The effect of silicon on the structure of Fe-40 at.% Al type alloys with high contents of carbon (1.9-3.8Â at.%)

The phase compositions of alloys based on Fe-40 at.% Al with additions of carbon ranging from 1.9 to 3.8 at.% and with and without the addition of 1.2 at.% Si were studied by light optical and transmission electron microscopies, X-ray diffraction and electron probe microanalysis. The structure of the matrix of all alloys is a B2 lattice, inherent to the intermetallic compound FeAl. Two main precipitating phases are observed: graphite and aluminium carbide Al4C3. The quantity of the phases depends in detail on the chemical composition and the heat treatment. The presence of Al4C3 in the alloys with the highest carbon content is in agreement with the recent theoretical prediction of a Fe-Al-C ternary phase diagram by Ohtani et al., as well as with older experiments of Vyklický and Tůma. The occurrence of Al4C3 is supported by the addition of silicon. Small traces of carbide κ of perovskite-type Fe3AlC are observed only in the alloys with the lowest aluminium and carbon contents.