Grain size determination in nano-scale polycrystalline aggregates by precession illumination-hollow cone dark field imaging in the transmission electron microscope

Precession illumination hollow cone dark field (PI-HCDF) transmission electron microscopy (TEM) provides high contrast multi-beam dark field images, which are suitable for effective and robust grain size measurements in nano-scale polycrystalline aggregates. Precession illumination with slightly converged electron beam probes and precession angles up to 3° has been produced using a computer-controlled system using a JEOL JEM 2000FX TEM instrument. Theoretical and practical aspects of the experimental technique are discussed using example precession illumination hollow cone diffraction patterns from single crystalline NiAl and the importance of selecting the appropriate precession angle for PI-HCDF image formation and interpretation is described. Results obtained for precession illumination are compared with those of conventional parallel beam illumination experiments. Nanocrystalline Al has been used to evaluate the influence of the precession angle on PI-HCDF image contrast with a focus on grain size analysis. PI-HCDF imaging has been applied for grain size measurements in regions of a nanocrystalline Al thin film adjacent to the edge of a pulsed laser melted and rapidly solidified region and determined the dimensions of a heat-affected zone.

► New TEM method for grain size measurements combines TEM resolution with obtainability of statistically significant data sets.
► We use precession illumination to produce time precession illumination hollow cone diffraction patterns PI-HCDP.
► Contrast in dark field images (PI-HCDF) formed from PI-HCDP is easy to interpret as dynamical effects are reduced.
► PI-HCDFs use several time-averaged g-rings simultaneously and contain more information than conventional DF-images.
► Easy contrast interpretation and less dark field images required, allows fast, robust and reliable grain size determination.