Visualization of three different phases in a multiphase steel by scanning electron microscopy at 1 eV landing energy

In this study, we investigated an observation technique by super low energy scanning electron microscopy (SLESEM) at below 5 eV and its contrast mechanism for analyzing complex microstructures of a multiphase steel consisting of ferrite, martensite and austenite. With SLESEM at 1 eV, the three phases were observed as different brightness levels, ferrite as the darkest contrast, martensite as the second brightest and austenite as the brightest. These contrasts disappeared at 2 eV or higher. Similar contrasts and phenomena were also observed in the results of low energy electron microscopy (LEEM). According to the energy dependences of the LEEM intensities of the three phases, the threshold energies of the transition from electron reflection to surface impact were determined to be 0.00 eV, 0.15 eV and 0.39 eV for ferrite, martensite and austenite, respectively. These differences in thresholds indicate that the potentials on the surfaces of each phase are different, which is considered to result in the different brightness of each phase. This potential differences are probably due to the contact potentials generated when phases with different work functions contact each other. Although the sample is covered by a thin native oxide film (several nm thickness), the potentials can affect the incident electrons through the oxide film.