Heterogeneous Chemical State of Carbon in Fe-C Binary Alloys

Abstract

In this study, we performed scanning transmission X-ray microscopy with a spatial resolution of approximately 50 nm to investigate the two-dimensional mapping of the chemical states of carbon in Fe–C alloy. The lamellar texture (pearlite) consisting of ferrite (α-Fe) and θ-Fe₃C with an interval of approximately 100 nm was identified by absorption from the carbon 1s→2p excitation in the X-ray absorption image. It was clearly observed that there exist more than two types of chemical states of carbon in θ-Fe₃C depending on the microtextures. The differences in chemical states were found between grained θ-Fe₃C and lamellar θ-Fe₃C in pearlite, which might have originated from the texture and morphology of the θ-Fe₃C. To consider the origins of the differences, we performed first-principles calculations by assuming the distortion and crystal anisotropy of the unit cell of the θ-Fe₃C structure. The results suggest that the anisotropy of the crystal structure of θ-Fe₃C and the lattice strain within lamellar θ-Fe₃C fail to explain the differences, and therefore, other factors should be considered.