Solubility of NbC in a Ni-16Cr-8Fe Alloy

抄録

The solubility of niobium carbide NbC in the nickel-rich phase of the Ni-16Cr-8Fe system, which is a base system of commercial Inconel alloy 600, was experimentally determined at a temperature range between 1273 and 1473 K. A ternary Ni-16Cr-8Fe alloy was prepared from three elemental metals. Thirteen Ni-16Cr-8Fe-Nb quaternary alloys with niobium concentrations between 0 and 6.0 mass% were prepared from the Ni-16Cr-8Fe alloy and pure niobium. The Ni-16Cr-8Fe-Nb alloys were carburized by a sealed capsule method at equilibrating temperatures of 1273, 1373 and 1473 K for various times between 3.1×10⁵ and 7.8×10⁵ sec. Carbon activity in each capsule was determined from the carbon concentration of the pure iron sealed in the capsule together with the alloys.
Only niobium carbide NbC_x was observed as a second phase in the equilibrated alloys under the present experimental conditions. The carbon concentration x of NbC_x was lower than that of the stoichiometric carbide NbC and was estimated to be x=0.81 from the experimentally determined solubility of NbC_x in the nickel-rich phase. The solubility was described as a solubility product and its temperature dependence was evaluated as (mass% Nb)(mass% C)^0.81=exp{7.5-(14000/T)} or Y_Nb(Y_C)^0.81=exp{0.053-(14000/T)}, where the concentration parameters of niobium and carbon, Y_Nb and Y_C, are related to the mole fractions, X_Nb and X_C, by the equations Y_Nb/(1-X_C) and Y_C=X_C/(1-X_C). The present results were compared with those for commercial Inconel alloy 600.