ISM 2022 (Microscopy)

M7C3 – A MISLEADING CARBIDE

author.DisplayName 1 author.DisplayName 2 author.DisplayName 1
1Faculty of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa, Israel
2Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA

The M7C3-type carbide plays an important role in the design of wear and corrosion resistant alloys in which Fe and Cr are major alloying elements. However, despite the large body of scientific literature dealing with the M7C3 carbide, its atomic structure, and hence its functional, kinetic and thermodynamic properties are still under debate. While early studies determined that the atomic structure can be either trigonal or hexagonal, later publications indicated that the structure was more likely to be orthorhombic. Even today, 8 decades since its discovery, publications claiming that the M7C3 carbide is hexagonal/orthorhombic “beyond doubt” are equally frequent. First principles calculations, aiming at settling this disagreement in the experimental literature, rely on the atomic positions in the previously suggested structures and are rather inconclusive. To date, no hard evidence has been presented to support either claim in an indisputable manner.

Based on evidence from several complementing characterization techniques, such as EBSD, TEM diffractions and high-resolution S/TEM lattice images, gathered from numerous zone-axes of the (Cr,Fe)7C3 phase found in the AlCrFe2Ni2 alloy, we show that the only possible structure is hexagonal, albeit not the one previously suggested. Furthermore, we demonstrate the co-existence of two crystallographically identical variants of the revealed atomic structure, which we believe were mistakenly identified as stacking faults in previous studies. The spatial arrangement of these variants obeys a high order periodicity that can potentially "break" the 6-fold symmetry and mimic an orthorhombic unit cell.