Formation mechanism of stacking faults and its effect on hardness in M7C3 carbides

Although stacking faults are commonly found in M7C3 carbides, their causes and effects on the properties of carbides are rarely reported. The purpose of this work is to explore the effect of solidification rate on the stacking fault density and study the effect of stacking faults on the carbide microhardness. Hypereutectic high-chromium cast iron samples were solidified at three different cooling rates, and optical microscopy, scanning electron microscopy, X-ray diffraction, electron probe microanalysis, transmission electron microscopy, and hardness testing were employed to study the changes in carbides. The results show that the density of stacking faults increases with the cooling rate. Partial dislocations due to stacking faults were found in carbides with high fault densities. The Cr/Fe ratio in the carbides continued to decrease with the cooling rate, while the microhardness of the carbides first increased slightly and then decreased. The analysis shows that the significant growth driving force owing to larger degrees of undercooling generated more stacking faults. Notably, hardness is related to stacking faults in carbides. Moreover, low-density stacking faults could increase the microhardness of carbides, while high-density stacking faults produce a large number of vacancies that could soften carbides.