Interface Segregation and Nitrogen Measurement in Fe–Mn–N Steel by Atom Probe Tomography

Abstract

Improved understanding of the interactions between solutes and the austenite/ferrite interface can benefit modeling of ferrite growth during austenite decomposition, as the transformation kinetic is significantly affected by solutes that influence interface mobility. Solute-interface interactions dominate solute segregation at the interface in binary systems, but in multi-component alloys, solute-solute interactions may also affect segregation. In this study, interface segregation in Fe-Mn-N is examined and compared with Fe-Mn-C, to reveal the extent to which C affects the segregation of Mn. Atom probe tomography (APT) is well-suited to analyze solute concentrations across the interface, as this technique combines high spatial resolution and compositional sensitivity. Measurements of Mn show that segregation is only observed for Fe-Mn-C. This demonstrates that Mn segregation is primarily driven by an affinity for C, which also segregates to the interface. However, the measurement of N in steels by APT may be affected by a variety of experimental factors. Therefore, in verifying the Fe-Mn-N result, systematic examination is conducted on the influence of pulsing method (voltage versus laser), sample preparation (ion milling versus electropolishing), and vacuum storage on the measured N concentration. Both laser pulsing and focused ion beam sample preparation are observed to decrease the apparent N concentration.

Quantitative Analysis of Nitrogen by Atom Probe Tomography Using Stoichiometric γ′-Fe4N Consisting of 15N Isotope

“…The origin of the missing nitrogen is considered to be the occurrence of neutral nitrogen (N) or nitrogen gas molecules (N 2 ) in field evaporation, which cannot be detected as signals by atom probe instruments (Langelier et al, 2017). In the Fe–N phase system, the N 2 gas phase preferably emerges under thermodynamic conditions.…”

“…They predicted that approximately 30% of nitrogen is obscured by the dominant peak of 56 Fe 2+ . Recently, Langelier et al (2017) analyzed Fe–Mn–N martensitic steels and found that the apparent concentration of solute nitrogen decreased when measured using pulsed laser atom probes. They predicted the formation of nitrogen gas molecules (N 2 ) and an outward diffusion of nitrogen during the fabrication of the focused ion beam (FIB) specimen, in addition to pile-up and overlapping effects.…”

Effects of Alloying Element on Bainitic Transformation in Fe-0.3N Alloy

“…Regarding the effect of alloying elements on bainitic transformation behavior, there are wide variety of reports on elements such as Mn 7)-9) , Si 8) -10) , and Mo 11), 12) , main alloying elements, in the current Fe-C based TRIP steels. Although there are several reports on AF formation during denitriding experiment in Fe-Mn-N alloys 13), 14) , reports on the bainitic transformation are limited to Fe-N binary alloys with at high nitrogen concentrations 15)-17) . Therefore, the effect of alloying element additions on the bainitic transformation behavior of low-nitrogen steels comparable to commercial Fe-C TRIP steels, which are practical steels, is still unclear.…”

Modelling of the diffusional austenite-ferrite transformation

“…High-precision dilatometry work on the massive transformation has led to better estimates of the interface mobility [6][7][8]. Improved precision and wider availability of electron probe microanalysis (EPMA) [9,10] and atom probe tomography (APT) [11][12][13] have provided better estimates of interfacial conditions (carbon content) and solute segregation at the ferrite/austenite interface. Decarburization [14][15][16] and cyclic transformation [17][18][19] experiments have delivered accurate growth kinetic measurements by minimising the complications from nucleation.…”