Oxidation behavior of three-phase Cu-25Ni-30Cr alloy at 700-800 degrees C under high oxygen pressures

The thermogravimetric analysis of a ternary Cu-25Ni-30Cr alloy prepared by conventional casting was performed in 0.1 MPa pure O-2 at 700 - 800 degrees C. The results show that the alloy is composed of three phases, where the phase with the largest copper and lowest chromium content forms the matrix, while the other two, much richer in chromium, form a dispersion of isolated particles. At variance with another three-phase Cu-20Ni-20Cr alloy, which forms complex scales containing the oxides of the various components and double oxides plus an irregular region composed of alloy and oxides, the present alloy can form a very irregular but continuous chromia layer at the base of the mixed internal region, producing a gradual decrease of the oxidation rate down to very low values. A larger chromium content needed to form chromia layer for a ternary three-phase alloy is attributed to the limitations to the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.