392 B. Langelier et al. a


4.5 5


3.5 4


2.5 3


1.5 2


0.5 1


0 Electropolishing b


10 20 30 40 50


-50 -40 -30 -20 -10 0


Electropolishing FIB 0.8 Matrix Segregation 0.6 0.4 0.2 0.0 0 25 50 75 Distance (nm) Matrix Segregation


Figure 12. 1D concentration profile of Ga taken along the APT analysis direction (i.e. parallel to the long axis of the sample) from a FIB-prepared Fe-Mn-N martensite sample, analyzed by VP.


FIB


of N loss to be promoted by FIB preparation, two possible means are considered here: (i) ejection of N by ion collisions and (ii) diffusion of N to the sample surface. It is well known that FIB milling can cause damage to


Figure 11. Comparison of N concentrations in Fe-Mn-N martensite analyzed by VP, prepared by electropolishing and FIB (for 3D atom maps see Figures 5 and 14b, respectively). a: N concentrations measured by 1NN method; (b) normalized N concentration difference from electropolishing result.


samples required FIB preparation, in order to achieve site- specific analysis, the possible influence of FIB processing on the tips is examined. To compare, the N content in Fe–Mn– N samples prepared by electropolishing is analyzed. As seen by the results in Figure 11, a small decrease in the N con- centration of both the matrix and segregation components is observed for a FIB prepared sample, when compared with a sample prepared by electropolishing. It is noted that the decrease appears more significant for the matrix component than for the segregation component. These results confirm that someNloss can be attributed to FIB sample preparation. As a possible mechanism forNloss in the sample during


processing, it is considered that as a relatively mobile inter- stitial element, some N in the near-surface regions of the APT needle may combine and exit the martensite as an N2 gas molecule. Such a denitriding process may become pos- sible in high vacuum, where the N concentration in the environment surrounding the tip is essentially 0. The greater proportion of N loss in the matrix phase, as shown in Figure 11, is consistent with such a mechanism. While the diffusion rate of N in the segregation regions would undoubtedly be higher, it is possible that the N in those regions is stabilized by lattice defects. Conversely, N in the matrix would be more highly supersaturated, and have a greater driving force for outward diffusion. For this process


the near-surface regions of a sample, resulting in atom dis- placements, vacancy generation, and the implantation of Ga ions (Giannuzzi & Stevie, 2005; Volkert & Minor, 2007). These effects may provide N atoms with sufficient energy to be ejected from the sample. As most damage is directly associated with the impacting Ga ions, high Ga concentra- tions in the APT sample may be a strong indicator of FIB damage. A profile of the Ga concentration down the long axis of a tip for a FIB-prepared sample is given in Figure 12. This figure shows that the maximum Ga concentration is extremely low, as even at the tip apex it only rises to ~0.5 at.%. This concentration decays rapidly with depth, until ~15–20 nm, after which the Ga content is only present at background levels. Assuming the FIB milling to be oper- ating in the linear cascade regime (Giannuzzi&Stevie, 2005), there is the possibility that the collision cascades caused by the incident Ga ions affect N at greater depths than those reached by the Ga ions themselves; however, in a relatively dense material such as steel, the depth of the collision cas- cade should not be far greater than the Ga ion penetration depth. An approximation of the collision depth in the Fe– Mn–N martensite samples is made by modeling with the stopping and range of ions in matter (SRIM) software package (Ziegler, 2013). The ion dose-normalized results are plotted in Figure 13 for 1× 104Ga ions at 10 keV (energy of the final FIB processing step) into Fe-1.4 Mn (at.%) and incident angles of 90 and 15° to the specimen surface, representing orientations of the tip apex and tip shank rela- tive to the FIB. The calculation involves several assumptions in modeling the material, such as disregarding channeling effects (Volkert & Minor, 2007), but it is noted that the dis- tribution profile results for Ga penetration depth at 90° matches fairly well with the APT measurement in Figure 12 (i.e., a maximum penetration of ~15nm down the sample axis). The results for the collision cascade depicting recoil


100 125 150


Normalized N Concentration Difference (%)


N Concentration (at.%)


Ga Concentration (at.%)


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