Atom column analysis of (Fe,Cr)₂B phase in high B containing ferritic steel

This paper depicts nuances of atomistic level characterization of (Fe,Cr)(2)B crystal in a high B containing P91 steel using probe aberration-corrected Scanning Transmission Electron Microscope (STEM) based techniques. Substitution of Cr atoms in Fe atom positions resulted in intensity variation of atomic columns of a STEM-High Angle Annular Dark Field (HAADF) image. Image simulation has been used to understand this variation. Relative estimation of Cr atom substitution in Fe atom columns has also been quantified through the analysis of changes in peak profile intensity ratio of STEM-HAADF image in comparison with simulated images. Imaging light atoms, such as B, has recently been made possible with the advent of the Differential Phase Contrast (DPC) detectors. In this work, STEM based Integrated DPC (iDPC) has been used to obtain a very detailed image of the (Fe,Cr)(2)B crystal showing projection of the Fe/Cr and B atom columns along [100]. However, the observed delocalization of B atom columns was intriguing. Computational analysis using density functional theory (DFT) modelling was employed to understand the effect of Cr substitution on BB bond length with actually little or no effect. Further theoretical calculations confirmed that the energy imparted by the incident electron during imaging was responsible for the B atom delocalization.