In situ TEM observations of initial oxidation behavior in Fe-rich Fe-Cr alloys

Two nanocrystalline alloys of Fe-4Cr and Fe-16Cr were sputter deposited and formed a solid solution A2 phase. Each alloy was then in situ annealed up to 800 °C with the phase and microstructure quantified by transmission electron microscopy (TEM). The low partial pressure of oxygen in the TEM column (~2.6 × 10−6Pa) oxidized both films at temperatures at and above 700 °C. However, the oxide morphologies were quite different. The Fe-4Cr revealed nanosphere Cr oxide clusters within the grains which have been explained by the thermodynamic preference for Cr to be in solution. The Fe-16Cr precipitated distinct Cr2O3 grains which have been described by Cr phase separation at the grain boundaries. Iron oxide was not observed and demonstrating Cr's strong preference for oxygen, even in very low oxygen environments. Using precession electron diffraction (PED), the grain characters of these boundaries were determined to reveal a significant effect of high angle grain boundaries on the growth and precipitation behavior of the oxides. The collective results reveal the initial stages of oxidation evolution in nanocrystalline grains, the role of Cr content on such behavior.