Effects of microstructure on native oxide scale development and electrical characteristics of eutectic Cu-Cu6La alloys

A combination of electron microscopy, focused ion beam and conductive atomic force microscopy techniques have been used to study the microstructure, oxide scale development, and electrical behavior of a Cu-9 at.% La alloy. The as-cast alloy exhibits a eutectic microstructure comprising 30 vol.% Cu rods in a Cu6La matrix. The eutectic colonies exhibit a singular orientation relationship with [0 1 0] Cu6La parallel to 〈0 1 1〉 Cu along the rod axis, and it is shown that this corresponds to lattice matching of the two phases along this direction (∼0.02% misfit). Oxidation of the alloy at 100 °C to accelerate formation of a native oxide scale led to the development of a Cu2O layer less than 25 nm thick on the Cu rods and a La-doped Cu2O scale up to 1 μm thick on the Cu6La matrix. The La-doped regions of the scale are more conductive despite being much thicker, which is consistent with previous contact resistance data obtained for this alloy. The mechanisms responsible for the formation of this non-equilibrium oxide scale structure and for the enhanced electrical conductivity of the La-doped regions are discussed.