technique to metals (Cu, W), ceramics (MgAl2O4, Al2O3, SiC), and a semi-conductor (GaN), it is possible to compare the size and intensity of rotations due to geometrically necessary dislocations. The results for the Cu and W can be compared to indent- induced lattice rotations measured with other techniques, such as EBSD and Synchrotron-source X-ray diffraction. New results have been found for the other materials, showing kinking in the hexagonal materials and rotations associated with the cracking around larger indents.

[1] J.F. Nye. Acta metall. 1, 153 (1953).

This talk is part of the Electron Microscopy Group Seminars series.

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