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

Abstract

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 V_O= --6590/T+2.892 and e exp Nb_O= --4066/T+1.502) and activity coefficients of vanadiun and niobium in dilute solution (ln gamma exp O_V= --35 320/T + 12.68 and ln gamma_Nbexp 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 VO_1+x.