Segregation and N Measurement in Steel by APT 387 a FM Mn,C Mn,N b
2 at. % C C
F
M
1 at. % N N
Mn
Mn
Fe
Fe
50 nm
50 nm
Figure 2. Typical APT datasets obtained in LP mode for (a) Fe-Mn-C and (b) Fe-Mn-N showing atom maps for C, Mn, and Fe, as well as C and N iso-concentration surfaces. Ferrite (F) and martensite (M) grains are labelled, with an arrow indicating the direction for the concentration profiles in Figure 3.
Data acquisition was performed in LP mode. Typical APT reconstructed volumes of Fe–Mn–Cand Fe–Mn–Nare shown in Figures 2a and 2b, respectively. These samples clearly show an interface between the grains of ferrite and martensite (i.e., austenite at high temperature). Fromthe atom maps, it appears that the interstitial elements C and N are strongly segregated to the interfaces in their respective alloys. Mn also appears to be present in higher density at the interface; however, the appearance of solute segregation in three-dimensional (3D) atom maps may be an artifact resulting fromoverall density variations at defects in the APT analysis, particularly as the 3D atom map for the solvent element Fe also appears to show a variation in density at the interface. To confirm the presence of interface segregation, the concentration and/or interfacial excessmust be quantified. One-dimensional concentration profiles normal to the
interfaces are generated for the solute elements, with results for the data sets shown in Figure 2, given in Figure 3. From the Fe–Mn–C profile, clear segregation can be observed for both C and Mn. C exhibits a peak concentration at the
interface of ~6 at.%, but lower concentrations of >0.5 at.% in the ferrite, and 2–3 at.% in the martensite, in the near- interface region. The Mn concentration is equal in both the ferrite and martensite, measured at ~1.5 at.%, but increases to a maximum of ~2.5 at.% at the interface. In comparing the Fe–Mn–C concentration profiles of
Figure 3a with those of the Fe–Mn–Nsample from Figure 3b, the behavior of the solute elements differs significantly between the two alloys. In the Fe–Mn–N sample, there is only a very small amount of N segregation observable. The concentration in each grain to either side of the interface is similar. In ferrite and martensite, the N content is <0.5 at.%. At the interface, the N content increases to a small peak at only ~1.0 at.%. The concentration profile for Mn appears completely flat, with a measured concentration in both ferrite and martensite, and at the interface of ~1.5 at.%. The varying levels of Mn interfacial excess in each alloy
can also be visualized by the ladder diagram in Figure 4. In that figure, the segregation ofMnin the Fe–Mn–Calloy is clear,while no similar Mn segregation is visible for the Fe–Mn–N alloy.
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