Comparative TEM investigation on the precipitation behaviors in Cu-15wt%Sn alloy

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 well-defined 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 δ 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 α phase. These contrasting precipitation behaviors are discussed from the viewpoint of crystallographic coherency of these phases.