Chromium and silicon react at high temperatures to form two stable compounds: CrSi and CrSi2. Because chromium silicides are more stable than its carbides, in the presence of silicon, some carbon will be replaced by silicon, forming carbosilicon complex chromium compounds, until silicides are formed. Yu.A. Pavlov studied the phase structure of Cr-Si-Fe-C cast alloys with a Cr:Fe ratio of 1.
When Si content in the alloy is <20%, it is essentially composed of a single phase (Cr,Fe)3(C,Si)2. This can be considered as the result of some Cr being replaced by Fe and some C by Si in Cr3C2. When the silicon content increases to >20%–29%, a new complex phase (Cr,Fe)(Si,C) is formed. Excess Cr and Fe form the intermetallic compound FeCr, i.e., the σ phase. Between 29% and 34% Si content, a new phase (Cr,Fe)Si is added. When Si exceeds 34%, chromium, iron, and silicon form silicides. The increased silicon content leads to the formation of CrSi2 and SiC phases. Chromium has a stronger affinity for silicon than iron, so CrSi2 is formed first. However, CrSi2 and FeSi2 have different crystal structures and cannot form a solid solution. When the Si content is 44%–51%, Cr reacts with Si to form CrSi2, and some FeSi reacts with Si to form FeSi2. When the Si content is 51%–60%, the alloy consists of Cr-Si2, FeSi2, SiC, and Si. From the above results, it can be seen that high-silicon chromium-ferrosilicon alloys are composed of chromium and iron silicides, SiC, and Si, meaning carbon exists in the SiC phase. The structural analysis of industrially produced silicon-chromium-ferrosilicon alloys is basically consistent with this. Carbon exists as the SiC phase, which is insoluble in the liquid phase of silicon-chromium-ferrosilicon.
