Isothermal oxidation behaviors of FeCoV and FeCoVNb alloys

The present study investigates the isothermal oxidation of two iron-cobalt based alloys at 500 °C for up to 5000 h. Both alloys exhibit crack-free oxide layers consist of a porosity-free iron rich outer layer and a solute rich inner layer populated with islands of solute oxides and porosities. X-ray diffraction measurements reveal the presence of MO–Fe2O3-type (where M=Co, Nb, and/or V) spinel structures along with Fe2O3. Analysis shows that the oxidation follows the parabolic rate law for both alloys with niobium containing alloy exhibiting a higher oxidation rate. Comparison of yield strengths (σ0) of oxidized specimens to those aged similarly under an inert atmosphere indicates that two competing mechanisms occur. Surface oxidation decreases the yield strength while precipitation reactions inside the iron-cobalt matrix increase the yield strength of the matrix.