Segregation and N Measurement in Steel by APT 393


0.0 1.0 2.0 3.0 4.0 5.0 6.0


0 5 10 Depth from Surface (nm)


Figure 13. Ga and lattice recoil atom depths (dose-normalized) for 10 keV Ga ions at incident angles of 90° and 15° into Fe-1.4Mn (at.%), calculated with SRIM(Ziegler, 2013).


lattice atoms (i.e., Fe or Mn knocked out of their lattice sites) indicate a depth that is a bit less than the Ga ion implanta- tion, and that it is further for 90° than for the 15° case. However, both of these results are on the same scale as the Ga ion implantation, i.e., affecting only the very near sample surface. Therefore, even when taken only as an approxima- tion of the ion interactions in the sample, the calculations suggest that the majority ofNin the APT needles is not likely to be affected by the collision cascade. The lower N mea- surement in the APT specimens thus cannot be solely attributed to direct interaction with the FIB. An alternative means by which FIB may promote N loss


is by sample heating. Such an increase in temperature at the APT sample tip may facilitate the escape of N from the sample by increasing the kinetics of outward N diffusion. During ion beam bombardment, the majority of the elastic energy interactions (i.e., from the initial kinetic energy of the ion) is converted to heat (Volkert&Minor, 2007). It has been argued that in most samples during milling, specimen heat- ing is largely confined to the collision cascade (Giannuzzi & Stevie, 2005); however, for the case of an APT sample, it is possible that some heating of the bulk needle still occurs. The shape of the APT sample has an extremely high aspect ratio at the tip apex, which yields a high amount of area exposed to the ion beam, per volume. In addition, the fine radius of the tip apex, required for analysis, by necessity limits the cross- sectional area bywhich heat may be removed from the tip via conduction. The possibility of specimen heating during FIB milling is thus a likely cause of some N loss, especially when considering that only modest increases in temperature could dramatically improve the rate of diffusion for the highly mobile N. Future experiments involving the sharpening of Fe–Mn–N martensite APT samples under cryogenic condi- tions would likely prove valuable in confirming or refuting this hypothesis. An additional factor involving the condition of the


specimen, unrelated to preparation, is the possible effect of outward diffusion and loss of N to vacuum during room temperature storage, before analysis. The bulk diffusivity of N in Fe at room temperature is relatively high at


b 1 h Fe


2 at.% N N


10.0 0.0 -10.0 -20.0


Segregation Matrix


-30.0 0 50 100 150 Distance (nm)


Figure 14. N concentrations in Fe-Mn-N martensite, analyzed by VP, measured by 1NN method. a: A comparison with vacuum storage time at room temperature for a FIB-prepared sample. b: A comparison of the same data, expressed by normalized difference in N concentration from an electropolished sample, plotted against analysis depth. The reconstructions for the three datasets are also shownin(b).


~2 ×10−20m2/s. This value puts the ffiffiffiffiffipDt diffusion distance


estimate for a 24-h period (typical for overnight pumping into the APT vacuum) at ~40 nm, which is close to the radius of the sharpened APT needle. This is also likely to be a conservative estimate, as it ignores the potential for fast diffusion of N along defects. To determine possible effects of denitriding to vacuum


at room temperature, a FIB-prepared specimen was analyzed in VP mode after a very quick pumping time in vacuum (~1 h). The same specimen was tested again after being stored for 1 week, then 2 weeks (3 weeks total) at room temperature in the APT vacuum (0.5–1×10−9Torr). The results are shown in Figure 14a, and appear counter-intuitive at first: N concentrations in both the matrix and segregation components measure higher for longer vacuum storage times. However, it must be noted that the specimen is FIB- prepared, and that each successive analysis probes deeper


200 250 300 (Electropolished)


90° Lattice Recoil 15° Lattice Recoil 90° Ga 15° Ga


0.0 3.0 6.0 9.0 12.0 15.0 18.0 21.0


15 20 a


4.5 5


3.5 4


2.5 3


1.5 2


0.5 1


0 1 h 1 week 2 weeks Time Stored in Vacuum at Room Temperature 1 week 2 weeks Matrix Segregation


1 x108 Lattice Atoms/cm3 / Atoms/cm2


1 x105 Ga Atoms/cm3 / Atoms/cm2


Normalized N Concentration Difference (%)


N Concentration (at.%)


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