Quantitative Composition Analysis of Spherical Cementite by Three-dimensional Atom Probe

Abstract

To examine the quantitative performance of three-dimensional atom probe (3D-AP) in steel materials, composition analysis of spherical cementite (Fe₃C) formed in an annealed pearlitic steel wire was conducted. It was visualized in three dimensions that manganese and chromium atoms were concentrated in cementite, while silicon atoms were concentrated in ferrite. Phosphor atoms were segregated at the cementite-ferrite interface. By taking all carbon molecular ions into consideration, the carbon concentration in cementite was in the range of 25-27 at%, which approximately coincided with the stoichiometric composition of 25 at%. On the other hand, the iron concentration in cementite was in the range of 70-73 at%, which was a few atomic % lower than 75 at%. The reduction of the iron concentration is caused by the substitution of manganese, chromium and silicon atoms for iron atoms in cementite. It was shown that the 3D-AP enables the quantitative composition analysis with high accuracy. Inhomogeneous distribution of manganese and chromium in cementite was observed, and it was explained by the change in the partition coefficient depending on the interface mobility of spherical cementite growth.