Study on Quantitative Analysis of Carbon and Nitrogen in Stoichiometric θ-Fe₃C and γ′-Fe₄N by Atom Probe Tomography

Abstract

The quantitative analysis performance of carbon and nitrogen was investigated using stoichiometric θ-Fe3C (25 at% C) and γ′-Fe4N (~20 at% N) precipitates in pulsed voltage and pulsed laser atom probes. The dependencies of specimen temperature, pulse fraction, and laser pulse energy on the apparent concentrations of carbon and nitrogen were measured. Good coincidence with 25 at% carbon concentration in θ-Fe3C was obtained for the pulsed voltage atom probe by considering the mean number of carbon atoms per ion at 24 Da and the detection loss of iron, while better coincidence was obtained for the pulsed laser atom probe by considering only the mean number of carbon at 24 Da. On the other hand, a lack of nitrogen concentration in γ′-Fe4N was observed for the two atom probes. In particular, the pulsed laser atom probe showed a significant lack of nitrogen concentration. This implies that a large amount of 14N2+ was obscured by the main iron peak of 56Fe2+ at 28 Da in the mass-to-charge spectrum. Regarding preferential evaporation or retention, carbon in θ-Fe3C exhibited little of either, but nitrogen in γ′-Fe4N exhibited definite preferential retention. This result can be explained by the large difference in ionization energy between carbon and nitrogen.

Determination of the Chemical Compositions of Fine titanium Carbide and Niobium Carbide Precipitates in Isothermally Aged Ferritic Steel by Atom Probe Tomography Analysis

“…The experimentally observed abundance of 13 C is 0.0104, which is almost the same as the natural abundance of 13 C (0.0107). It indicates that there was little detection loss of C in TiC during this experiment using 3DAP with a delay-line detector (Takahashi et al, 2020). Note that the detected number of ions of 6.5 Da is somewhat smaller and that of 13 Da is larger than the natural abundance.…”

Quantitative Analysis of Nitrogen by Atom Probe Tomography Using Stoichiometric γ′-Fe4N Consisting of 15N Isotope

“…In the voltage mode, the apparent nitrogen concentrations of γ′ -Fe 4 N were confirmed to be lower than the stoichiometric composition. Although not shown here, even in the laser mode, the apparent nitrogen concentrations of γ′ -Fe 4 N were lower than the stoichiometric composition, and decreased with increasing laser pulse energy (Takahashi et al, 2020). These results suggest that the nitrogen concentrations decrease with the actual specimen temperature during field evaporation.…”

“…6), whereas the temperature dependence is similar. In the voltage mode, the detection loss of Fe 2+ was estimated based on the relative atomic ratio of Fe 2+ , and the corrected values are plotted in the figure (Miyamoto et al, 2012; Takahashi et al, 2020). In the laser mode, the detection loss could not be estimated because of the large tails of the Fe 2+ peaks; nevertheless, it is considered to be very small (Takahashi et al, 2017).…”

“…The authors previously performed a quantitative analysis of carbon using stoichiometric θ -Fe 3 C under various conditions with pulsed voltage and pulsed laser atom probes (Takahashi et al, 2011, 2020). The quantitation of carbon was realized primarily by considering the mean number of carbon atoms per ion at 24 Da and the detection loss of the matrix iron when using pulsed voltage atom probes.…”

“…The current authors also previously conducted a quantitative analysis of nitrogen in Braunite steel using stoichiometric γ ′-Fe 4 N precipitates under various analytical conditions using pulsed voltage and pulsed laser atom probes (Takahashi et al, 2020). Both atom probes presented large nitrogen deficiencies, which increased with the increasing specimen temperature and laser pulse energy.…”