Dark-field electron spectroscopic imaging of aged microstructures in an aluminium lithium base alloy

Abstract

The examination of microstructures make very high demands on the imaging quality and, therefore, on the instrumentation. In Al-Li base alloys it is of great interest to determine parameters such as size, distribution, morphology and coherency of precipitate phases as they dictate their mechanical behavior. In order to reveal morphological features with high quality the electron spectroscopic imaging (ESI) in dark field mode has shown to be quite a powerful technique.The ESI technique in the TEM is based on the possibility that accelerated electrons can be elastic and inelastically scattered by the sample atoms, as recently reviewed. The electron distribution in the transmitted and diffracted beams through a crystalline sample is such that both energy loss and elastic electrons will enter a typical objective aperture and thus contribute to both bright field and dark field images. The effect of the polyenergetic electrons is that the image is affected by chromatic aberration of the objective lens. In CTEM’s this effect is enhanced the lower the accelerating voltage and the thicker the sample.