388 B. Langelier et al. a
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
-20 b
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
-20
Mn C
interaction between Mn and C. The highest concentration of C is found at the interface, due to the strong affinity of C to segregate there. The segregation peak for Mn thus comes as an effect of that high C concentration, which attracts Mn to it. Mn is also attracted to N solutes, but as the N interface segregation is significantly lower than for C, no similar Mn peak is created. The validity of this result, however, depends on the
-15 -10 -5 0 5 10 15 20
Mn N
-15
-10
-5
0
5 Distance from Interface (nm)
Figure 3. 1D Concentration profiles of Mn and N across the ferrite-martensite interfaces depicted in Figure 2 for (a)Fe-Mn-C and (b) Fe-Mn-N. Ferrite grains are on the left side in each plot.
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
0 0
50000 100000 150000 200000 250000 Total Atoms (Cumulative Total)
Figure 4. Ladder diagram for Mn along 1D profile across interfaces in Fe-Mn-N and Fe-Mn-C. The location of the interface, and the excessMn in Fe-Mn-C, are indicated.
that is absent in Fe–Mn–N, reflects the relative strengths of the binding energy between Mn and the interface and the binding energy betweenMnandCorNpresent at the interface. Thermo- dynamic models suggest that Mn is strongly attracted to both C and N in steel (Sozinov & Gavriljuk, 1999). This, along with the fact that the ferrite/austenite interface in Fe–C–Mn has a high carbon content whereas the interface in Fe–Mn–Nhas a relatively smaller N content, suggests that theMn segregation peak in the Fe–Mn–C alloy is due to an attractive solute–solute
The presence of a segregation peak for Mn in Fe–Mn–C,
Fe-Mn-N Fe-Mn-C
10
15
20
Excess Interface
measured segregation profiles in the Fe–Mn–Nsamples actually representing the martensite (former austenite)/ferrite interface. Tracking of the interface during sample preparation by SEM and subsequentTEM imaging of the sharpened tip suggests this to be the case, such as evidenced by the overlappingTEMimage and APT data in Figure 1. Additional details on how the transformation interface is tracked and located during specimen preparation and in the APT data are provided in the Supple- mentary Sections 2 and 3. The large ferrite grains (typically >100µm) also make it unlikely that boundaries in that phase could be mistaken for the transformation interface. However, APT measurement at the interfaces analyzed in Fe–Mn–N produce somewhat unexpected results when quantifying N. Whereas in the Fe–Mn–C sample, the C concentration clearly differentiates between the martensite and the ferrite phases, in the Fe–Mn–N samples, the N concentrations are not as well- matched to their predicted values. These are calculated as 0.04 at. % in ferrite and 1.69 at.% in martensite (i.e., in prior austenite) for the material near the transformation interface, as a result of denitriding (Zurob et al., 2013; Guo et al., 2015). Given thatAPT measurements are made very close to the interface, it is possible that small amounts ofNdiffuse from the martensite and enrich the ferrite phase, which may partially explain the differences between the theoretical and observed values, particularly the small difference for ferrite. However, this effect cannot account for the dramatically lower N content of the martensite phase. Various segregation features observed in the martensite phase (e.g., see Fig. 2b), may also alter local measurements of N qconcentration, but even considering this factor, the bulk N content of the martensite remains deficient. Therefore, analysis and specimen parameters which could affect the N content measured in the Fe–Mn–N alloy are investigated, as detailed in the following sections of this work. The study of these factors to explain the apparent loss of N in Fe–Mn–N martensite is necessary for interpreting the Mn segregation results, by confirming that the measurements were indeed performed across the targeted ferrite–martensite interphase interfaces.
Supplementary Section 3
Supplementary Section 3 can be found online. Please visit
journals.cambridge.org/jid_MAM
Characterizing N Measurement in Fe–Mn–N Martensite Determining which analysis and specimen parameters affect the quantification of N in Fe–Mn–N martensite is compli- cated by its heterogeneous distribution in the microstructure.
Mn Atoms (Cumulative Total)
Concentration (at.%)
Concentration (at.%)
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