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

Abstract

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 NbCx was observed as a second phase in the equilibrated alloys under the present experimental conditions. The carbon concentration x of NbCx was lower than that of the stoichiometric carbide NbC and was estimated to be x=0.81 from the experimentally determined solubility of NbCx 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 YNb(YC)0.81=exp{0.053-(14000/T)}, where the concentration parameters of niobium and carbon, YNb and YC, are related to the mole fractions, XNb and XC, by the equations YNb/(1-XC) and YC=XC/(1-XC). The present results were compared with those for commercial Inconel alloy 600.