Microsc. Microanal. 23, 385–395, 2017 doi:10.1017/S1431927617000150
© MICROSCOPY SOCIETY OF AMERICA 2017
Interface Segregation and Nitrogen Measurement in Fe–Mn–N Steel by Atom Probe Tomography
Brian Langelier,1,* Hugo P. Van Landeghem,2 Gianluigi A. Botton,1 and Hatem S. Zurob2
1Department of Materials Science and Engineering, McMaster University, 1280 Main St. W., Hamilton, ON, Canada 2SIMaP, UMR 5622, Grenoble INP – CNRS – UGA, 1130 rue de la piscine, BP75, F-38420 St Martin d’Hères, France
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–Nis 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.
Key words: atom probe tomography, interface segregation, nitrogen quantification, martensite, steel INTRODUCTION
The processing for many modern steels requires knowledge of the austenite to ferrite phase transformation. To model this transformation, the factors affecting its thermodynamics and kinetics must be properly understood. A key parameter influencing the reaction kinetics is the interface mobility, which is strongly affected by solute drag (Cahn, 1962; Hillert & Sundman, 1976; Purdy & Brechet, 1995; Gouné et al., 2015). The interaction between a solute and the interface can be described by a binding energy, which is related to the amount of solute segregation at the transfor- mation interface. However, in multi-component alloys (i.e., all steels), the segregation behavior of a given solute may be strongly influenced by the presence of other solutes at the interface. As part of a broader study on solute segregation to
transformation interfaces (Van Landeghem et al., 2016), this work examines the segregation of the substitutional element Mn to austenite/ferrite interfaces. To examine Mn segrega- tion in the most straightforward case, its segregation is measured from a transformation interface of known velocity in a model Fe–Mn–C ternary alloy. However, to discern the effects of possible Mn–C interactions, a similarly-prepared Fe–Mn–N alloy is also examined. Atom probe tomography (APT) is an excellent method by which to study such segregation, because of its high spatial
*Corresponding author.
langelb@mcmaster.ca Received June 29, 2016; accepted January 15, 2017
resolution and elemental sensitivity. Nevertheless, quantifica- tion of interstitial elements, particularly N, can be problematic (Sha et al., 1992; Gault et al., 2012a). The focus of this work therefore lies in two areas (i) APT analysis of interface segre- gation in Fe–Mn–Cand Fe–Mn–N, and (ii) evaluation of analysis and specimen parameters affecting N quantification. By systematic study of N quantification in Fe–Mn–N, the validity of observations made on the Fe–Mn–N system and used for comparison with Fe–Mn–C, can be determined.
MATERIALS ANDMETHODS
The Fe–Mn–N and Fe–Mn–C alloys examined in this work were prepared, respectively, by nitriding and carburizing the same high purity binary Fe–Mn alloy. The compositions are reported in Table 1, as verified by inductively coupled plasma optical emission spectroscopy and combustion analysis. Austenite–ferrite transformation interfaces were created by decarburizing or denitriding using wet hydrogen at 750°C. During quenching to room temperature, the austenite transforms to martensite. Polished cross-section of an Fe– Mn–N sample is shown in Figure 1a, with additional images in Supplementary Sections 1 and 2. The full procedures for these processes are given by Guo et al. (2015).
Supplementary Sections 1 and 2
Supplementary sections 1 and 2 can be found online. Please visit
journals.cambridge.org/jid_MAM
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