Thermodynamics and phase relations in refractory metal solid solutions containing carbon,nitrogen, and oxygen

The refractory metals Nb, Ta, Mo, and W dissolve C, N, and O by forming interstitial solid solutions. The concentration of these components can be increased or decreased by annealing treatments, depending on the partial pressure of gases such as N₂, O₂, H₂O, CH₄, or CO and on the temperature of the specimen. New results in binary and ternary systems combined with those obtained in the last few years now provide most of the data needed for the thermochemical analysis of the solid solution phases and for the establishment of p-T-c phase diagrams. The mechanisms of the gas-metal reactions and the general feature of the constitution diagrams are similar for all refractory metals. However, marked differences exist between group Va metals niobium and tantalum and group VIa metals molybdenum and tungsten in the absolute values and temperature dependence of the equilibrium pressure of gaseous species and therefore also in the amount of gas absorbed. The data now available for the estimation of the final content of interstitials in solid solution after annealing treatments are compiled and discussed. On leave from the Max-Planck-Institut fur Metallforschung, Institut fur Sondermetalle, Stuttgart, Germany