Abstract
Dilatometer, resistivity, and electron-microscope measurements have been made on aluminium-4½% copper alloys containing additions of 0.1% indium or tin and 1% silver. Activation-energy measurements indicated the presence of mobile solute-vacancy clusters in the Al-Cu-In and Al-Cu-Sn alloys and nucleation of θ′ probably took place by the coalescence of these clusters. Silver additions to the tin-bearing alloy had very little influence on precipitation characteristics, but a similar addition to the indium-bearing alloy delayed the nucleation of θ′ and produced a coarse precipitate dispersion. The results for Al-Cu-In-Ag suggested that nucleation of θ′ in this alloy took place by the growth of individual solute clusters to a critical nucleus size (activation energy ∼ 1.45 eV).
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