The decomposition and precipitation behaviors of a quenched Cu-15wt%Sn alloy as a function of aging temperature were investigated using transmission electron microscopy (TEM). Focused ion beam The decomposition and precipitation behaviors of a quenched Cu-15wt%Sn alloy as a function of aging temperature were investigated using transmission electron microscopy (TEM). Focused ion beam (FIB) was employed to assist TEM specimen preparation. At 300 °C, the decomposition of the supersaturated α’ phase occurred at grain boundaries, displaying a cellular morphology. The lamellae were found with ζ and α phases, rather than with the equilibrium ε and α phases. The ζ and α phases exhibit a welldefined orientation relationship (OR) as (110)α//(0001)ζ; [112]α//[1210]ζ . On the other hand, at 320 °C, only incipient lamellar structures of several micron meters were observed, which were composed of the  and α phases. At the same time, abundant intragranular precipitation of the ε phase in the form of platelets was observed, and OR as (111)α//(001)ε ; [110]α //[100]ε exists between ε phase and the a phase. These contrasting precipitation behaviors are discussed from the viewpoint of crystallographic coherency of these phases. The decomposition and precipitation behaviors of a quenched Cu-15 wt% Sn alloy as a function of aging temperature were investigated using transmission electron microscopy (TEM). Focused ion beam The decomposition and precipitation behaviors of a quenched Cu-15 wt% Sn alloy as a function At 300 ° C, the decomposition of the supersaturated α ’phase occurred at grain boundaries, displaying a cellular morphology. The lamellae were found with ζ and α phases, rather than with the equilibrium ε and α phases. The ζ and α phases exhibit a well defined orientation relationship (OR) as (110) α // (0001) ζ; / [1210] ζ. On the other hand, at 320 ° C, only incipient lamellar structures of several micron meters were observed, which were composed of the  and α phases. At the same time, abundant intragranular precipitation of the ε pha (110) α // [100] ε exists between ε phase and the a phase. These contrasting precipitation activities are discussed from the viewpoint of crystallographic coherency of these phases.