386 B. Langelier et al.


Table 1. Solute Compositions of Experimental Alloys Before Decarburizing or Denitriding, Given in at% (wt%).


Alloy Designation Mn Fe–Mn–C


Fe–Mn–N 1.42 (1.43)


N –


1.43 (1.43) 2.08 (0.53) C Fe


2.73 (0.60) Balance –


APT specimens were prepared using either of two


methods: focused ion beam (FIB) liftout or electropolishing. FIB-prepared specimens were made by a standard liftout procedure (Thompson et al., 2007; Felfer et al., 2011) using a Zeiss NVision 40 (Carl Zeiss, Jena, Germany) dual-beam FIB scanning electron microscope (SEM). Specimens were protected by W deposition and extraction. As the transfor- mation interface typically runs perpendicular to the sample surface, liftouts would be rotated ~90°, then mounted to pre-sharpened Si or W posts. This step would orient the transformation interface approximately normal to the final tip axis, as shown by the Fe–Mn–N specimen before annular milling, depicted in Figure 1b. Specimens were annularmilled with a 30 kVGa ion beam, at a probe currents of 150, down to 40pA. Final sharpening of the needles was performed using a defocused Ga ion beam at 10 kV and 80pA. Additional details on the FIB liftout and sample rotation procedures, and how the transformation interface is tracked during sample preparation, are provided in the Supplementary Section 2. When site-specific analysiswas not required (e.g., analysis


of interfaces), specimen preparation could be performed by electropolishing. This was conducted by first mechanically grinding sections of the alloy into pillars, ~0.50×0.50×10mm in dimensions. These pillars were subsequentlymade into fine nano-tipped needles by electropolishing in two stages, similar


a


to the procedures described by Miller (2000) and Gault et al. (2012b). The first stage was done by repeatedly dipping one end of the pillar intro a solution of 25% perchloric acid (70%) in glacial acetic acid at 20V-DC until a point was formed. The point was then refined by the second stage of electro- polishing, which used an electrolyte of 2% perchloric acid in 2-butoxyethanol and a potential of 15–20 V-DC. BeforeAPTexamination, several specimens were imaged by


transmission electron microscopy (TEM), using a JEOL 2010F microscope(JEOL Ltd.,Tokyo,Japan)at200kV.Such animage for a FIB-prepared Fe–Mn–N sample is shown in Figure 1c. APT testing was performed using a CAMECA local


electrode atom probe (LEAP, CAMECA Instruments Inc., Madison, WI, USA) 4000X HR. Testing was performed at a stage temperature of ~43K and under ultrahigh vacuum (~3.0×10−11Torr). Evaporation was induced either by laser pulsing (LP) or voltage pulsing (VP).WithVP, a pulse fraction of 20% and a pulse rate of 200kHz was used. With LP, a UV (λ=355nm) laser was used at 50 pJ pulse energy (unless otherwise specified) and a 250kHz pulse rate. Evaporation was set to a target rate of 0.005 ions/pulse (0.5%).AllAPT data was reconstructed using the Integrated Visualization and Analysis Software package (IVAS) v3.6.12 and established algorithms.


RESULTS AND DISCUSSION


Fe–Mn–C and Fe–Mn–N Ferrite Growth Interfaces APT analysis of Fe–Mn–C and Fe–Mn–N ferrite growth interfaces, successfully conducted on several samples, yields consistent results. In targeting the ferrite growth interface to be within the APT analysis volume, site-specific sample preparation via FIB was required for all interface samples.


b FM


F M


100 µm c F M


500 nm d


50 nm


50 nm


Figure 1. SEM micrographs of Fe-Mn-N alloy, showing (a) ferrite (F) and martensite (M) regions in the denitrided sample and (b) a cross-section from a FIB liftout, which contains the interface. c: Bright-field TEM micrograph of a typi- cal FIB-sharpened APT needle, containing the ferrite-martensite interface, and (d) the same TEM micrograph including an overlay of the reconstructed APT volume (Fe atom density map). The interface is indicated by arrows in (b-d).


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