Ternary Phase Equilibria in Transition Metal-Boron-Carbon-Silicon Systems. Part Ii. Ternary Systems. Volume Xviii. Constitution of Niobium-Tungsten-Carbon Alloys

摘要

The ternary alloy system niobium-tungsten-carbon was investigated by means of X-ray, melting point, DTA, and metallographic techniques on chemically and thermally defined specimens. A phase diagram from 1500C through the melting range was established. The solid state equilibria in the system are characterized by limited metal exchanges in the subcarbides and the occurrence of a two-phase equilibrium between the monocarbide and the metal phase. Above 2530C, niobium monocarbide and the cubic high temperature phase in the W-C system form an uninterrupted series of solid solutions; below this temperature, the tungsten exchange in NbC is temperature-dependent and decreases to approximately 25 At% W at 1500C. The tungsten exchange in Nb2C is less than 5 At% at all temperatures, whereas the solubility in W2C increases from 24 mole% Nb2C at 1500C, to a maximum of approximately 37 mole% Nb2C at 2600C. The order-disorder reaction, as well as the change-of-order transition, temperature in W2C and Nb2C are lowered by niobium additions, but the details of the transition reactions in the ternary could not be delineated. Five isothermal reactions, all of which involve a liquid phase, occur in the system. Two correspond to limiting tie lines and the remaining three are class II (two-over-two) ternary reactions. The results of this phase diagram investigation are discussed and the phase equilibria thermodynamically analyzed and interpreted. (Author)