Morphology of [011]-tilt Σ3 boundaries in pure copper and copper–aluminum alloy tricrystals

Tricrystals of pure Cu, Cu-3at.%Al and Cu-9at.%Al having [011]-tilt Σs=3, 3 and 9 CSL boundaries were grown and the morphology of the grain boundaries was investigated. In pure Cu and Cu-3at.%Al tricrystals, the Σ3 boundaries were composed of flat planar surfaces of the {111} coherent plane and short incoherent segments. Frequently observed incoherent steps are {322} and {311}. As the aluminum content of the alloy was increased to 9at.%, the morphology of the Σ3 boundaries changed drastically. The average step height became larger and the ratio of the incoherent segments in the total length of the Σ3 boundary increased to about 40%. To explain these experimental results, a simple model is proposed which attributes the step formation to the nucleation and subsequent sidewise growth of microscopic twins close to the Σ3 boundary during the solidification process.