The precipitation sequence in a Cu-20 mass%Ni-8 mass%Sn alloy on aging at temperatures between 623 and 723 K after quenching from 1123 K and the equilibrium phase diagram at 823 K of the Cu-Ni rich Cu-Ni-Sn system were investigated by means of optical and electron microscopic observations, X-ray analysis, and hardness measurements. Results obtained are as follows:
(1) The equilibrium precipitate phase in a Cu-20%Ni-8%Sn alloy was δ phase (β-Cu₃Ti type) with the lattice parameters of a=0.451, b=0.538 and c=0.427 nm.
(2) The precipitation sequence of this alloy is as follows;
\ oindentα-supersatulated solid solution
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(3) The values of apparent activation energy for intragranular and cellular precipitation of this alloy are 170 and 180 kJ/mol, respectively.
(4) The phase boundaries among (α+γ(DO₃)), (α+γ+δ), (α+δ), (α+δ+θ(DO₁₉)) and (α+θ) regions at 823 K in the Cu-Ni rich Cu-Ni-Sn system were determined.