ON THE OXIDATION OF ALPHA-FE AND EPSILON-FE2N1-Z .2. RESIDUAL STRAINSAND BLISTERS IN THE OXIDE LAYER

The development of residual strains in the iron-oxide layers growing o n alpha-Fe and epsilon-Fe2N1-z at 673 K in O-2 at 1 atm was investigat ed by X-ray diffraction at room temperature. After correction for ther mal-strain development due to cooling after oxidation, it was found th at tensile growth strains occur in magnetite and compressive growth st rains occur in hematite. The growth strains in the oxides on alpha-Fe are (in absolute sense) 2-3 times as large as those in the oxides on e psilon-Fe2N1-z. Buckling of the oxide layer occurs in the case of an a lpha-Fe substrate, which is attributed mainly to relaxation of the gro wth strains in magnetite and hematite. Thermal-strain development duri ng cooling enhances the tendency for buckling. Buckling is nor observe d for oxide layers on epsilon-Fe2N1-z, which could be due to the small er values of absolute strain in the oxide layer on epsilon-nitride. Th e absolute values of the growth strains in the oxide layer on epsilon- nitride being smaller is attributed to microstructural changes in the nitride layer during oxidation.