Oxidation of a three-phase Cu-45Ni-30Cr alloy at 700-800 degrees C under 1 atm O-2

The oxidation of a ternary Cu-Ni-Cr alloy containing approximately 45 wt.% Ni and 30 wt.% Cr has been studied in 1 atm O-2 at 700-800degreesC. The alloy contains a mixture of three phases: the one with the largest copper and lowest chromium content forms the matrix, the one with an intermediate chromium content has a rather large volume fraction and forms large islands, while the phase richest in chromium forms isolated particles dispersed in the other two phases. At variance with another Cu-Ni-Cr ternary three-phase alloy containing only 20 wt.% Cr and 20 wt.% Ni, which formed complex scales containing mixtures of the oxides of the various components and double oxides, plus an irregular region composed of a mixture of alloy and oxides, the present alloy is able to form protective, external chromia scales. A similar result could be obtained with alloys containing about 20 wt.% Cr, but composed of either a single phase (Cu-60Ni-20Cr) or of a mixture of two phases (Cu-40Ni-20Cr). The need for a larger chromium content for producing chromia scales for three-phase as compared to two-phase Cu-Ni-Cr alloys is attributed to the limitations of the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.