Cruciform pattern formation in Sn/Co couples

Abstract

CoSn3 phase is formed in Sn/Co couples reacted at 200 °C. The reaction phase shows a unique cruciform pattern that is observed for the first time in the solid/solid reaction. The reaction phase layers are thick and uniform along the edges of the Co substrate, and there are no reaction phases at the corners. A continuous reaction layer is formed in Sn/Co couples reacted at 180 °C. A metastable CoSn4 phase is formed at the corner, and the reaction phase along the edge of the Co substrate is the CoSn3 phase. The reaction CoSn3 phase region shows a cruciform pattern if the CoSn4 phase region is ignored. It is concluded that the Sn flux is much faster than the Co flux, and the cruciform pattern of the reaction CoSn3 phase layer is formed either by cracking or transformation to the CoSn4 phase at the corners where stresses are most intensified.