Characterization of complex phase steel using backscattered electron images with controlled collection angles

For optimizing the microstructure of complex phase (CP) steels, characterization using scanning electron microscopy (SEM) is powerful because it allows observations from very low to high magnification. SEM specimens of steels are often etched in order to distinguish between the different phases by producing topographic information. This is however an 'indirect' method of characterization, which does not give precise structural information. We have developed a new technique for the selective imaging of the martensite (M) phase in a ferritic (F)-M complex phase steel. Backscattered electron (BSE) images at 15-20 kV were recorded by systematically changing the collection angle θ, where θ is measured from the specimen surface. When θ was 30-45°, strong channeling contrast was observed. For lower values of θ, it is the low energy loss electrons that mainly contribute to the contrast. As θ increases, the M phase exhibits brighter contrast. When θ exceeds 60°, a selective imaging of the M phase is achieved. This is not because martensite has a larger mean atomic number than ferrite, but is due to the fact that martensite has a high crystallographic defect density. Anomalously bright M contrast is due to multiple scattering of BSE due to the high density of planar defects and dislocations. Low angle BSE allows high resolution characterization of complex microstructures, while high angle BSE gives quantitative assessment of the distribution and the volume fraction of the martensite phase.