390 B. Langelier et al.


a correction factor, k is applied to the segregation region density. The magnitude of this factor is calculated to achieve consistency between the component concentrations and the experimentally- measured bulk concentration of theAPTdata set, as determined by equation (6). The magnitude of k is found to typically range from 1.35 to 1.85. This finally leads to the result:


X0 =ð1 - fMÞ


Cm ρ + fM


Cs kρ :


(6) The outcome of the methodology is the atomic


fractions of N for both the segregation and matrix compo- nents (Xs and Xm, respectively). It should be noted that the compositions of the segregated phase may exhibit natural variation in N composition, but are being modeled here with a single peak to the distribution of their 1NN distances. Therefore, the final value obtained, Xs, represents that phase only by a single average concentration. Finally, the values of the N concentrations are corrected for Fe ion detection loss, as detailed by Miyamoto et al. (2012), using the procedure developed for C in that work. The potential for 14N2


+ overlap


with 56Fe2+ at 28Da interfering with this correction procedure has been evaluated, and is found to be negligible (see Supplementary Section 4). These corrected values are used in the following sub-sections to evaluate the influence of specimen and analysis parameters on N concentration.


Supplementary Section 4


Supplementary Section 4 can be found online. Please visit journals.cambridge.org/jid_MAM


Effect of Analysis Mode The mode of acquisition used for APT data, either VP or LP, can have a significant effect on the field at the tip. This may result in differing evaporation behavior for different elements, such as N. To examine this effect, Fe–Mn–N bulk martensite samples were analyzed by APT under VP mode as well as LP mode with two different laser pulse energies: 15 and 50 pJ. These tests were performed on the same FIB- prepared sample. While the composition of N in this sample originally deviated from the electropolished case (described later in Effects of Sample Preparation section) experiments comparing these analysis modes were conducted after con- sistent compositions were obtained in VP mode. Results for the N concentrations in these data sets, as determined by the procedure described in Measuring N Content by Nearest- Neighbor Analysis section, are given in Figure 7a. It is clear from this data that by operating in LP mode, the amount of N measured by APT significantly decreases. This effect is exacerbated by increasing the laser energy, as the 50 pJ results show far lower N concentrations than the 15 pJ results. To better clarify the differences in measured N con- tent, the changes in concentration from the VP mode result, normalized between the matrix and segregation components and expressed as percentages, are shown in Figure 7b.


a


3.5 4


2.5 3


1.5 2


0.5 1


0 Voltage Pulse b 100


20 40 60 80


-100 -80 -60 -40 -20 0


Voltage Pulse Matrix Segregation


Laser Pulse 15 pJ


Laser Pulse 50 pJ


Laser Pulse 15 pJ


Laser Pulse 50 pJ


Matrix Segregation


Figure 7. Comparison of the N concentrations in FIB-prepared Fe-Mn-N martensite analyzed by VP and by LP with 15 pJ and 50 pJ pulse energies. a: N concentrations measured by 1NN method; (b) normalized N concentration differences from voltage pulsing result.


a


H+ H2+


N+


Fe2+ Mn2+


FeN2+


Fe+ Mn+


Voltage Pulse


b


Laser Pulse 50 pJ


Fe2N2+ FeN+


010


20


30


40


50 Mass / Charge (Da)


Figure 8. Typical mass spectra for Fe-Mn-N martensite, analyzed in (a) VP mode and (b) 50 pJ LP mode. Major ions are labelled, with unlabelled peaks corresponding to contaminant ions.


These results show that the same relative decrease in N content is observed for both the matrix and segregation components at 15 pJ, but at 50 pJ, the segregation component exhibits a greater decrease. A comparison of the mass spectra obtained by VP and


LP modes is given in Figure 8. With VP, the only major peaks containing N are the N+ peaks, with the major isotope at 14 Da, and FeN2+ molecular ions, with the major isotope


60


70


80


Normalized Counts


Normalized Counts


Normalized N Concentration Difference (%)


N Concentration (at.%)


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