The microstructure and mechanical properties of alloy systems are known to be influenced by the size and distribution of sub-micron dispersoids through boundary pinning and Orowan strengthening. Explaining and accurately predicting the behaviour of these systems relies in part on the measurement of the volume fraction of the second phase precipitates. The transmission electron microscope (TEM) provides the necessary resolving power to measure the dispersoids of interest, and when coupled to Electron Energy Loss Spectroscopy (EELS) based filtering systems, the thickness of the observed volumes can be determined using the log-ratio technique.

A series of nominally AA5754 alloys with increasing Manganese content (0%, 0.2%, 0.5%, 0.8%) was used to evaluate the method. The data was collected using a Philips FEGSTEM equipped with a Gatan Imaging Filter (GIF). A series of images were collected from each of a number of contiguous fields in each specimen. These included an unfiltered image, an image collected from the zero-loss region of the transmitted spectrum, and a filtered image generating sufficient precipitate contrast to allow subsequent spatial measurement. Using a filtered image, the contribution of diffuse scattered electrons is minimized, thus enabling the analysis of thicker regions.