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Atom Probe Sample Preparation


Analytical transmission electron microscopy . (S)TEM imaging together with energy dispersive X-ray spectroscopy (EDS) was conducted using an FEI Tecnai F20ST fi eld emission (S)TEM equipped with an EDAX Apollo XLT silicon-driſt EDS system and a STEM-HAADF (high-angle annular dark-fi eld) detector. For X-ray quantifi cation, a nanotip prepared from the North Chile meteorite (Field Museum specimen ME 2595) was used for measuring the k-factors [ 23 ] of Fe and Ni using the Cliff -Lorimer ratio method [ 24 ] and the parameterless method [ 25 ]. North Chile meteorite has homogenous kamacite composition and is therefore commonly used as a standard [ 26 ]. T e k NiFe value determined was 1.3±0.03 at 200 kV. To improve the precision, total counts of > 100,000 were obtained for the Fe peak and total counts of > 10,000 were obtained for minor elements (Ni and Co) in measuring the North Chile meteorite standard and the kamacite nanotips. Background under the peaks was subtracted manually using the routine within the TEAM TM EDS analysis soſt ware as explained in [ 27 ]. Atom Probe Tomography . A Cameca LEAP 4000X Si at Northwestern University Center for Atom-Probe Tomography (NUCAPT) with a straight fl ight path of 90 mm was used for all APT observations. All the APT analyses were conducted in laser mode with a pulse frequency of 250–500 Hz, using UV laser pulse energies ranging from 20–33 pJ with maximum DC voltages of 13 kV. T e sample base temperature was kept at 30–35 K.


Results


Alloy standards . T e APT atomic reconstructions of the nanotips prepared from the IARM 341A and CRM 182C steel alloy standards show few carbide inclusions and a Ni-rich region near the apex of the nanotip, respectively ( Figure 4 ). T e measured elemental concentrations from the APT mass spectra of IARM 341A and CRM 182C specimens did not match with the reference compositions measured using other analytical techniques (for example, SEM-WDS and optical spectroscopy data from ARMI). T is is because the volumes sampled by the APT nanotips were smaller than the length scales of compositional heterogeneities within typical steel samples. For detailed information about the steel samples and other reference specimens refer to [ 14 ]. Bristol meteorite . Figure 5a shows the TEM image of a nanotip (Tip B) prepared from the kamacite region of the Bristol iron meteorite. Kamacite was identifi ed fi rst from its TEM selected area electron diff raction pattern (α -iron; ferrite; bcc; Figure 5a insert) and also from its composition measured using STEM-EDS X-ray elemental analysis: Fe=93.5±0.2 wt%, Ni=6.5±1.4 wt%. Another nanotip (Tip C) prepared from a diff erent kamacite region had a composition of: Fe=94.1±0.2 wt%, Ni=5.9±1.4 wt%. T e average Ni content in kamacite varies from ~5 wt% to ~10 wt% and depends on the cooling rates of the meteorite [ 28 ]. Within a kamacite crystal the Ni concentration is nearly homogeneous, but the Ni content decreases near the


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kamacite (ferrite)-taenite (austenite) interface because of the slow diff usion at low temperatures (< 350°C) during the formation of the Widmanstätten pattern [ 28 ]. Although, kamacite has on average <1 wt% Co, the concentration of Co in our nanotips could not be measured by STEM-EDS because of a small Co systems peak from the objective lens pole piece. T e APT reconstruction and elemental data revealed that the kamacite phase contained about 94 wt% Fe as expected for this phase, and all elements (Fe, Ni, Co, P, Cr, and Mn) were uniformly distributed throughout the nanotip ( Figures 5 b, 6 ). T e composition measured from the APT mass spectrum of Tip B was: Fe=93.8±0.4 wt%, Ni=5.9±0.5 wt%, Co=0.27±0.05


Figure 4 : (a) APT reconstruction of the nanotip prepared from the steel standard IARM 341A. (b) APT reconstruction of the nanotip prepared from the steel standard CRM 182C. Each sphere represents a single atom that was detected. Only 5% of the detected atoms are shown for better illustration of the element distributions.


Figure 5 : (a) Bright-fi eld TEM image of a nanotip (Tip B) prepared from a kamacite region in the Bristol IVA iron meteorite. The region that was analyzed by APT is shown by a dashed box. Inset shows the selected area electron diffraction pattern acquired from the nanotip. (b) 3D APT reconstruction of the nanotip where each sphere represents a single atom. Only 5% of the detected atoms are shown for clarity. The APT reconstruction does not match with the shape of the studied region in the TEM image because some of the fi eld-evaporated atoms were not captured by the detector during the APT analysis.


www.microscopy-today.com • 2018 March


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