High resolution imaging and analysis of grain boundaries in steel using a field emission auger microprobe

A standard cast low alloy steel sample, with known levels of grain boundary phosphorus and tin segregation, has been analysed after impact-fracture in a dedicated Auger microprobe. The instrument utilised a thermally assisted, Schottky diode field emission electron source. The examination illustrates the extra information that can be obtained by the high spatial resolution, in both secondary electron imaging and Auger electron spectroscopy and imaging, offered by this kind of source, when compared with more conventional tungsten and lanthanum hexaboride filaments. Cavities, with diameters of less than 1 μm, are observed on a proportion of the grain surfaces. Where cavitation occurs, tin is found to be present within the cavities and phosphorus on the areas of smooth grain boundary surface between them. During previous examinations of the same material, in a number of different laboratories, these features had not been resolved. In addition, elemental Auger mapping and analysis with a spatial resolution down to a few tens of nanometres are demonstrated by elemental imaging and spectroscopy of individual boron nitride particles on the cavitated grain boundary surfaces. It is suggested that, in the material examined, localised creep cavitation occurs during heat treatment. This is driven by residual stresses from the initial solution treatment and water quenching.