Abstract
The composition of carbides in steel, measured by atom probe tomography, can be influenced by limitations in the ion detector system. When carbides are analyzed, many ions tend to field evaporate from the same region of the specimen during the same laser or voltage pulse. This results in a so-called multiple event, meaning that several ions impact the detector in close proximity both in time and space. Due to a finite detector dead-time not all ions can be detected, a phenomenon known as detector pile-up. The evaporation behavior of carbon is often different than the evaporation behavior of metals when analyzing alloy carbides, leading to preferential loss of carbon ions, and a measured carbon concentration below the expected value. This effect becomes stronger as the overall detection efficiency gets higher. Here, the detection efficiency was deliberately reduced by inserting a grid into the flight-path, which resulted in a higher and more correct carbon concentration, accompanied by an increase in the statistical uncertainty.
Highlights
Atom probe tomography (APT) is a useful technique for measuring local composition on a nanometer scale
The specimen is held at a high DC potential and field evaporation is initiated by either a voltage pulse or a laser pulse that serves as the start signal for the time-of-flight measurement
The influence of using a grid-local electrode (GLE) on the detector hit map was studied by analyzing a standard pre-sharpened microtip (PSM) sample
Summary
Atom probe tomography (APT) is a useful technique for measuring local composition on a nanometer scale. The method relies on identifying, by time-of-flight mass spectroscopy, and positioning individual atoms that have been ionized by field evaporation from the tip of a needle-shaped specimen (Miller & Forbes, 2014). The specimen is held at a high DC potential and field evaporation is initiated by either a voltage pulse or a laser pulse that serves as the start signal for the time-of-flight measurement. The detector consists of a channel plate, which turns the incoming ion to an electron burst, and a delay-line for positioning the electron burst. The position of the atom in the specimen is calculated from the impact position on the detector using a reconstruction algorithm, basically assuming a point-projection, with some further modifications (Gault et al, 2012).
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