M7C3: The story of a misunderstood carbide

Since the discovery of the M7C3-type carbide in 1935, it has been the subject of many studies owing to it extensive usage in wear sensitive applications. Despite the technological importance of (Cr,Fe)7C3 carbides, attempts at using computational methods for tailoring its growth characteristics, stability and mechanical properties are hindered by the ambiguity associated with its atomisitic structure. The presented work aims at removing the former ambiguity regarding the atomistic structure of M7C3-type carbides. To this end we utilized EBSD (electron backscattered diffraction), TEM (transmission electron microscopy) diffractions and high-resolution STEM (scanning transmission electron microscopy) imaging, gathered from 9 zone-axis of the (Cr,Fe)7C3 phase formed in the as-cast AlCrFe2Ni2 high entropy alloy. Our experimental findings, supplemented by DFT calculations, indicates that only the hexagonal structure is a viable option. Additionally, we show that a carbon atom has to populate the center of a octahedral complex, a cite which was previously considered vacant. Furthermore, we demonstrate the co-existence of two variants of the newly revealed atomic structure which we believe were mistakenly identified as stacking faults in previous studies. The higher structural complexity of the newly identified structure, strengthened by the existence of spatially alternating variants, is suggested to partially justify the preference of this structure over the orthorhombic one. We believe that this study, and its like, are vital for aiding computational efforts aimed at controlling the growth, stability and performance of (Cr,Fe)7C3 carbides.