Segregation and N Measurement in Steel by APT 391
at 35 Da. With LP there are greater numbers of both molecular ions and single-charged species. At 50 pJ pulse energy, Fe2N2+ molecular ions are also present at 63 Da. There is also evidence for a greater number of H-containing molecular ions, as the 57Fe-containing peaks for FeN2+
and Fe+ appear abnormally high, and thus are more likely 56FeH-containing molecular ions. A higher number of single-charged ions is evident in the increased peak height for Fe+ and the appearance of FeN+. The higher number of single-charged and molecular ions
in LP mode also gives evidence towards the mechanism by which N measurement is decreased. Being an Fe-based alloy, the largest peak in the mass spectrum is for 56Fe2+,which appears at 28Da. If a largeamount ofNevaporates as an 14N2
+
single-charged molecular ion, it will also appear at 28Da. The relative size of the two overlapping peaks will completely obscure the N2 evaporates as N2
+ signal. It is unknown what fraction of N + in VP mode, but some insight can be taken
from the report of (Sha et al., 1992). Those researchers con- ducted VP mode experiments with an Fe sample enriched in the 15N isotope, so as to be able to de-convolute the peak at 28Da. Their results suggested mapping the 28Da peak solely as 56Fe2+ leads to a 26% lower measurement of N content. Based on the behavior of other ions when comparing VP mode with LP mode, it can safely be assumed that the fraction of N “lost” as N2
+ will be greater in LP mode.
increases surface migration of N towards high electric field regions (Gault et al., 2012a). This behavior can increase the tendency to evaporate as a molecular ion, possibly con- tributing to an increased fraction ofN2
It has also been reported that the use of LP mode +. Such molecular ions
may also postionize and dissociate, arriving as multiple ion hits to the detector. However, if these ions are of the same isotope (e.g., 14N) and arrive very close to each other on the detector, one ion may be lost to the pile-up effect (Thuvander et al., 2011; Miyamoto et al., 2012). To determine the influence that pulsing mode has on N
measurement across the ferrite–martensite interface, inter- face samples are prepared and analyzed using amethodology
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Voltage Pulse Laser Pulse (50 pJ)
Effects of Sample Preparation As the use of LP mode analysis cannot fully explain the apparent loss of N content in Fe–Mn–N martensite, it must be considered that some N loss may be caused by sample preparation. In this case, the lower than expected N content measured by APT accurately reflects a real reduction in N concentration of the sample. As all interface-containing
25 20 15 10 5 -20 -15 -10 -5 0 5 Distance from Interface (nm)
Figure 9. 1D concentration profiles of N across ferrite-martensite interfaces in Fe-Mn-N, obtained by VP and 50 pJ LP. Ferrite grains are on the left side of the plot.
10 15 20 0 Voltage Pulse
Laser Pulse 15 pJ
Laser Pulse 50 pJ
Figure 10. Calculated Fe ion detection loss in FIB-prepared Fe-Mn-N martensite analyzed by VP and by LP with 15 pJ and 50 pJ pulse energies.
identical to the data presented in the Fe–Mn–C and Fe–Mn– N Ferrite Growth Interfaces section, but with the data acquired in VP mode. The N concentration profiles across the transformation interface for both LP and VP data sets are presented in Figure 9. As clearly shown by the N profiles, the concentration peak of ~2.5 at.% at the interface is far higher in the VP sample than in the LP sample. However, the con- centrations in the grains appear approximately equal between VP and LP modes, at >0.5 at.%. Furthermore, the VP mode data also shows no significant difference in the N concentration between the ferrite and the martensite grains. These results indicate that while pulsing mode has a sig- nificant effect on N measurement by APT, this effect alone cannot explain the abnormally low N concentration measured for Fe–Mn–N martensite, compared with ferrite. It is also worth noting that the correction procedure for Fe
ion detection loss, as laid out in the work of Miyamoto et al. (2012) for Fe–C, indicates a far greater detection loss in VP mode than in LP mode (Fig. 10). This trend of increased detection loss is found linked to a greater fraction of multi-hit events, as has been noted by others (Marceau et al., 2013; Kitaguchi et al., 2014). As the data presented in this work is corrected for Fe detection loss, the higher N measurement in VP mode is
notsimplyanartifactofthiseffect.Inaddition, these results on Fe ion detection loss suggest that for steels in which N detection is not relevant, compositional accuracy in APT analysis, for noncorrected data, is improved using LP mode over VP mode. For analysis of N content however, VP mode analysis provide more accurate results, even though post- acquisition correction of the composition data will be required.
N Concentration (at.%)
Fe Ion Detection Loss (%)
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