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requirment of specimen preparation

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The goal of specimen preparation for atom probe tomography is to produce a sharp needle that contains the region of interest in the analyzable apex region of the specimen. This specimen should have a circular cross section with no protrusions, an end diameter of less than ~100 nm, and a taper angle of 1–10°. Ideally, the specimen should also be free of surface films or contamination. However, field evaporation at the start of the experiment can normally remove these surface layers and produce a pristine sample. These specimen parameters are important to acquire high quality data and also minimize the introduction of artifacts during the reconstruction of the atom positions. The thickness of the specimen in the apex region is amenable to examination in standard transmission electron microscopes so that it is possible to identify and characterize the features in the specimen prior to analysis by atom probe tomography. The correct choice of analysis conditions in the three-dimensional atom probe is critical for steels, as steels normally contain several elements with different evaporation fields (Miller et al., 1996a). The standing and pulse voltages (pulse fraction of 0.2), specimen temperature (typically 40–60 K), pulse repetition rate (200 kHz), and evaporation/collection rate (1–2%) should be selected to prevent preferential field evaporation or retention of all the elements in the steel. The new generation of atom probes with high pulse repetition rates minimizes the time spent at the standing voltage, and therefore helps to minimize preferential evaporation between the field evaporation pulses. The selection of specimen temperature is particularly important in laser-pulsed mode, where surface migration of carbon may occur even at cryogenic temperatures, and the local heating produced from the laser pulse may significantly affect the microstructure. Therefore, the lowest practical base temperature, typically less than 30 K, and minimum laser power that are consistent with no preferential field evaporation or retention of all elements in the steel should be used in laser-pulsed mode. It should be noted that holding a specimen at these cryogenic temperatures may transform any retained austenite present into martensite, thereby altering the microstructure, especially when the same specimen is resharpened for additional analyses. Specimen analysis procedures should also take into account the possibility of natural aging that may occur in some steels.

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