Morphology of [011]-tilt Sigma 3 boundaries in pure copper and copper-aluminum alloy tricrystals

Tricrystals of pure Cu, Cu-3at.%Al and Cu-9at.%Al having [011]-tilt Sigma s = 3, 3 and 9 CSL boundaries were grown and the morphology of the grain bou ndaries was investigated. In pure Cu and Cu-3at.%Al tricrystals, the Sigma 3 boundaries were composed of flat planar surfaces of the {111} coherent pl ane 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 Sigma 3 boundaries changed drastically. The avera ge step height became larger and the ratio of the incoherent segments in th e total length of the Sigma 3 boundary increased to about 40%. To explain t hese experimental results, a simple model is proposed which attributes the step formation to the nucleation and subsequent sidewise growth of microsco pic twins close to the Sigma 3 boundary during the solidification process. (C) 2000 Elsevier Science S.A. All rights reserved.