Microstructural effects on the high temperature oxidation of two-phase Cu–Cr alloys in 1 atm O 2

Abstract

The oxidation of a Cu–Cr alloy containing about 60 wt% Cr and of two Cu–Cr alloys containing about 40 wt% Cr was studied at 700 and 800 °C in 1 atm O 2. The 60 wt% Cr alloy was prepared by powder metallurgy (PM) and had a phase particle size of 50–150 μm. One of the two alloys containing about 40 wt% Cr was prepared by mechanical alloying (MA) and had a phase grain size ranging from 10–50 nm to 200–300 nm, depending on the location, while the other was prepared by magnetron sputtering (MS) and had a phase grain size around 5–10 nm. The most important difference between the oxidation behavior of the three alloys is the formation of an exclusive chromia scale on the surface of the Cu–40 wt% Cr alloy prepared by magnetron sputtering and of a continuous chromia layer beneath an outermost layer of copper oxides on the corresponding alloy prepared by mechanical alloying, while the Cu–60 wt% Cr alloy prepared by powder metallurgy formed complex scales composed mostly of CuO, Cu 2O with some Cu 2Cr 2O 4 and Cr 2O 3. Thus, the microstructure of two-phase binary alloys has a strong effect of their oxidation behavior. In particular, a decrease of the alloy grain size favors the exclusive external oxidation of the most reactive component, reducing the corresponding critical content in the alloy. This effect is attributed to the presence of larger concentrations of rapid diffusion paths for the migration of the components in the alloy as well as to a faster dissolution of the particles of the Cr-rich phase in the copper matrix.