Investigation on vanadium carbide transformations in M23C6 carbide by experimental observation and crystallographic analysis

An investigation based on high-resolution transmission microscopy (HRTEM) and crystallographic analysis was performed to gain insights into vanadium (V) carbide transformations in M23C6 carbide. Vanadium carbides including V6C5, V8C7 and VC were observed in or adjacent to M23C6 carbides, and the orientation relationships for V6C5/M23C6, VC/ M23C6 and V8C7/VC were revealed. Energy dispersive spectrometry (EDS) and atomic-scale configuration analysis on M23C6 contained a certain amount of V indicated that VV atomic pair and local V-rich domain were gradually formed in M23C6 cell as the diffusion of V in M23C6, contributing to formation of vanadium carbides in M23C6. Crystallographic analysis on vanadium carbides observed in M23C6 clarified that the mechanism of vanadium carbide transformations in M23C6 was fulfilled by decreasing of metal atom number in the carbon coordination polyhedron from eight to six, as well that V6C5 and V8C7 containing carbon vacancies precipitated preferentially in M23C6.