Phase Equilibria in Ni-Cr-W-C quaternary system at low carbon activity at 1,100 deg C

Isothermal iso-carbon-activity sections of an equilibrium phase diagram in a Ni-Cr-W-C quaternary system were experimentally determined at a temperature of 1,100 deg C at two carbon activities of aC = 0.01 and 0.03. Twelve ternary Ni-Cr-W alloys were carburized using a sealed capsule technique. Phases in the equilibrated specimens were identified by X-ray powder diffraction. Chemical compositions of the constituent phases were determined by means of electron probe microanalysis. Nickel-rich fcc γ phase is in equilibrium with three carbides : M23C6, M6C and M12C, at aC = 0.01. One more carbide: M7C3, emerged at aC = 0.03. A computer calculation of the phase equilibrium diagram has been carried out on the basis of the experimentally determined phase equilibria. The calculation has been proved to form a good estimate of the phase equilibria in the activity range from aC = 0 to 0.05. Calculated phase boundaries well explain observed microstructures in the newly developed Ni-Cr-W superalloys. (author)