An atom probe study of the anomalous field evaporation of alloys containing silicon

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Abstract
In some atom probe microanalytical investigations of homogeneous alloys, the apparent concentrations of certain elements have been found to differ from the expected values. Both the time of flight atom probe and the imaging atom probe have been used to study this phenomenon in detail, in the case of an iron 3 wt.% silicon transformer steel. The apparent concentration of silicon is found to depend on pulse fraction, time between pulses, specimen temperature, and crystallographic area. Low pulse fractions and long periods between pulses both give apparent increases in the silicon level. When the probe aperture is positioned inside the central ring of a (110) pole, the silicon atoms are found to be retained until the end of the evaporation of that layer, and the overall apparent level of silicon is approximately three times higher than expected. These results suggest that certain elements may be preferentially removed from an alloy by dc field evaporation during the interval between the H. T. pulses if incorrect experimental conditions are used. For quantitative atom probe analysis of silicon-containing alloys, it appears that a pulse fraction of at least 15% is required and that analysis inside the central ring of low index poles should be avoided.

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