Oxidation of two-phase Cu–50 Cr alloy at low oxygen pressure

Oxidation behaviour of a cast two-phase Cu–50 Cr alloy and a nano-crystalline two-phase Cu–50 Cr alloy coating were studied at 1073 K at an oxygen partial pressure of 10−8 Pa. Microstructural observations clearly showed that at the initial oxidation stage, discrete Cr2O3 scales were formed only on the β-phase (Cr) areas of the cast Cu–50 Cr alloy, however, surface diffusion of Cr would assist the lateral growth of these discrete Cr2O3 scales from the β-phase regions, a continuous Cr2O3 scale could be developed to cover the whole sample surface if the oxidation time is long enough (the Cr content should also be high enough). Formation and growth of oxide scales on the nano-crystalline two-phase Cu–50 Cr alloy coating were quite different. A continuous Cr2O3 scale could form and cover the whole surface after only 15 min oxidation, indicating that a phase grain size reduction could promote the formation of a protective scale on the surface through enhanced diffusion along grain boundaries and shortened lateral surface diffusion distance. Based on these morphological results, a physical model describing the role of Cr surface diffusion in the scaling behaviour of two-phase alloys was proposed.