Initial stages of decomposition in Cu–9Ni–6Sn

The early stages of the decomposition process in the ternary Cu-9 wt.% Ni-6 wt.% Sn alloy have been studied by transmission electron microscopy. The supersaturated solid solution was found to decompose through the spinodal mechanism into periodic and aligned regions which are Sn-rich and Sn-lean. It was also found that in addition to this clustering reaction, an ordering reaction also progressed with the ageing treatment to produce a metastable phase with the DO₂₂ (Al₃Ti type) structure. However, on prolonged ageing, a discontinuous reaction started at the grain boundaries and consumed the matrix leaving behind colonies of a pearlitic product composed of the equilibrium α (A₁ structure) and the γ (DO₃ structure) phases. This paper discusses the mechanism of formation of the DO₂₂ phase in the alloy.