Alloy/oxide equilibria in iron--vanadium--oxygen and iron--niobium--oxygen systems

An experimental characterization of three-phase equilibria in Fe--V--O and Fe--Nb--O systems at 1823, 1873 and 1923K has been carried out using a solid state cell and by analysis of quenched samples. The oxygen potentials corresponding to these three-phase equilibria were monitored by a solid state cell incorporating Y2O3 doped ThO2with Cr+r2O3 as reference electrode. Similar measurements were carried out for Fe--Nb--O alloys in equilibrium with a mixture of FeNb2O6 and NbO2 . These measurements permit evaluation of interaction parameters (e exp VO= --6590/T+2.892 and e exp NbO= --4066/T+1.502) and activity coefficients of vanadiun and niobium in dilute solution (ln gamma exp OV= --35 320/T + 12.68 and ln gammaNbexp O=--12 386/T + 4.34) in liquid iron. The results obtained in this study resolve a number of discrepancies in thermodynamic data reported in the literature, especially regarding the activity coefficients of V and Nb and the stability ranges for V2O3 and VO1+x