Mechanism of SiMo DCI HT oxidation based on complementary LM/SEM/TEM and XRD methods

Abstract Cast irons intended for high temperature (HT) applications like the SiMo family are widely used in the automotive industry, however further progress in that area requires a thorough understanding of their oxidation behavior. In the present work, measurements of weight gains during cyclic HT exposition (1000 °C in air) of the SiMo5-1 (5 wt. % Si, 1 wt. % of Mo) were explained by microstructure investigation with light, scanning and transmission electron microscopy (LM, SEM, TEM). The results showed that ferrite areas and regions filled with carbides promoted two different oxidation paths. First one triggered formation of amorphous SiO 2 layer covered with Fe 3 O 4 , while the latter restricted development of the silica diffusion barrier, leading to an accelerated growth of globular Fe 2 O 3 crystallite agglomerates. With time, the oxide scale over ferrite areas thickened and transformed into a mixture of Fe 3 O 4 and α-SiO 2 phases, eventually surrounding the globules and slowing down their expansion.