Atom-probe tomography and transmission electron microscopy of the kamacite-taenite interface in the fast-cooled Bristol IVA iron meteorite

We report the first combined atom-probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite-tetrataenite (K-T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K-T interface with focused ion beam scanning electron microscopy (FIB-SEM) and then studied using TEM followed by APT. Near the K-T interface, we found 3.8 +/- 0.5 wt% Ni in kamacite and 53.4 +/- 0.5 wt% Ni in tetrataenite. High-Ni precipitate regions of the cloudy zone (CZ) have 50.4 +/- 0.8 wt% Ni. A region near the CZ and martensite interface has <10nm sized Ni-rich precipitates with 38.4 +/- 0.7 wt% Ni present within a low-Ni matrix having 25.5 +/- 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K-T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3-D of the K-T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast-cooled iron meteorite below 400 degrees C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast-cooled iron meteorites.