On the Oxidation of α-Fe and ε-Fe2N1-z: II. Residual Strains and Blisters in the Oxide Layer

The development of residual strains in the iron-oxide layers growing on α-Fe and ε-Fe2N1-z at 673 K in O2 at 1 atm was investigated by X-ray diffraction at room temperature. After correction for thermal-strain development due to cooling after oxidation, it was found that tensile growth strains occur in magnetite and compressive growth strains occur in hematite. The growth strains in the oxides on α-Fe are (in absolute sense) 2-3 times as large as those in the oxides on ε-Fe2N1-z. Buckling of the oxide layer occurs in the case of an α-Fe substrate, which is attributed mainly to relaxation of the growth strains in magnetite and hematite. Thermal-strain development during cooling enhances the tendency for buckling. Buckling is not observed for oxide layers on ε-Fe2N1-z, which could be due to the smaller values of absolute strain in the oxide layer on ε-nitride. The absolute values of the growth strains in the oxide layer on ε-nitride being smaller is attributed to microstructural changes in the nitride layer during oxidation.