The iron silicocarbide in cast irons revisited

Abstract

Fe silicocarbides (SCs) in high-Si alloyed cast irons and steels have been frequently investigated in literature. The nature of SCs in these materials, however, has been controversially discussed und is hitherto uncertain. In particular, various crystal structures and chemical compositions were reported, several of which have been considered in different thermodynamic descriptions of the metastable Fe–C–Si system.In the current work, the occurrence of SCs within the microstructure of metastable-solidified Fe–C–Si model alloys (2.2 wt.% - 3.6 wt.% C; 3.0 wt.% - 11.0 wt.% Si) as well as their crystallographic nature have been studied by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Among the SCs reported in literature, the triclinic ‘Fe8Si2C’-type SC is the only SC existing in the investigated materials. This SC was identified as primary phase formed from the melt and in eutectics together with α-Fe, γ-Fe and cementite. The results of the microstructural investigations have been discussed in regard to recent thermodynamic descriptions of the metastable Fe–C–Si system. Moreover, crystallographic defects of the SC were observed for the first time and reasoned at atomic scale employing the concept of plane nets. These defects are stacking faults located on (001) and twins characterized by rotations of 180° about [010] and the (012) plane normal, exhibiting the habit planes (001) and (012), respectively. Stacking faults and multiple twinning might allow a composition range extending around Fe23Si5C4, in which the ‘Fe8Si2C’-type SC is stable.